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Behavioural mechanisms of population regulation in the limpet, Acmaea digitalis Breen, Paul Allan 1970

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BEHAVIOURAL, MECHANISMS  OF POPULATION REGULATION  THE LIMPET,  A C M A E A  DIGITALIS  by  PAUL A L L A N B.Sc,  BREEN  U n i v e r s i t y of B r i t i s h C o l u m b i a ,  A THESIS S U B M I T T E D  1968  IN P A R T I A L F U L F I L M E N T  THE REQUIREMENTS MASTER  FOR THE DEGREE OF  OF  SCIENCE  in the D e p a r t m e n t of Zoology  We a c c e p t this t h e s i s a s c o n f o r m i n g to the required  standard.  THE UNIVERSITY  OF BRITISH  October,  1970  COLUMBIA  OF  IN  In p r e s e n t i n g t h i s  thesis  in p a r t i a l  f u l f i l m e n t o f the requirements f o r  an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, the I  Library shall  freely available  f u r t h e r agree t h a t p e r m i s s i o n  for  for  r e f e r e n c e and  f o r e x t e n s i v e copying o f  this  that  study. thesis  s c h o l a r l y purposes may be granted by the Head of my Department or  by h i s of  make i t  I agree  this  written  representatives. thesis  It  is understood that copying o r p u b l i c a t i o n  f o r f i n a n c i a l gain s h a l l  not be allowed without my  permission.  Paul  Department of  Zoology  The U n i v e r s i t y o f B r i t i s h Vancouver 8, Canada  Date  Columbia  Allan  B R E E N  ii  ABSTRACT  Two hypotheses  that d e s c r i b e d possible m e c h a n i s m s  regulation in A c m a e a digitalis were examined in this  The first hypothesis  of p o p u l a t i o n  study.  stated that l o c a l populations c o u l d be  b y m e a n s of c h a n g e s i n t h e h o m i n g  behaviour  regulated  of i n d i v i d u a l l i m p e t s .  c h a n g e s w o u l d o c c u r a s a r e s u l t of f e e d b a c k f r o m d e n s i t y c h a n g e s . f r o m experiments which tested this hypothesis  The second hypothesis  were  These Results  inconclusive.  stated that local populations  c o u l d be  regulated  b y m e a n s of u p w a r d m i g r a t i o n of l i m p e t s i f s u c h m i g r a t i o n w e r e  density  dependent;  density  and that m o r t a l i t y during s u m m e r  dependent way.  This hypothesis  was  would also act in a  substantiated experimentally,  it was not shown that the m e c h a n i s m does indeed regulate l o c a l  Possible ted and  populations.  e x p l a n a t i o n s of h o m i n g a n d m i g r a t i o n b e h a v i o u r a r e  discussed.  although  presen-  iii  TABLE  OF  CONTENTS  Page  TITLE PAGE  1  ABSTRACT TABLE  OF  ii CONTENTS  LIST OF  FIGURES  LIST OF  TABLES  iii v  v  i  i  GENERAL INTRODUCTION  1  EXPERIMENTAL METHODS  3  . Introduction D e t e r m i n a t i o n of t i d a l h e i g h t s  3 3  Marking Enclosures Displacement  of l i m p e t s  3 4 4  Measurement  of l i m p e t s  6  DESCRIPTION OF  THE  STUDY AREA  Location Physical factors  7 7  D i s t r i b u t i o n of A c m a e a d i g i t a l i s Distribution and abundance Summary STUDIES OF  of f o o d s p e c i e s  HOMING BEHAVIOUR  Introduction O c c u r r e n c e of h o m i n g I d e n t i f i c a t i o n of t w o b e h a v i o u r a l t y p e s R e l a t i o n between density and homing behaviour Relation between homing and local environmental conditions Conclusion STUDIES OF SEASONAL MIGRATION BEHAVIOUR Introduction O c c u r r e n c e of s e a s o n a l  7  8 13 14 I 6 1° 18 19 31 35 42 45 45  migration  R e l a t i o n of u p w a r d m i g r a t i o n w i t h d e n s i t y  46 52  iv  Page  Mortality  59  Conclusion  67  RELATION QF UPWARD MIGRATION DISCUSSION LITERATURE  WITH GROWTH  68 75  CITED  ACKNOWLEDGMENTS  77 79  V  LIST OF  FIGURES  Figure  Page  1  D e t a i l of e n c l o s u r e c o n s t r u c t i o n  5  2  Density  3  P e r c e n t a g e of l i m p e t s i n e a c h s i z e c l a s s at d i f f e r e n t t i d a l  of l i m p e t s a g a i n s t t i d a l h e i g h t  heights.  10  T h e t i d a l h e i g h t of e a c h s a m p l e  is given  in  parentheses 4  11  T h e n u m b e r s of l i m p e t s s e e n i n s i d e a n d o u t s i d e t h e where they were marked,  5  The number  f r o m July,  1969.  to J u l y ,  of l i m p e t s s e e n i n s i d e a n d o u t s i d e t h e  area 1970  47  area  w h e r e t h e y w e r e m a r k e d , f r o m A u g u s t , 1969, to M a r c h , 1970.. 6  The number  of l i m p e t s w h i c h m i g r a t e d i n e a c h d i r e c t i o n  d u r i n g the p e r i o d f r o m S e p t e m b e r 1970.  migrate;  d o t s i n t h e u p p e r p a r t of t h e c i r c l e  The number  The number  10  11  49  1, 1969> t o F e b r u a r y  16 t o J u n e 5,  16, 1 9 7 0 , . . .  50  1970  51  N u m b e r s of l i m p e t s i n the e x p e r i m e n t a l a n d c o n t r o l g r o u p s r e m a i n i n g i n the p a r t i a l e n c l o s u r e s o v e r t i m e . (See t e x t for explanation.)  54  T h e n u m b e r of l i m p e t s r e m a i n i n g i n e a c h p a r t of t h e c o n t r o l a r e a d u r i n g t h e c o u r s e of t h e e x p e r i m e n t . ( S e e t e x t f o r explanation.)  55  The number  of l i m p e t s r e m a i n i n g i n e a c h p a r t of t h e e x p e r i -  m e n t a l a r e a d u r i n g the c o u r s e for explanation.) 12  represent  a n d so on  of l i m p e t s w h i c h m i g r a t e d i n e a c h d i r e c t i o n  d u r i n g the p e r i o d F e b r u a r y 9  16,  of l i m p e t s w h i c h m i g r a t e d i n e a c h d i r e c t i o n  d u r i n g the p e r i o d S e p t e m b e r 8  1, 1 9 6 9 . t o F e b r u a r y  D o t s i n the c e n t r e r e p r e s e n t l i m p e t s that d i d not  l i m p e t s that m i g r a t e d upward; 7  48  of the e x p e r i m e n t .  The relation between relative density  text 57  and the percentage  m a r k e d l i m p e t s that did not m i g r a t e f r o m the a r e a fall and winter.  (See  (See t e x t f o r e x p l a n a t i o n . )  of  during 61  Figure  13  14  Page  T h e p e r c e n t a g e of l i m p e t s f o u n d i n the u p p e r t h r e e c e n s u s s q u a r e s o v e r the s u m m e r . Tidal heights a r e given in parentheses for each square  64  T h e p e r c e n t a g e of l i m p e t s r e m a i n i n g i n the t h r e e l o w e r c e n s u s s q u a r e s d u r i n g the s u m m e r . Tidal heights are given.in parentheses for each square  65  vii  LIST OF  TABLES  T A B L E  I  II  Page  T h e n u m b e r of l i m p e t s w h i c h o c c u r r e d i n e a c h c a t e g o r y of m i g r a t i o n f r e q u e n c y . T h e d a t a w e r e c o l l e c t e d f r o m the f i r s t m a r k e d s a m p l e of l i m p e t s , f r o m J u n e 5 t h r o u g h J u n e 10, 1 9 6 9 . T h e o b s e r v e d d i s t r i b u t i o n of m i g r a t i o n f r e q u e n c i e s was c o m p a r e d w i t h the P o i s s o n d i s t r i b u t i o n c a l c u l a t e d f r o m the m e a n n u m b e r of m i g r a t i o n s p e r l i m p e t . T h e l a s t f o u r c a t e g o r i e s w e r e g r o u p e d to m e e t t h e r e q u i r e m e n t s of the C h i - s q u a r e d t e s t f o r g o o d n e s s of f i t The number  of l i m p e t s w h i c h o c c u r r e d i n e a c h  of m i g r a t i o n f r e q u e n c y .  category  The data w e r e c o l l e c t e d f r o m /  the s e c o n d m a r k e d s a m p l e of l i m p e t s , t h r o u g h A u g u s t 16,  1969.  from August  11  T h e o b s e r v e d d i s t r i b u t i o n of  migration frequencies was  c o m p a r e d with the  d i s t r i b u t i o n c a l c u l a t e d f r o m the m e a n n u m b e r tion frequencies per limpet.  The last four  Poisson of m i g r a -  categories  w e r e g r o u p e d to m e e t the r e q u i r e m e n t s of the  Chi-  s q u a r e d t e s t f o r g o o d n e s s of f i t III  IV  V  21  22  T h e n u m b e r of l i m p e t s w h i c h o c c u r r e d i n e a c h c a t e g o r y of m i g r a t i o n f r e q u e n c y . T h e d a t a w e r e c o l l e c t e d f r o m F e b r u a r y 1 5 t h r o u g h F e b r u a r y 19. 1 9 7 0 . T h e o b s e r v e d d i s t r i b u t i o n was c o m p a r e d w i t h the P o i s s o n d i s t r i b u t i o n c a l c u l a t e d f r o m the m e a n n u m b e r of m i g r a t i o n s p e r l i m p e t . T h e l a s t t h r e e c a t e g o r i e s w e r e g r o u p e d to m e e t t h e r e q u i r e m e n t s of the C h i - s q u a r e d t e s t f o r g o o d n e s s of fit  23  T h e n u m b e r of l i m p e t s w h i c h o c c u r r e d i n e a c h c a t e g o r y of m i g r a t i o n f r e q u e n c y . T h e d a t a w e r e c o l l e c t e d f r o m A p r i l 2 9 t h r o u g h M a y 2, 1 9 7 0 . T h e o b s e r v e d d i s t r i b u t i o n w a s c o m p a r e d w i t h the P o i s s o n d i s t r i b u t i o n c a l c u l a t e d f r o m the m e a n n u m b e r of m i g r a t i o n s p e r l i m p e t . The l a s t t w o c a t e g o r i e s w e r e g r o u p e d to m e e t t h e r e q u i r e m e n t s o f t h e C h i - s q u a r e d t e s t f o r g o o d n e s s of f i t  24  The number  of l i m p e t s w h i c h o c c u r r e d i n e a c h  of m i g r a t i o n f r e q u e n c y . June 1 through June was  5,  category  The data w e r e c o l l e c t e d f r o m 1970.  The observed  distribution  c o m p a r e d w i t h the P o i s s o n d i s t r i b u t i o n c a l c u l a t e d  viii  T A B L E  Page  f r o m the m e a n n u m b e r  of m i g r a t i o n s p e r l i m p e t .  The  l a s t t h r e e c a t e g o r i e s w e r e g r o u p e d to m e e t the r e q u i r e m e n t s o f t h e C h i - s q u a r e d t e s t f o r g o o d n e s s of f i t VI  The number  of l i m p e t s w h i c h o c c u r r e d i n e a c h  of m i g r a t i o n f r e q u e n c y .  category  The date w e r e c o l l e c t e d f r o m  J u n e 16 t h r o u g h J u n e 19, 1 9 7 0 . was  25  The observed  distribution  c o m p a r e d w i t h the P o i s s o n d i s t r i b u t i o n c a l c u l a t e d  f r o m the m e a n n u m b e r  of m i g r a t i o n s  per limpet.  The  l a s t t h r e e c a t e g o r i e s w e r e g r o u p e d to m e e t the r e q u i r e ments VII  of t h e C h i - s q u a r e d t e s t f o r g o o d n e s s of f i t  The number  of l i m p e t s w h i c h o c c u r r e d i n e a c h  of m i g r a t i o n f r e q u e n c y . J u l y 23 t h r o u g h J u l y 27,  category  The data were c o l l e c t e d f r o m 1970.  The observed distribution  c a l c u l a t e d f r o m the m e a n n u m b e r limpet.  26  of m i g r a t i o n s  per  T h e l a s t t h r e e c a t e g o r i e s w e r e g r o u p e d to m e e t  t h e r e q u i r e m e n t s of the C h i - s q u a r e d t e s t f o r g o o d n e s s  VIII  IX  X  of  fit  27  A c o m p a r i s o n b e t w e e n the m i g r a t i o n f r e q u e n c y d i s t r i b u t i o n s of l i m p e t s i n i t i a l l y f o u n d i n l a r g e a n d s m a l l c l u m p s . A s m a l l c l u m p was defined as that containing l e s s than ten l i m p e t s ; a l a r g e c l u m p as that containing ten or m o r e l i m p e t s . T h e data w e r e c o l l e c t e d f r o m the f i r s t s a m p l e o f m a r k e d l i m p e t s , f r o m J u l y 23 t h r o u g h J u l y 27, 1970. The last two migration frequency categories were grouped t o m e e t t h e r e q u i r e m e n t s of t h e C h i - s q u a r e d t e s t f o r i n d e p e n d e n c e of s a m p l e s  29  A c o m p a r i s o n between the m i g r a t i o n f r e q u e n c y d i s t r i b u t i o n s of l i m p e t s i n i t i a l l y f o u n d i n l a r g e a n d s m a l l c l u m p s . A s m a l l c l u m p was defined as that containing l e s s than f i f t e e n l i m p e t s ; a l a r g e c l u m p as that containing fifteen or m o r e l i m p e t s . The data w e r e c o l l e c t e d f r o m the s e c o n d s a m p l e o f m a r k e d l i m p e t s , f r o m A u g u s t 11 t h r o u g h A u g u s t 16, 1 9 6 9 . T h e l a s t t h r e e m i g r a t i o n f r e q u e n c y c a t e g o r e i s w e r e g r o u p e d to m e e t the r e q u i r e m e n t s of t h e C h i - s q u a r e d t e s t f o r i n d e p e n d e n c e of s a m p l e s  30  The number  of l i m p e t s w h i c h o c c u r r e d i n e a c h  of m i g r a t i o n f r e q u e n c y . T a b l e III,  IV,  V,  and VI.  The data a r e combined  category from  The combined distribution  IX  T A B L E  XI  XII  Page  w a s c o m p a r e d w i t h the P o i s s o n d i s t r i b u t i o n c a l c u l a t e d f r o m t h e m e a n n u m b e r of m i g r a t i o n s p e r l i m p e t . T h e l a s t f o u r c a t e g o r i e s w e r e g r o u p e d to m e e t the r e q u i r e m e n t s of t h e C h i - s q u a r e d t e s t f o r g o o d n e s s of f i t .  32  C o m p a r i s o n b e t w e e n r e l a t i v e d e n s i t y a n d p e r c e n t a g e of l i m p e t s t h a t m i g r a t e d d u r i n g the p e r i o d of o b s e r v a t i o n , a n d b e t w e e n d e n s i t y a n d t h e m e a n n u m b e r of m i g r a t i o n s per l i m p e t per day. R e l a t i v e density was c a l c u l a t e d by d i v i d i n g t h e n u m b e r of l i m p e t s b y t h e n u m b e r i n t h e e n c l o s u r e on F e b r u a r y 15. C o r r e l a t i o n c o e f f i c i e n t s f o r both comparisons were calculated  34  The number  of l i m p e t s r e m a i n i n g , the n u m b e r  g r a n t s and the m e d i a n d i s t a n c e t r a v e l l e d by  of e m i -  emigrants  i n t h e e x p e r i m e n t a l ( E ) a n d c o n t r o l ( C ) g r o u p s of e x p e r i m e n t d e s i g n e d to m e a s u r e reduced food levels. the  XIV  XV  1,  an  of l i m p e t s to  removed from  e x p e r i m e n t a l g r o u p o n J u l y 3 0 , A u g u s t 2,  and September XIII  Algae was  response  around  10,  15  1969  37  T h e n u m b e r of l i m p e t s r e m a i n i n g , t h e n u m b e r of e m i g r a n t s a n d the m e d i a n d i s t a n c e t r a v e l l e d b y e m i g r a n t s i n t h e e x p e r i m e n t a l ( E ) a n d c o n t r o l (C) g r o u p s o f a n e x p e r i m e n t d e s i g n e d to m e a s u r e t h e r e s p o n s e of l i m pets to r e d u c e d food l e v e l s . A l g a e was r e m o v e d f r o m a r o u n d t h e e x p e r i m e n t a l g r o u p o n J u n e 4 a n d 5, 1 9 7 0  38  T h e i n i t i a l n u m b e r of l i m p e t s i n e a c h of f o u r t r e a t m e n t g r o u p s of a n e x p e r i m e n t d e s i g n e d to m e a s u r e response of l i m p e t s to c r o w d i n g , a n d the n u m b e r that r e m a i n e d i n e a c h g r o u p a f t e r one day. L i m p e t s r e m o v e d f r o m e l s e w h e r e w e r e a d d e d to the e x p e r i m e n t a l g r o u p s ; d i s t u r b e d g r o u p s w e r e r e m o v e d f r o m the r o c k and r e p l a c e d at the s a m e s i t e  40  The number  of l i m p e t s t h a t r e m a i n e d i n e a c h of  t r e a t m e n t g r o u p s a f t e r one day. The number  explanation.)  r e m a i n i n g in the h i g h d e n s i t y t r e a t m e n t  b e e n d i v i d e d b y 2, pared.  (See t e x t f o r  four has  so that r e s u l t s c a n be d i r e c t l y c o m -  T h e f i r s t two r e p l i c a t e s w e r e obtained f r o m J u l y  23 t o 24;  t h e s e c o n d t w o f r o m J u l y 24 t o 2 5 ,  1970  43  TABLE  XVI  Page  A c o m p a r i s o n b e t w e e n t h e m e a n l e n g t h of l i m p e t s t h a t i m m i g r a t e d to a n a r e a d u r i n g f a l l and winter, a n dthe m e a n l e n g t h of l i m p e t s w h i c h r e m a i n e d o n t h e s a m e a r e a f r o m the previous were measured  XVII  XVIII  XIX  XX  summer  on February  (residents).  Lengths 53  16, 1 9 7 0  A c o m p a r i s o n between the g r o w t h of l i m p e t s i n the e x p e r i m e n t a l a n d c o n t r o l g r o u p s of a n e x p e r i m e n t d e s i g n e d to t e s t t h e m i g r a t i o n r e s p o n s e s of l i m p e t s to h i g h d e n s i t y . T h e d e n s i t y of the e x p e r i m e n t a l g r o u p w a s m a d e high by adding l i m p e t s . In this c o m p a r i s o n the r e g r e s s i o n s of a t t a i n e d l e n g t h o n i n i t i a l l e n g t h f o r e a c h g r o u p arre c o m p a r e d . I n i t i a l l e n g t h w a s d e t e r m i n e d o n J u l y 5 a n d a t t a i n e d l e n g t h o n N o v e m b e r 22, 1969  58  C o m p a r i s o n between density change and the percentage of l i m p e t s t h a t d i d n o t m i g r a t e d u r i n g the p e r i o d A u g u s t 11, 1 9 6 9 t h r o u g h F e b r u a r y 16, 1 9 7 0 . C h a n g e i n d e n s i t y w a s c a l c u l a t e d b y d i v i d i n g t h e t o t a l n u m b e r of l i m p e t s present i n F e b r u a r y by the total number in August. S i x different locations on the beach a r e compared. A c o r r e lation coefficient between p e r cent r e m a i n i n g and density change is calculated  60  T h e o r i g i n a l n u m b e r of l i m p e t s , t i d a l height a n d a r e a of six m a r k e d squares, i n w h i c h censuses w e r e c a r r i e d out d u r i n g t h e s u m m e r of 1970 t o d e t e r m i n e m o r t a l i t y r a t e s at different tidal heights  63  A c o m p a r i s o n between the m e a n i n i t i a l lengths tory and non-migratory limpets. m i n e d o n J u l y 1,  Lengths  were  of m i g r a deter-  1969. T h e c o m p a r i s o n i s made i n two 70  s e p a r a t e g r o u p s of m a r k e d l i m p e t s XXI  A c o m p a r i s o n b e t w e e n the m e a n g r o w t h i n c r e m e n t of m i gratory and non-migratory  limpets. The initial  lengths  w e r e d e t e r m i n e d J u l y 1, 1 9 6 9 ; s u b s e q u e n t l e n g t h s ruary  on Feb-  16, 1 9 7 0 . T h e c o m p a r i s o n i s m a d e i n t h e s a m e t w o 71  g r o u p s of m a r k e d l i m p e t s a s T a b l e X X XXII  Comparisons  a m o n g the m e a n g r o w t h i n c r e m e n t of t h r e e  g r o u p s of l i m p e t s .  (See t e x t f o r e x p l a n a t i o n . ) T h e c o n t r o l  group is c o m p a r e d with the high density group,  a n dthe  high density group with the m i x e d high density group  74  1  GENERAL INTRODUCTION A p r o b l e m of g e n e r a l i n t e r e s t to e c o l o g i s t s n u m b e r of a n i m a l s w i t h i n a p o p u l a t i o n . tion as "the maintenance within a certain range, the e n v i r o n m e n t ,  c o n c e r n s r e g u l a t i o n of t h e  E i s e n b e r g (1966) d e f i n e d s u c h r e g u l a -  of the n u m b e r s of a n i m a l s i n a n a t u r a l p o p u l a t i o n  a r o u n d s o m e l e v e l t h a t i s d e t e r m i n e d b y t h e w h o l e of  a n d w h i c h i s b r o u g h t a b o u t a s a r e s u l t of f e e d b a c k f r o m p o p u -  lation density." F e e d b a c k f r o m p o p u l a t i o n d e n s i t y h a s b e e n s u p p o s e d to o c c u r i n a n u m b e r of w a y s .  N i c h o l s o n (1933) d i s c u s s e d s e v e r a l f a c t o r s t h a t m i g h t v a r y  w i t h population density i n their effect on i n d i v i d u a l s . 1954) a n d m o r t a l i t y ( L a c k ,  B i r t h rates  (Ricker,  1966) h a v e e a c h b e e n , s u g g e s t e d a s d e n s i t y d e p e n -  dent f a c t o r s w h i c h m i g h t r e g u l a t e p o p u l a t i o n s . F e e d b a c k f r o m p o p u l a t i o n d e n s i t y m a y a l s o o c c u r t h r o u g h the b e h a v i o u r of i n d i v i d u a l s t h a t a f f e c t s d i s p e r s a l .  H u f f a k e r , S h e a a n d H e r m a n (1963) s h o w e d  that a p r e d a t o r s p e c i e s a n d a p r e y s p e c i e s f o r m e d a m o r e stable s y s t e m when the l a b o r a t o r y e n v i r o n m e n t w a s m a d e m o r e c o m p l e x .  The more  complex  " u n i v e r s e " a l l o w e d t h e p r e y to d i s p e r s e m o r e w i d e l y , a n d h e n c e t h e p r e d a t o r d i d n o t o v e r - e x p l o i t the p r e y i n t h e w h o l e s y s t e m , do w i t h i n a s m a l l e r ,  less complex system.  a s i t w a s p r e v i o u s l y a b l e to  W e l l i n g t o n (1964) s h o w e d t h a t d i f -  f e r e n c e s i n d i s p e r s i n g b e h a v i o u r a m o n g adult a n d l a r v a l tent c a t e r p i l l a r s resulted i n a complex distribution within a mixed environment. of b e h a v i o u r a n d d i s t r i b u t i o n e n s u r e d t h a t f o o d r e s o u r c e s exploited,  were never  a n d that the s p e c i e s w a s able to s u r v i v e u n f a v o u r a b l e  conditions.  The patterns over-  weather  D i s p e r s a l c a n t h u s be e x t r e m e l y i m p o r t a n t to t h e s t a b i l i t y of a  population. R e g u l a t i o n of p o p u l a t i o n s b y m e a n s of d i s p e r s a l h a s b e e n w i d e l y c o n sidered.  T e r r i t o r i e s m a y be r e q u i r e d b y i n d i v i d u a l s f o r b r e e d i n g (Watson a n d  Jenkins,  1968) o r f e e d i n g ( C o n n e l l , 1 9 6 3 ) .  E m i g r a t i o n f r o m a r e a s of h i g h  2  d e n s i t y w a s s u g g e s t e d a s a n i m p o r t a n t f a c t o r i n c o n t r o l l i n g t h e p o p u l a t i o n of M i c r o t u s , s. s m a l L r o d e n t ( K r e b s ,  et al_., 1 9 6 9 ) .  Such e m i g r a t i o n w a s thought  to be b r o u g h t a b o u t b y c h a n g e s i n a g g r e s s i v e b e h a v i o u r w h i c h a c c o m p a n i e d changes in density (Krebs,  1970).  S t u d i e s s u c h a s t h e s e i n d i c a t e the n e e d f o r  r e c o g n i t i o n of i n d i v i d u a l d i f f e r e n c e s i n b e h a v i o u r w h e n p o p u l a t i o n s of a n i m a l s are being examined. T h e p u r p o s e of t h i s s t u d y w a s to e x a m i n e b e h a v i o u r a l m e c h a n i s m s d i s p e r s a l i n the l i m p e t A c m a e a d i g i t a l i s .  Although t e r r i t o r i a l behaviour has  b e e n r e p o r t e d i n a t l e a s t one s p e c i e s of l i m p e t ( S t i m s o n , demonstrated i n this limpet, occurs (Galbraith,  1965).  to be h o m i n g ( W e l l s ,  of  1970),  i t h a s not b e e n  and it a p p e a r s u n l i k e l y that a g g r e s s i v e  behaviour  T h e t w o m a j o r t y p e s of d i s p e r s i n g b e h a v i o u r  1917) a n d s e a s o n a l m i g r a t i o n ( F r a n k ,  appear  1965).  F r a n k s u g g e s t e d t h a t r e g u l a t i o n of l o c a l p o p u l a t i o n s c o u l d o c c u r t h r o u g h changes i n homing behaviour. below,  This hypothesis is discussed i n a section  and was tested i n this study.  A second hypothesis,  describing a way  i n w h i c h l o c a l p o p u l a t i o n s m i g h t be r e g u l a t e d b y s e a s o n a l m i g r a t i o n , w a s d e v e l o p e d f r o m s t u d i e s p u b l i s h e d b y F r a n k (1965) a n d L e w i s ( 1 9 5 4 ) .  This  h y p o t h e s i s w a s a l s o e x a m i n e d i n t h i s study, a n d i s d i s c u s s e d b e l o w .  Possible  e x p l a n a t i o n s of t h e t w o t y p e s of b e h a v i o u r a r e a l s o c o n s i d e r e d . -  3  EXPERIMENTAL, METHODS Introduction Special techniques, employed i n this study.  such as m a r k i n g and fencing, were developed and  T o a v o i d r e p e t i t i o n a n d d i s r u p t i o n of c o n t i n u i t y i n  the s e c t i o n s d e a l i n g w i t h e x p e r i m e n t s , in this section.  the g e n e r a l t e c h n i q u e s a r e  presented  P r o c e d u r e s p e c u l i a r to one e x p e r i m e n t a r e p r e s e n t e d i n the  a p p r o p r i a t e s e c t i o n d e a l i n g w i t h the  experiment.  D e t e r m i n a t i o n of t i d a l h e i g h t s T h e t i d a l h e i g h t s at s p e c i f i c l o c a t i o n s on the b e a c h w e r e d e t e r m i n e d i n May,  1970,  i n conjunction with a mapping s u r v e y conducted by s e v e r a l m e m -  b e r s of the U n i v e r s i t y of B r i t i s h C o l u m b i a . were determined with a surveyor's level.  T h e r e l a t i v e h e i g h t s of p o i n t s T h e h e i g h t of one p o i n t w a s d e t e r -  m i n e d b y o b s e r v i n g the t i m e at w h i c h i t w a s r e a c h e d by the t i d e , t h e n c a l c u l a t i n g the h e i g h t of the t i d e a t t h a t t i m e u s i n g t i d e t a b l e s . eleven times,  a n d the c a l c u l a t e d h e i g h t s w e r e a v e r a g e d .  This was  repeated  T h e h e i g h t s of  p o i n t s o n t h e b e a c h w e r e t h e n d e t e r m i n e d b y c o m p a r i s o n w i t h the k n o w n p o i n t . Marking L i m p e t s were m a r k e d individually with adhesive plastic wire m a r k e r s m a d e b y the W . H . B r a d y C o m p a n y . f r o m 0 t h r o u g h 99. coloured plastic,  These s m a l l numbers were available  T o o b t a i n a h i g h e r n u m b e r a s m a l l p i e c e of a d h e s i v e  m a d e b y the s a m e c o m p a n y ,  w a s p l a c e d w i t h the n u m b e r .  T h e n u m b e r w a s t h e n c o a t e d w i t h t w o c o a t s of p l a s t i c c e m e n t  ('Dekophane').  M a r k i n g m o r t a l i t y w a s n e v e r h i g h e r t h a n f i v e p e r c e n t a n d a p p e a r e d to be c a u s e d c h i e f l y b y c e m e n t c o m i n g i n c o n t a c t w i t h the m a n t l e . G r o u p s of l i m p e t s w e r e m a r k e d w i t h f a s t - d r y i n g p a i n t l a c q u e r a c r y l i c a r t i s t ' s paint.  U n l i k e the i n d i v i d u a l m a r k i n g s ,  the a n i m a l s f o r p e r i o d s i n e x c e s s of a y e a r ,  or  which remained on  1  p a i n t m a r k i n g s l a s t e d no l o n g e r  4 than four months after application.  A r e q u i s i t e f o r m a r k i n g was For the  this r e a s o n m a r k i n g  that the s h e l l of the l i m p e t be quite d r y .  c o u l d be done o n l y i n s u m m e r .  a n i m a l s w i t h a l c o h o l o r a c e t o n e was  D r y i n g wet s h e l l s of  attempted without s u c c e s s .  of wet s h e l l s w i t h a c r y l i c putty, a l c o h o l m a r k i n g pens, e p o x y putty a n d a c r y l i c p a i n t w e r e a l l u n s u c c e s s f u l .  Marking  oil pastel crayons,  F i l i n g m a r k s into the  s h e l l s p r o v e d to be i m p r a c t i c a l b e c a u s e the l i m p e t s m i g r a t e d a w a y f r o m the place where they were marked,  a p p a r e n t l y i n r e s p o n s e to the d i s t u r b a n c e  c a u s e d by f i l i n g .  Enclosures L i m p e t s w e r e c o n t a i n e d i n a r e a s or r e s t r i c t e d i n m o v e m e n t by m e a n s of f e n c e s a t t a c h e d t o the r o c k .  Fences were constructed f r o m 7cm  wide  s t r i p s of a p l a s t i c m e s h s o l d a s ' V e x a r ,  w h i c h i s m a n u f a c t u r e d f o r use i n  produce  s t r i p s w e r e f o l d e d a n d a t t a c h e d to  1  the  s e c t i o n s of s u p e r m a r k e t s .  The  r o c k b y m e a n s of r o c k n a i l s o r m e t a l s c r e w s a n d s c r e w a n c h o r s ,  d i n g on the h a r d n e s s of the r o c k .  depen-  T h e u p p e r edge of the f o l d e d s t r i p was  held  a b o v e the l o w e r edge by m e a n s of a r u b b e r t a p w a s h e r i n s e r t e d onto the n a i l or  s c r e w b e t w e e n the two e d g e s ( F i g u r e 1).  L i m p e t s w e r e a b l e to m o v e  e a s i l y on the m e s h i t s e l f but w e r e u n a b l e to c r o s s the s h a r p u p p e r edge of the fence.  It was was  n e c e s s a r y to s e a l the l o w e r edge of f e n c e s to the r o c k .  done by u s i n g a m i x t u r e of f i b r e g l a s  w h i c h f o r m e d a s o r t of f i b r e g l a s successfully,  resin,  concrete  sand and  This  catalytic hardener,  O r d i n a r y c o n c r e t e was  used  once  but t h i s r e q u i r e d a m u c h l o n g e r s e t t i n g t i m e a n d m o r e w o r k .  A c r y l i c m o d e l l i n g putty a n d a n a c e t i c a c i d b a s e d c e m e n t w e r e b o t h t e s t e d f o r the  p u r p o s e a n d f o u n d to be i n a d e q u a t e .  D i s p l a c e m e n t of l i m p e t s  L i m p e t s w e r e r e m o v e d to e x p e r i m e n t a l s i t e s i n s e v e r a l e x p e r i m e n t s .  5  Nail  F i g u r e 1.  D e t a i l of e n c l o s u r e  construction.  6  T o m i n i m i z e m o r t a l i t y , the f o l l o w i n g p r o c e d u r e w a s u s e d .  A l i m p e t was  r e m o v e d b y the i n s e r t i o n of a d u l l b l a d e u n d e r the s h e l l m a r g i n , f o l l o w e d b y gentle l i f t i n g .  L i m p e t s w e r e p l a c e d i n f r e s h s e a w a t e r f o r a p e r i o d of f i v e to  t e n m i n u t e s p r i o r to r e p l a c e m e n t . sea water,  The  replacement  area  was wetted  t h e n the l i m p e t s w e r e p l a c e d on the a r e a a n d , i f n e c e s s a r y ,  until they had taken hold.  with held  A f t e r a l l l i m p e t s h a d b e e n r e p l a c e d , the a r e a w a s  wetted a g a i n w i t h sea w a t e r .  It w a s n e c e s s a r y to a l l o w a t l e a s t t h i r t y m i n u t e s  b e t w e e n the t i m e of r e p l a c e m e n t a n d the a r r i v a l of the t i d e . M e a s u r e m e n t of l i m p e t s L i m p e t s w e r e m e a s u r e d i n s i t u so that d i s t u r b a n c e s c a u s e d b y r e m o v a l w o u l d n o t be i n t r o d u c e d i n t o e x p e r i m e n t s .  B e c a u s e the a n i m a l s w e r e o f t e n  p a c k e d c l o s e l y t o g e t h e r o r f o u n d i n n a r r o w c r a c k s i n the r o c k , p o s s i b l e to u s e v e r n i e r c a l i p e r s .  i t w a s not  M e a s u r e m e n t s w e r e made by c l o s i n g a  d r a w i n g c o m p a s s u n t i l it t o u c h e d b o t h ends of the s h e l l a n d d r a w i n g the a r c i n a field book. to  T h e l e n g t h of the a r c w a s l a t e r m e a s u r e d w i t h v e r n i e r c a l i p e r s  .01cm. A s t a n d a r d l e n g t h w a s d e f i n e d a s the d i s t a n c e f r o m the m o s t a n t e r i o r  p o r t i o n of the s h e l l to the l o n g e r of the t w o r i b s a t the p o s t e r i o r m a r g i n .  7  DESCRIPTION OF  THE  STUDY  AREA  Location  This  study was  Port Renfrew.  c a r r i e d o u t o n t h e w e s t c o a s t of " V a n c o u v e r I s l a n d  T h e s e c t i o n of s h o r e  l o c a l l y as B o t a n i c a l Beach,  on w h i c h the study w a s done i s  i n r e f e r e n c e to the S e a s i d e  Station  t h e r e b y t h e U n i v e r s i t y of M i n n e s o t a f r o m 1900 to 1910. B e a c h i s a c c e s s i b l e by road,  disturbed.  L i v i n g and r e s e a r c h facilities,  old Station site by m e m b e r s  known  maintained  Although Botanical  it is r e m o t e f r o m human habitation and  v i s i t e d by people e x c e p t d u r i n g the s u m m e r .  near  seldom  The a r e a was thus l a r g e l y  un-  which were constructed near  of t h e U n i v e r s i t y of B r i t i s h C o l u m b i a ,  the  were  used  d u r i n g v i s i t s to the study, a r e a .  Physical  factors  Botanical Beach consists slope.  of a b r o a d , r o c k y s h e l f of v a r y i n g w i d t h a n d  T h e i n t e r t i d a l z o n e i s 100 m e t r e s w i d e i n s o m e  The shelf,  c o m p o s e d of s a n d s t o n e  intruded by h a r d m e t a m o r p h i c s ,  c a r v e d i n t o a c o m p l e x a r r a y of p o o l s , topography  v a r i e s f r o m v e r y flat,  pitted surfaces  of s o m e  d e s c r i p t i o n has been given by H a l l  A l t h o u g h the s h o r e bounds Fuca,  it is an exposed  access  smooth sandstone  coast.  and benches.  has  been  The  microrough,  T h e b e a c h i s b o u n d e d at Detailed geological  (1906).  the outer r e a c h e s  Pacific storms  of t h e S t r a i t of J u a n de  and seas have  unrestricted  The tides are m i x e d  w i t h two u n e q u a l h i g h t i d e s a n d l o w t i d e s e v e r y 25 h o u r s .  semi-  The  range  the l o w e r l o w tide o c c u r s b e t w e e n dawn and noon at  spring  o f t h e t i d e v a r i e s f r o m 6.1 t o 1 2 . 6  In s u m m e r ,  exposed.  to the e x t r e m e l y  w h i c h w e r e not studied.  to the b e a c h f r o m the w e s t a n d n o r t h w e s t .  diurnal,  tides,  prominences  metamorphic projections.  both ends by upturned shale beds,  places when  feet.  a n d the h i g h e r h i g h t i d e o c c u r s at night.  During  the w i n t e r ,  lower  low  8  t i d e o c c u r s b e t w e e n s u n s e t a n d m i d n i g h t at s p r i n g t i d e s , tide o c c u r s i n the late m o r n i n g .  Thus the upper i n t e r t i d a l is exposed  l o n g e r p e r i o d s to sunlight d u r i n g s u m m e r tide patterns occur in September and  than winter.  patterns as w e l l as w a r m e r weather,  S h i f t s of the  are further exaggerated by sea  t i m e t h e y w a s h the r o c k s at l e v e l s f a r above the tide l e v e l .  seasonal  a r e w a s h e d o n l y to the a c t u a l t i d e height.  conditions.  and at any  The  swells a r e r a r e l y g r e a t e r than two feet high,  during neap tides,  for  w h i c h a r e b r o u g h t about by tide  s w e l l s o c c u r a l m o s t c o n s t a n t l y d u r i n g the c o l d e r months,  is much calmer;  high  March.  P e r i o d s of d r y i n g d u r i n g t h e s u m m e r ,  Large  while the higher  summer  and the  If t h e w e a t h e r i s c a l m a n d  sea  rocks dry  t h e r o c k s a b o v e t h e 11 f o o t l e v e l m a y b e d r y f o r u p t o 20  days.  Rainfall occurs 1969 a n d 1970, by forest.  m o s t l y in the colder months.  the s u m m e r s  c a m p f i r e p e r m i t s w e r e s u s p e n d e d f o r l a c k of r a i n i n t h e  The weather was  not d a m p e n the  During  s u n n y e x c e p t f o r p e r i o d s of s e a - f o g ,  near-  which did  rocks.  D u r i n g the one w i n t e r (1969-1970) i n c l u d e d i n t h i s study, as that o b s e r v e d by F r a n k masses  (1965),  occurred.  Frank  of i c e g r o u n d l i m p e t s f r o m t h e s u b s t r a t e .  no i c e ,  such  r e p o r t e d that floating The only adverse  condi-  tions for intertidal organisms observed during winter were large amounts f r e s h water f r o m r a i n and runoff, p i e c e s of d r i f t w o o d a n d l o g s a c r o s s been atypical;  and heavy  seas,  the beach.  d u r i n g the p r e c e d i n g w i n t e r f r e e z i n g c o n d i t i o n s at  D i s t r i b u t i o n of A c m a e a  1969.  of  w h i c h often c a r r i e d large  The winter observed may  p e r s i s t e d for several weeks and large amounts  In M a y ,  of  have  Vancouver  of i n t e r t i d a l i c e w e r e  observed.  digitalis  l i m p e t s w e r e counted and s a m p l e d along a line that  r u n d o w n a g e n t l y s l o p i n g p a r t of t h e s h o r e . w e r e m a r k e d on the r o c k w i t h c h a l k .  Intervals  of l m  was  along the line  L i m p e t s w i t h i n 1 0 c m on e i t h e r side  of  9  the l i n e w e r e counted w i t h i n e v e r y i n t e r v a l , and s a m p l e s limpet within five counting areas  were taken.  samples  The tidal heights  were measured later.  were determined in May, above  chart datum,  1970.  consisting  The lengths  made.  of p o i n t s a l o n g t h e  ( t i d a l h e i g h t r e f e r s t o t h e e l e v a t i o n of a p o i n t  p o r t i o n of t h e s h o r e t h a t w a s  d i r e c t l y e x p o s e d to surf,  the s p e c i e s  s e r v e d b e t w e e n the two a r e a s . difference in exposure  Figure  series  species were also  w h i c h m u s t b e m o d i f i e d b y c o n s i d e r a t i o n of l o c a l  increases  l o w v a l u e o f d e n s i t y a t 8.2 5 f e e t .  r a p i d l y f r o m 6.0 t o 7 . 2 5 f e e t , t h e n d e c l i n e s . i n w h i c h A . d i g i t a l i s w e r e not found,  A  T h i s a c c o u n t s f o r the  s e c t i o n of u n i f o r m s u b s t r a t e w o u l d  The modal  of e a c h s i z e c l a s s at d i f f e r e n t t i d e l e v e l s size class increases  with tidal height.  observation is consistent with those r e p o r t e d by F r a n k and Lewis  (1965) f o r A .  (1954) f o r the e c o l o g i c a l l y s i m i l a r P a t e l l a v u l g a t a .  l a r g e s t l i m p e t s a t t h e u p p e r p a r t of t h e s h o r e , shore.  Frank  and s m a l l e r  Both  This  digitalis found  ones l o w e r on the  The l i m p e t s he studied showed net u p w a r d  d u r i n g the f a l l and w i n t e r ,  and a l e s s e r net downward m o v e m e n t  made  several migrations  f r o m the p l a c e w h e r e t h e y  was  movement in  spring.  T h u s t h e l a r g e r l i m p e t s a t t h e u p p e r e n d of t h e v e r t i c a l d i s t r i b u t i o n a r e  settled.  is  s u g g e s t e d t h a t t h i s d i f f e r e n t i a l d i s t r i b u t i o n of s i z e c l a s s e s  caused by m i g r a t i o n .  w h i c h have  pro-  d e c l i n e i n d e n s i t y f r o m 7.25 to 10.0 f e e t .  The relative abundance 3.  factors.  of l i m p e t s p e r u n i t a r e a a t d i f f e r e n t h e i g h t s  shallow tidepools,  bably show a continuous  ob-  that t i d a l height is only a  o c c u r r e d o n t h e t r a n s e c t f r o m 8.0 t o 8 . 7 5 t i d a l f e e t .  shown in Figure  more  w a s f o u n d f r o m 9.0 t o 1 4 . 0 t i d a l f e e t .  to s u r f , a n d d e m o n s t r a t e s  Density  of v e r y  transect  T h e d i f f e r e n c e a p p e a r e d to be c a u s e d by the  2 s h o w s the n u m b e r  on the t r a n s e c t . A  steeper and  d i f f e r e n c e s i n the d i s t r i b u t i o n of a l g a e  relative measure  transect  0.0 t i d a l f e e t ) .  A t another  Corresponding  every  of l i m p e t s i n the  A . d i g i t a l i s w a s f o u n d f r o m 6.0 t o 1 0 . 0 t i d a l f e e t w h e r e t h e was  of  those  originally  F i g u r e 3.  P e r c e n t a g e of l i m p e t s i n each s i z e c l a s s at different t i d a l heights. The t i d a l height of e a c h sample i s g i v e n i n parentheses.  12  Very  small  l i m p e t s b e g a n to a p p e a r i n A p r i l a n d M a y ,  1970.  w e r e n o t f o u n d a b o v e a p p r o x i m a t e l y 8.0 f e e t o n t h e t r a n s e c t l i n e .  They  Doty  (1946)  s u g g e s t s t h a t c r i t i c a l t i d e l e v e l s m a y d e f i n e the u p p e r l i m i t of d i s t r i b u t i o n of species  in the intertidal zone.  limpets,  8.0 f e e t ,  T h e u p p e r l i m i t of d i s t r i b u t i o n of n e w l y  corresponds  w i t h t h e l o w e s t l e v e l of l o w e r h i g h t i d e  T h i s i s the h i g h tide w h i c h o c c u r s shore  d u r i n g the day i n s u m m e r ,  w h i c h a r e not c o v e r e d by L H T  settled (LHT).  a n d p a r t s of t h e  a r e e x p o s e d f o r m o s t of t h e d a y .  There  i s a l a r g e d i f f e r e n c e i n the m a x i m u m l e n g t h of d a y t i m e e x p o s u r e b e t w e e n of t h e s h o r e a b o v e a n d b e l o w L H T .  T h a t y o u n g l i m p e t s s e t t l e b e l o w 8.0  parts feet  m a y i n d i c a t e t h e y a r e not a b l e to w i t h s t a n d the l o n g p e r i o d s of d e s i c c a t i o n encountered above that level,  whereas l a r g e r limpets can.  Davies  (1969)  has  shown that s m a l l e r P a t e l l a l o s e w a t e r at a g r e a t e r r a t e than l a r g e r ones.  The  a b i l i t y o f s m a l l l i m p e t s t o s u r v i v e a t l e v e l s a b o v e 8.0 t i d a l f e e t w a s n o t t e s t e d in this study because  i t w a s i m p o s s i b l e to r e m o v e a n d r e p l a c e t h e v e r y  limpets without damaging for very small limpets,  them.  If i t i s t r u e t h a t a c r i t i c a l t i d e l e v e l  of a d a p t a t i o n to p h y s i c a l f a c t o r s .  g r a t i o n w i l l be d i s c u s s e d  more fully in later  A. digitalis occurs  The upward  mi-  sections.  on e x p o s e d r o c k r a t h e r t h a n i n p o o l s .  o f A . d i g i t a l i s w e r e i n h a b i t e d b y A_. p a r a d i g i t a l i s  I 9 6 0 ) a n d A_. p e l t a .  exists  then u p w a r d m i g r a t i o n i n f a l l a n d w i n t e r w o u l d be  partly explained in terms  w i t h i n the range  small  A . t e s t u d i n a l i s b e g a n to appear  Pools  located  (Fritchman,  in pools t o w a r d the l o w e r  l i m i t of d i s t r i b u t i o n a n d w a s f o u n d b e l o w t h e l o w e r l i m i t of A . d i g i t a l i s .  There  w a s no a p p a r e n t c o m p e t i t i o n b e t w e e n A . d i g i t a l i s and o t h e r l i m p e t s ,  for either  food or shelter space,  separately.  and L.  because  A. digitalis  almost always  occurred  Possible  c o m p e t i t o r s f o r food w e r e the l i t t o r i n e s L i t t o r i n a s c u t u l a t a  sitkana.  B o t h s p e c i e s o c c u r r e d w i t h i n the v e r t i c a l r a n g e  a n d f r e q u e n t l y o c c u p i e d the s a m e s h e l t e r sites at l o w tide.  o f A_. d i g i t a l i s  However,  compe-  t i t i o n f o r s h e l t e r space d i d not s e e m to e x i s t b e c a u s e l i t t o r i n e s o c c u p i e d either  s h e l t e r sites without l i m p e t s or the i n t e r s t i c e s b e t w e e n  limpets.  clumped  O n one f a l l i n g tide l i t t o r i n e s w e r e o b s e r v e d f e e d i n g a f t e r the l i m p e t s  13  had already become immobile limpet  and  c e a s e d f e e d i n g l a t e r to s h e l t e r i n t h e  clumps.  D i s t r i b u t i o n and abundance  of f o o d  species  T h e s u b s t r a t e on w h i c h A . d i g i t a l i s was found s u p p o r t e d no scopic algae in s u m m e r above w h i c h this l i m p e t was found,  9.0 f e e t .  o n l y t u f t s of G e l i d i u m sp.  l i m p e t s d i d n o t a p p e a r to e a t . m i c r o s c o p i c plants.  B e l o w that level,  macro-  i n the z o n e  w e r e found,  The substrate was v e r y thinly coated  In June,  1969.  a s m a l l fence was  sampled.  s e v e r a l s p e c i e s of c o l o n i a l d i a t o m s ,  In e a r l y October,  1969,  the  w h i c h w e r e not i d e n t i f i e d f u r t h e r .  t h e d i a t o m f i l m a b o v e 8.0 t i d a l f e e t o n a l l composed  of s t r a n d s  up to 3 c m  s i m i l a r t h i c k e n i n g of the a l g a l m a t i n f a l l w a s n o t e d b y C a s t e n h o l z  who suggested that it was  with  It w a s f o u n d to c o n s i s t e n t i r e l y of  p a r t s of the s h o r e b e c a m e a t h i c k m a t , A  which  c o n s t r u c t e d to p r e v e n t  l i m p e t s and l i t t o r i n e s f r o m g r a z i n g the a r e a within; after two weeks thickened algal covering was  in  caused by d e c r e a s e s  in littorine density.  long.  (1961), However,  the t h i c k c o v e r a p p e a r e d on the study b e a c h i n an a r e a w h e r e l i t t o r i n e s w e r e never  observed;  consequently,  this explanation seems unlikely.  Appearance  of the t h i c k m a t f o l l o w e d the c h a n g e f r o m s u m m e r t i d e p a t t e r n s to w i n t e r p a t t e r n s a n d m a y h a v e b e e n r e l a t e d t o t h e i n c r e a s e d d a m p n e s s of t h e i n t e r t i d a l zone at this t i m e .  Soon after it appeared,  the m a t d i s a p p e a r e d a r o u n d d e p r e s s i o n s  that  contained limpets or littorines, apparently having been grazed down.  The  m a t d e c l i n e d g r a d u a l l y during the next four months,  been  n e a r l y r e d u c e d to its s u m m e r l e v e l .  after w h i c h it had  The t h i c k m a t r e m a i n e d in the a r e a  w h i c h c o n t a i n e d no g r a z e r s f o r two m o n t h s a f t e r i t h a d d i s a p p e a r e d It a p p e a r s  l i k e l y that g r a z i n g a c t i o n was  In N o v e m b e r , Prophyra  sp.  elsewhere.  p a r t l y r e s p o n s i b l e f o r the d e c l i n e .  t h e z o n e b e t w e e n 8.0 a n d 1 5 . 0 f e e t b e c a m e c o v e r e d w i t h B y the t i m e these plants r e a c h e d 5 c m i n length, they  had  v i r t a u l l y d i s a p p e a r e d i n t h e z o n e b e l o w 12.0 f e e t , w h e r e m o s t l i m p e t s o c c u r r e d ;  14  but t h e y r e m a i n e d a b o v e that l e v e l u n t i l l a t e F e b r u a r y w h e n t h e y b e g a n to  dis-  colour and die.  Fucus  sp. o c c u r r e d i n a f e w s h a d e d p l a c e s w h e r e l i m p e t s w e r e  but s u c h p l a c e s w e r e not c o m m o n .  One plant was found i n an a r e a w i t h s o m e  m a r k e d l i m p e t s a n d r e m a i n e d t h e r e t h r o u g h o u t the study. a p p e a r to eat l a r g e F u c u s  found,  L i m p e t s do n o t  plants.  U l v a sp. a n d E n t e r o m o r p h a  sp. b e g a n to a p p e a r d u r i n g w i n t e r ,  l o c a t e d a b o v e t h e u p p e r l i m i t of_A_. d i g i t a l i s .  but w e r e  T h a t they d i d not a p p e a r i n the  r e g i o n i n h a b i t e d by l i m p e t s m a y i n d i c a t e that these plants a r e eaten by as  soon as they  settle.  Colonial diatoms, early winter,  limpets  p r e s e n t throughout the year but m o s t abundant  t h u s a p p e a r to be the m a j o r f o o d s p e c i e s of A . d i g i t a l i s .  limpets may also graze m a c r o s c o p i c plants Enter omor pha.  Ulva  The  and  Since these a r e not p r e s e n t w i t h l i m p e t s e x c e p t i n the w i n t e r ,  and a r e then quickly g r a z e d away, of A . d i g i t a l i s .  such as P o r p h y r a ,  in  t h e y m u s t f o r m o n l y a s m a l l p a r t of the d i e t  T h e l i m p e t s m a y h e l p t o d e t e r m i n e t h e v e r t i c a l d i s t r i b u t i o n of  some m a c r o s c o p i c algae,  such as  Porphyra.  Summary  P h y s i c a l c o n d i t i o n s o b s e r v e d at the study a r e a , weather and sea conditions,  including tides,  a p p e a r e d to be m o r e f a v o u r a b l e f o r l i m p e t s  t h e w i n t e r of 1 9 6 9 - 1 9 7 0 t h a n d u r i n g t h e s u m m e r s of t h o s e y e a r s . winter heavy  seas,  factors observed. c o m b i n i n g w i t h hot, ccation.  During  d r i f t w o o d a n d f r e s h w a t e r w e r e the only p o s s i b l e During  the s u m m e r s ,  during  adverse  l o w t i d e s o c c u r r e d d u r i n g the day,  d r y w e a t h e r a n d l o w s e a s to p r o d u c e l o n g p e r i o d s  of d e s i -  The winter observed m a y have been atypical.  A . d i g i t a l i s w a s f o u n d to be d i s t r i b u t e d f r o m j u s t a b o v e m i d - t i d e l e v e l n e a r l y to the h i g h t i d e l e v e l . height,  The density decreased with increasing  and the m o d a l length i n c r e a s e d w i t h i n c r e a s i n g height.  tidal  It w a s  con-  15  eluded that c o m p e t i t i o n w i t h other  species  competition with littorines for food may  F o o d f o r the l i m p e t s , diatoms,  was  of l i m p e t s d o e s not o c c u r ,  occur.  w h i c h w a s f o u n d to c o n s i s t m a i n l y of c o l o n i a l  p r e s e n t t h r o u g h o u t the y e a r but b e c a m e  October through January.  although  Other  species  most abundant  of a l g a e w e r e o b s e r v e d ,  from  a n d it  was  c o n c l u d e d t h a t t h e y w e r e n o t a s u b s t a n t i a l p a r t of the l i m p e t s ' d i e t . A . d i g i t a l i s m a y a f f e c t the d i s t r i b u t i o n of a l g a e b e t w e e n  tide-marks.  16  STUDIES OF  HOMING  BEHAVIOUR  Introduction  L i m p e t s t h a t a r e e x p o s e d to a i r d u r i n g p a r t of t h e t i d a l c y c l e a r e usually i m m o b i l e during that p e r i o d . a r e r e p o r t e d to r e t u r n to a h o m e (Wells,  1917).  Such behaviour  suitable place f r o m desiccation. graphy  In  s e v e r a l s p e c i e s of A c m a e a ,  site, i n w h i c h they r e m a i n during l o w tide may assure  the l i m p e t of b e i n g p r o t e c t e d i n a  L i m p e t s a r e thought to u t i l i z e l o c a l  r a t h e r t h a n c h e m i c a l s t i m u l i to r e t u r n to the h o m e  Galbraith,  limpets  topo-  site (Jessee,  1968;  1965).  A t l o w t i d e A_. d i g i t a l i s r e m a i n i n a c t i v e i n c r a c k s a n d d e p r e s s i o n s the s u b s t r a t e ,  which provide  (termed clumps),  which may  seen in such places. (Millard,  1968),  within large,  The  shade and m o i s t u r e .  Aggregations  c o n t a i n up to s e v e r a l h u n d r e d a n i m a l s ,  often  A  limpet  t i g h t l y p a c k e d c l u m p s m i g h t be l e s s v u l n e r a b l e to p r e d a t i o n t h a n m u c h as a f i s h i n a s c h o o l i s l e s s l i k e l y to be s e l e c t e d by a I960).  (1965) a n d M i l l a r d (1968) f o u n d t h a t s o m e  r e p e a t e d l y o b s e r v e d i n the s a m e t i o n on the r o c k . not o c c u r ,  1964).  are  clump may increase protection f r o m desiccation  predator than a solitary fish (Brock and Riffenburgh,  does  of l i m p e t s  and m a y also provide protection f r o m predators.  a single limpet,  Frank  in  Frank  clump,  individuals  even though the c l u m p changed  (1965) r e p o r t s t h a t h o m i n g to a s p e c i f i c h o m e  but r a t h e r the l i m p e t s r e m a i n w i t h i n a " h o m e  However,  Miller  were  (1968) f o u n d t h a t  posisite  range"  (Frank,  2 5 p e r c e n t of a g r o u p o f  marked  l i m p e t s r e t u r n e d to the s i t e a t w h i c h t h e y w e r e m a r k e d o v e r n i n e  days.  G a l b r a i t h ( 1 9 6 5) r e p o r t e d 54 p e r c e n t h o m i n g w i t h i n a g r i d s y s t e m o v e r days.  D i f f e r e n t t e c h n i q u e s w e r e u s e d b y t h e v a r i o u s a u t h o r s to s t u d y  this might account for discrepancies among  It w a s  their  seven homing;  findings.  of i n t e r e s t t h a t s o m e l i m p e t s i n the s t u d i e s r e p o r t e d a b o v e  showed homing behaviour  w h i l e others d i d not.  Frank  (1965) s u g g e s t s that  17  when environmental conditions are favourable, a home  range.  their home  l i m p e t s t e n d to r e m a i n w i t h i n  W h e n c o n d i t i o n s d e t e r i o r a t e , l i m p e t s b e g i n to m i g r a t e  range.  In t h i s w a y l o c a l p o p u l a t i o n d e n s i t y c o u l d be  i n t h e s e n s e of E i s e n b e r g  (1966).  Because  in which self-regulation might occur,  those limpets within a few square  it is  (Frank,  1965).  o p e r a t e at a w i d e r a n g e thesis is correct,  of l i m p e t s n o r m a l l y  this m e c h a n i s m  range.  "local  emigrate.  conditions  of r e g u l a t i o n w o u l d  of d e n s i t i e s a n d l o c a l c o n d i t i o n s .  If F r a n k ' s  hypo-  t h e n a t a n y d e n s i t y s o m e l i m p e t s a r e t o l e r a n t of l o c a l  ditions and r e m a i n within a home range, no h o m e  A  from which .  F r a n k a l s o s u g g e s t e d t h a t the t h r e s h o l d of t o l e r a n c e to l o c a l consequently,  the  c o u l d thus be d e f i n e d as  m e t r e s of r o c k s u r f a c e a r e a ,  o n l y a s m a l l p r o p o r t i o n of the t o t a l n u m b e r  varies among limpets;  difficult  After settlement, however,  l i m p e t s do n o t u s u a l l y m o v e m o r e t h a n a f e w m e t r e s population",  self-regulated  A. digitalis have pelagic larvae,  w h i c h d r i f t a b o u t i n the p l a n k t o n f o r a n u n k n o w n l e n g t h of t i m e , to d e f i n e a " p o p u l a t i o n " of t h e s e a n i m a l s .  from  while others are intolerant and  When local density increases,  have  local conditions deteriorate,  a n d a g r e a t e r p r o p o r t i o n of the p o p u l a t i o n b e c o m e s not display homing behaviour.  con-  intolerant and thus  does  T h e s e l i m p e t s t e n d to m i g r a t e f r o m the a r e a ,  a n d l o c a l c o n d i t i o n s t e n d to i m p r o v e .  A  similar hypothesis  that land snails  was  s u g g e s t e d by L o m n i c k i (1969).  who  reported  (Helix pomatia) d e m o n s t r a t e d two b e h a v i o u r a l types.  Some  s n a i l s t e n d e d t o r e m a i n b e n e a t h t h e s u r f a c e of t h e g r o u n d i n a s m a l l a r e a , w h i l e o t h e r s m i g r a t e d about on the s u r f a c e . m o b i l e snails a r e the e x c e s s l a t e d by m i g r a t i o n .  L o m n i c k i suggests that  of t h e p o p u l a t i o n , a n d t h a t l o c a l d e n s i t y i s  Neither author tested his  The hypothesis  was  more regu-  hypothesis.  examined i n several stages.  Small groups  A. digitalis were  studied to d e t e r m i n e whether  strated.  g r o u p s w e r e m a r k e d to d e t e r m i n e w h e t h e r the two t y p e s  Larger  b e h a v i o u r p r e d i c t e d f r o m the hypothesis  homing behaviour was  of  c o u l d be i d e n t i f i e d .  The  demonof  proportion  18  of l i m p e t s of e a c h b e h a v i o u r a l t y p e w a s  measured  o v e r a r a n g e of d e n s i t i e s  i n a n e n c l o s e d a r e a to d e t e r m i n e w h e t h e r the p r o p o r t i o n of n o n - h o m i n g d u a l s i n c r e a s e d a s the d e n s i t y i n c r e a s e d .  Finally,  environmental  indivi-  conditions  a r o u n d g r o u p s of l i m p e t s w e r e m a d e u n f a v o u r a b l e b y d i r e c t m a n i p u l a t i o n , determine whether changes in homing behaviour would  O c c u r r e n c e of  occur.  homing  Forty limpets were marked individually in May,  1969.  tions w e r e r e c o r d e d on e l e v e n subsequent days at l o w tide. s e e n i n the o r i g i n a l p o s i t i o n at e v e r y r e c o r d e d o b s e r v a t i o n , p o s i t i o n once,  and ten changed position m o r e than once.  n o t c h a n g e p o s i t i o n w e r e k n o w n to h a v e m o v e d , twenty-two were in small clumps  presumably  of up to t e n a n i m a l s ,  clump remained  and their p o s i Twenty-six  five  were  changed  Limpets which did in feeding,  because  and the r e l a t i v e p o s i -  t i o n s of l i m p e t s w i t h i n the c l u m p s w e r e s e e n t o c h a n g e .  A  T h e p o s i t i o n of e a c h  constant.  s e c o n d g r o u p of n i n e l i m p e t s w a s  were very large  marked in August,  1969-  These  A . d i g i t a l i s ( m o r e t h a n 3 . 5 c m ) i n a n a r e a of l o w d e n s i t y .  p o s i t i o n of e a c h l i m p e t w a s m a r k e d b y d r i l l i n g a h o l e i m m e d i a t e l y b e s i d e limpet.  A f t e r one m o n t h , f i v e w e r e s t i l l i n p l a c e , t w o h a d c h a n g e d  and two were m i s s i n g .  After nearly a year,  had changed position and three were missing. been frequently observed, the  to  main beach.  three were  The the  position  still in place,  The three still in place  three had  as t h e y w e r e b e s i d e the p a t h n o r m a l l y t a k e n onto  They were never  o b s e r v e d a w a y f r o m t h e o r i g i n a l p o i n t of  marking.  I t i s s h o w n l a t e r t h a t r e m o v a l of l i m p e t s f r o m t h e r o c k a f f e c t s even when they a r e r e p l a c e d where they w e r e found.  Thus,  homing  homing,  c o u l d not  be s t u d i e d b y d i s p l a c i n g l i m p e t s a n d o b s e r v i n g w h e t h e r t h e y r e t u r n to t h e i r original sites.  It w a s  homing behaviour  concluded f r o m the evidence above,  however,  i s d e m o n s t r a t e d by s o m e i n d i v i d u a l l i m p e t s .  s p e c i f i c s i t e a n d h o m i n g to a c l u m p w e r e b o t h  observed.  that  H o m i n g to a  19  I d e n t i f i c a t i o n of t w o b e h a v i o u r a l  types  To determine whether homing be d i s t i n g u i s h e d , larger  samples  outcropping,  l i m p e t s and non-homing  as p r e d i c t e d f r o m the hypothesis, were marked.  The first sample  limpets  the i n d i v i d u a l s i n two  o c c u r r e d on a v e r t i c a l  o n w h i c h a p e r m a n e n t g r i d c o n t a i n i n g 28 s q u a r e s w a s  drilling holes  20cm apart.  and letters respectively,  Columns  and rows  could  rock  l a i d by  of t h e g r i d w e r e g i v e n  numbers  a n d w e r e u s e d to d e s c r i b e the p o s i t i o n of e a c h  indivi-  dual.  Observations June  10,  1969.  were made  on six c o n s e c u t i v e days,  E a c h d a y ' s o b s e r v a t i o n i n c l u d e d the s q u a r e  limpet was  found and the n u m b e r  clump was  defined as four or m o r e l i m p e t s grouped  m o r e than  1cm f r o m another limpet.  were recorded across number  f r o m June in which  of l i m p e t s w i t h w h i c h i t w a s  of c h a n g e s o f p o s i t i o n c o u l d b e  low tides,  shelter If,  Thus,  f r o m each limpet  The  seen  the t e r m  "migration"  it had not  migrated.  it had m i g r a t e d .  A  migration or  it  to a p o s i t i o n o u t s i d e the  clump  square.  second sample  soft sandstone depressions,  as  a l i m p e t c o u l d m i g r a t e to a n e w s q u a r e ;  c o u l d m i g r a t e f r o m w i t h i n a c l u m p i n a. s q u a r e i n the s a m e  the  if a l i m p e t moved f r o m its  it adopted a new shelter site,  could occur in two ways:  was  seen.  site, to f e e d a n d t h e n r e t u r n e d to the s a m e s i t e ,  after feeding,  A  s o t h a t a t t h e e n d of s i x d a y s  and feeding movements,  w a s adopted to d e s c r i b e the f o r m e r .  each  clumped.  To avoid confusion between changes in shelter position, between successive  through  s u c h that no l i m p e t  The observations  a s h e e t of d a t a p a p e r  5  of l i m p e t s w a s l o c a t e d o n a p r o j e c t i o n of  in which there were  30 s m a l l ,  hollow depressions.  i n w h i c h the l i m p e t s t e n d e d to f o r m c l u m p s ,  a n d u s e d i n p l a c e of the g r i d s y s t e m to d e s c r i b e p o s i t i o n s  were  These numbered  of l i m p e t s .  other procedures  w e r e the s a m e a s t h o s e d e s c r i b e d f o r the f i r s t  This  o b s e r v e d f r o m A u g u s t 11 t h r o u g h A u g u s t 16,  sample was  smooth  All  sample.  1969.  ZO  The number  of m i g r a t i o n s  made by each l i m p e t was  d i s t r i b u t i o n of m i g r a t i o n f r e q u e n c i e s a n d II.  In b o t h s a m p l e s ,  These  are seen in Tables  t h e r e w e r e m o r e l i m p e t s w h i c h d i d not m i g r a t e  a l l than in any other category, e a c h c a t e g o r y of h i g h e r  compiled.  determined and a  and there were progressively  migration  I at  fewer limpets  in  frequency.  If a l l l i m p e t s w e r e e q u a l l y l i k e l y to m i g r a t e o n a g i v e n night,  and if  t h e p r o b a b i l i t y of a n y l i m p e t m i g r a t i n g w e r e not m o d i f i e d by w h e t h e r i t h a d m i g r a t e d on p r e v i o u s would follow a Poisson  nights,  t h e n the d i s t r i b u t i o n of m i g r a t i o n  distribution.  This  tions were produced by l o c a l disturbances  frequencies  s i t u a t i o n m i g h t be f o u n d if  migra-  affecting limpets randomly,  a n i n n a t e t e n d e n c y of t h e l i m p e t s to m i g r a t e at r a n d o m i n t e r v a l s .  or  In s u c h a  s i t u a t i o n the m i g r a t i o n f r e q u e n c y p a t t e r n w o u l d have l i t t l e i m p o r t a n c e r e s p e c t to r e g u l a t i o n of l o c a l d e n s i t y . f r e q u e n c i e s w i t h the P o i s s o n of m i g r a t i o n s  per limpet,  distributed among  distribution,  comparing  the o b s e r v e d  Poisson  tions a r e not d i s t r i b u t e d r a n d o m l y not e q u a l l y l i k e l y to m i g r a t e ;  and independently.  are  with  migration  c a l c u l a t e d using the m e a n  one c a n t e s t w h e t h e r the m i g r a t i o n s  the l i m p e t s r a n d o m l y  a r e not i n a c c o r d w i t h the  dently,  By  number  indeed  If the  observations  d i s t r i b u t i o n , it i n d i c a t e s either that among  the l i m p e t s ,  or \ that m i g r a t i o n s  by  migra-  i n that a l l l i m p e t s  are  a r e not d i s t r i b u t e d i n d e p e n -  i n that the p r o b a b i l i t y of a l i m p e t m i g r a t i n g i s a f f e c t e d by w h e t h e r  not it has previously migrated. responsible  A  c o m b i n a t i o n of b o t h f a c t o r s c o u l d a l s o  f o r d e v i a t i o n f r o m the P o i s s o n  The observations  were compared  T a b l e s I a n d II. the P o i s s o n .  B o t h s e t s of o b s e r v a t i o n s  Both samples  These  comparisons  d i f f e r e d f r o m the P o i s s o n  In 1970,  in that m o r e  means  shown in  and  limpets more  o b s e r v a t i o n of the f i r s t s a m p l e w a s  t i n u e d i n a s s o c i a t i o n w i t h a n e x p e r i m e n t to b e d i s c u s s e d of o b s e r v a t i o n s  are  by  were significantly different f r o m  than expected did not m i g r a t e at a l l , fewer m i g r a t e d only once, m i g r a t e d m o r e than once.  w e r e o b t a i n e d ( T a b l e s III,  be  distribution.  w i t h the c a l c u l a t e d P o i s s o n  of t h e C h i - s q u a r e d t e s t of g o o d n e s s of f i t .  or  IV,  V,  below.  VI and VII).  Five All  con-  sets  showa  21  T A B L E  I.  The number  of l i m p e t s w h i c h o c c u r r e d i n e a c h c a t e g o r y of  tion frequency. sample  T h e data w e r e c o l l e c t e d f r o m the f i r s t  of l i m p e t s ,  f r o m J u n e 5 t h r o u g h J u n e 10,  s e r v e d d i s t r i b u t i o n of m i g r a t i o n f r e q u e n c i e s w a s  1969-  marked The  Number  of  limpets  observed  Number limpets  49.28  1  19  28.53  2  8  8.26  3  4  .1.59  4  1  0.23  5  0  0.03  88  87.92  =  4.94,  S i g n i f i c a n t at a l p h a =  of  expected  56  Chi-squared  d.f.  of  0  Total  to  of the C h i - s q u a r e d t e s t f o r g o o d n e s s of f i t .  Number  migrations  of m i -  The last four categories were grouped  m e e t the r e q u i r e m e n t s  ob-  compared with  the P o i s s o n d i s t r i b u t i o n c a l c u l a t e d f r o m the m e a n n u m b e r grations per limpet.  migra-  =  .0 5,  1.  T h r o u g h o u t t h i s study, the l e v e l of r e j e c t i o n (alpha) w a s .05. W h e r e the n u l l h y p o t h e s i s c o u l d h a v e b e e n r e j e c t e d at a l o w e r v a l u e of a l p h a , the l o w e r v a l u e i s g i v e n i n s t e a d of .0 5.  22  T A B L E  II.  T h e n u m b e r of l i m p e t s w h i c h o c c u r r e d i n e a c h c a t e g o r y of m i g r a tion frequency. The data w e r e c o l l e c t e d f r o m the second m a r k e d s a m p l e o f l i m p e t s , f r o m A u g u s t 11 t h r o u g h A u g u s t 16, 1 9 6 9 . The o b s e r v e d d i s t r i b u t i o n of m i g r a t i o n f r e q u e n c i e s w a s c o m p a r e d w i t h t h e P o i s s o n d i s t r i b u t i o n c a l c u l a t e d f r o m t h e m e a n n u m b e r of m i gration frequencies per limpet. The last four categories were g r o u p e d to m e e t the r e q u i r e m e n t s of the C h i - s q u a r e d t e s t f o r g o o d n e s s of f i t .  Number  of  Number  migrations  of  limpets observed  Number limpets  0  173  145.45  1  24  61.49  2  16  13.02  3  2  1.82  4  4  0.19  5  3  0.02  Total  222  221.99  Chi-squared  =  15.22,  S i g n i f i c a n t at A l p h a = .001, d.f.  =  1.  of  expected  23  T A B L E  III.  The number  of l i m p e t s w h i c h o c c u r r e d i n e a c h c a t e g o r y  migration frequency.  The data w e r e c o l l e c t e d f r o m  15 t h r o u g h F e b r u a r y  19.  1970.  of  February  The observed distribution was  c o m p a r e d w i t h the P o i s s o n d i s t r i b u t i o n c a l c u l a t e d f r o m the mean number gories  of m i g r a t i o n s p e r l i m p e t .  The last three cate-  w e r e g r o u p e d t o m e e t the r e q u i r e m e n t s of the  Chi-  s q u a r e d t e s t f o r g o o d n e s s of f i t .  Number  of  Number  migrations  limpets  observed  Number limpets  35  30.65  1  9  15.60  2  5  3.97  3  1  0.67  4  1  0.09  51  50.98  Chi-squared  =  4.50,  Significant at alpha =  1.  =  .0 5,  of  expected  0  Total  d.f.  of  24  T A B L E IV.  The number  of l i m p e t s w h i c h o c c u r r e d i n e a c h c a t e g o r y  of  migration frequency.  The data were c o l l e c t e d f r o m A p r i l  through May  The observed distribution was  2,  w i t h the P o i s s o n  1970.  d i s t r i b u t i o n c a l c u l a t e d f r o m the m e a n  of m i g r a t i o n s p e r l i m p e t .  29  compared number  The l a s t two c a t e g o r i e s w e r e  grouped  to m e e t the r e q u i r e m e n t s of the C h i - s q u a r e d t e s t f o r g o o d n e s s fit.  Number  of  Number  migrations  limpets  observed  Number limpets  72  64.40  1  15  28.34  2  10  6.23  3  3  0.91  100  Chi-squared  =  12.05,  S i g n i f i c a n t at a l p h a =  1.  =  .001,  of  expected  0  Total  d.f.  of  99.88  of  25  T A B L E  V.  The number  of l i m p e t s w h i c h o c c u r r e d i n e a c h c a t e g o r y - o f  migration frequency.  The data w e r e c o l l e c t e d f r o m June  t h r o u g h J u n e 5, 1 9 7 0 .  The observed distribution was  p a r e d w i t h the P o i s s o n number  d i s t r i b u t i o n c a l c u l a t e d f r o m the  of m i g r a t i o n s p e r l i m p e t .  The last three  w e r e g r o u p e d t o m e e t t h e r e q u i r e m e n t s of t h e  mean  categories  Chi-squared  t e s t f o r g o o d n e s s of f i t .  N u m b e r of migrations  Number limpets  of  observed  Number limpets  63  51.25  1  6  25.92  2  11  6.55  3  5  1.11  4  0  0.14  Total  85  84.97  = 26.63,  S i g n i f i c a n t a t a l p h a = .001, d.f.  =  1.  of  expected  0  Chi-squared  1  com-  26  T A B L E  VI.  The number  of l i m p e t s w h i c h o c c u r r e d i n e a c h c a t e g o r y  of  migration frequency.  The data w e r e c o l l e c t e d f r o m June  t h r o u g h J u n e 19, 1 9 7 0 .  The observed distribution was  with the P o i s s o n  distribution  of m i g r a t i o n s p e r l i m p e t .  c a l c u l a t e d f r o m the m e a n  16  compared number  The last three categories were  grouped  to m e e t t h e r e q u i r e m e n t s of t h e C h i - s q u a r e d t e s t f o r g o o d n e s s fit.  Number  of  Number  migrations  limpets  of  observed  Number limpets  expected  0  73  60.39  1  6  27.35  2  11  6.20  3  5  0.94  4  0  0.11  Total  95  94.99  Chi-squared  =  29.91,  Significant at alpha = d.f.  =  1.  .001,  of  of  27  T A B L E  VII.  The number  of l i m p e t s w h i c h o c c u r r e d i n e a c h c a t e g o r y o f m i g r a -  tion frequency. J u l y 27,  1970.  mean number gories  T h e d a t a w e r e c o l l e c t e d f r o m J u l y 23  through  T h e o b s e r v e d d i s t r i b u t i o n c a l c u l a t e d f r o m the of m i g r a t i o n s p e r l i m p e t .  The last three cate-  w e r e g r o u p e d to m e e t the r e q u i r e m e n t s of t h e  Chi-squared  t e s t f o r g o o d n e s s of f i t .  Number  of  Number  migrations  limpets  of  observed  Number limpets  0  76  62.75  1  19  37.43  2  12  11.17  3  7  2.22  4  0  0.33  Total  114  113.90  Chi-squared  =  13.90,  S i g n i f i c a n t at a l p h a = .001, d.f.  -  1.  of  expected  28  s i m i l a r p a t t e r n of d e v i a t i o n f r o m the  Poisson.  If m i g r a t i o n s w e r e n o t d i s t r i b u t e d r a n d o m l y a m o n g t h e l i m p e t s , m i g h t be e x p l a i n e d by d i f f e r e n c e s i n b e h a v i o u r a m o n g the l i m p e t s o r w i t h the s i z e s red.  this  patterns associated with size  of t h e c l u m p s i n w h i c h the l i m p e t s  occur-  If t h e m i g r a t i o n p a t t e r n of s m a l l e r l i m p e t s d i f f e r e d f r o m t h a t of l a r g e r  limpets, larger  or if limpets i n s m a l l e r clumps behaved differently f r o m those  clumps,  in  a d i s t r i b u t i o n of m i g r a t i o n f r e q u e n c i e s w h i c h d i f f e r e d f r o m  the P o i s s o n w o u l d be o b s e r v e d .  Checks  w e r e m a d e to d e t e r m i n e  whether  l i m p e t s i z e o r c l u m p s i z e c o u l d a c c o u n t f o r t h e t y p e of d i s t r i b u t i o n w h i c h observed.  T h e e f f e c t of s i z e w a s  d i s c o u n t e d i m m e d i a t e l y b e c a u s e a l l the  l i m p e t s on the f i r s t a r e a w e r e a p p r o x i m a t e l y the s a m e s i z e , 2.00cm.  was  b e t w e e n 1.50  T o check w h e t h e r c l u m p size a f f e c t e d the m i g r a t i o n behaviour,  following test was p e r f o r m e d in both samples  of m a r k e d l i m p e t s .  d i v i s i o n between " l a r g e " and " s m a l l " clumps was the l i m p e t s f e l l into e a c h group,  made  An  the  arbitrary  so t h a t r o u g h l y  half  a n d t h e d a t a w e r e d i v i d e d a c c o r d i n g to t h e  initial c l u m p i n w h i c h each l i m p e t was found.  T h e d i s t r i b u t i o n s of  migration  f r e q u e n c i e s f r o m the l a r g e and s m a l l c l u m p s w e r e then c o m p a r e d by of the C h i - s q u a r e d t e s t f o r i n d e p e n d e n c e These comparisons  and  of s a m p l e s  are shown in T a b l e s VIII and IX.  (contingency  means  tables).  In n e i t h e r s a m p l e  was  t h e r e a s i g n i f i c a n t d i f f e r e n c e b e t w e e n the m i g r a t i o n f r e q u e n c i e s f r o m the two t y p e s of  clumps.  The d e v i a t i o n s f r o m the e x p e c t e d P o i s s o n d i s t r i b u t i o n m i g h t a l s o r e s u l t e d f r o m u s i n g s h o r t p e r i o d s of o b s e r v a t i o n .  have  The p r o b a b i l i t y that a  lim-  pet w o u l d m i g r a t e m i g h t be a f f e c t e d by w h e t h e r or not it h a d m i g r a t e d i n the p r e v i o u s f e w days; but s u c h b u r s t s  of m i g r a t i o n a c t i v i t y c o u l d be  of a c t i v i t y m a d e a m o n t h p r e v i o u s l y .  If t h i s w e r e t h e c a s e ,  independent  migrations  could  b e c o n s i d e r e d to be i n d e p e n d e n t o v e r t h e l o n g e r t e r m a n d t h e d e v i a t i o n f r o m the P o i s s o n w o u l d have l i t t l e i m p o r t a n c e . e x a m i n i n g data c o l l e c t e d over same limpets,  m a d e at v a r i o u s  six months.  T h i s p o s s i b i l i t y was t e s t e d by Four  s e t s of o b s e r v a t i o n s  t i m e s during the six months and  of t h e  presented  29  T A B L E  VIII.  A  comparison  b e t w e e n the m i g r a t i o n f r e q u e n c y d i s t r i b u t i o n s  l i m p e t s initially found i n large and s m a l l c l u m p s . clump was  A  defined as that containing l e s s than ten l i m p e t s ;  large  c l u m p as that containing ten or m o r e l i m p e t s .  were  c o l l e c t e d f r o m the f i r s t s a m p l e  J u l y 23 t h r o u g h  July  27,  1970.  The  The last two m i g r a t i o n  small  Number  of  migrations  No. •  limpets  test for independence  from fre-  No.  large  limpets  No.  limpets  clumps  No.  limpets  observed  expected  0  38  42.67  38  33.33  1  12  10.67  7  8.33  2  8  6.74  4  5.26  3  6  3.93  1  3.07  Total  64  64.01  50  49.99  Chi-squared  =  3.73,  Not significant at alpha d.f.  =  1.  =  .0 5,  of  samples.  clumps  expected  - observed  of  a  data  of m a r k e d l i m p e t s ,  quency^ c a t e g o r i e s w e r e g r o u p e d to m e e t the r e q u i r e m e n t s the C h i - s q u a r e d  of  small  30  T A B L E  IX.  A  c o m p a r i s o n b e t w e e n the m i g r a t i o n f r e q u e n c y d i s t r i b u t i o n s  limpets  initially  clump was a large  found in large and s m a l l c l u m p s .  data w e r e c o l l e c t e d f r o m the second s a m p l e gration frequency categories of t h e C h i - s q u a r e d  small  Number  of  No.  limpets  1969.  of m a r k e d  The  limpets,  The last three  mi-  w e r e g r o u p e d to m e e t the r e q u i r e -  test for independence  clumps  No.  limpets.  large  limpets  No.  limpets  of  samples.  clumps  No.  limpets  migrations  observed  expected  observed  expected  0  61  61.56  112  111.44  1  8  8.54  16  15.46  2  6  5.70  10  10.31  3  1  0.71  1  1.29  4  2  1.42  2  1.93  5  1  1.07  2  1.93  Total  79  79.00  143  142.36  Chi-squared  =  0.39,  Not significant at alpha d.f.  =  1.  of  small  defined as that containing l e s s than fifteen l i m p e t s ;  c l u m p as that containing f i f t e e n or m o r e  f r o m A u g u s t 11 t h r o u g h A u g u s t 16, ments  A  =  .0 5,  31  a b o v e i n T a b l e s III,  IV,  V a n d VI,  were combined.  The resulting distribution  of m i g r a t i o n f r e q u e n c i e s c o n t a i n e d f i f t e e n p o s s i b l e m i g r a t i o n c a t e g o r i e s . observations  m a d e on consecutive days w e r e i n c l u d e d ; m i g r a t i o n s  b e t w e e n s e t s of o b s e r v a t i o n s servations  were ignored.  Comparison  of l i m p e t s  of the c o m b i n e d  ob-  w i t h the c a l c u l a t e d P o i s s o n d i s t r i b u t i o n ( T a b l e X) i n d i c a t e d that the  observations  w e r e not i n a c c o r d w i t h the e x p e c t e d P o i s s o n .  T h e p a t t e r n of  d e v i a t i o n f r o m the P o i s s o n was the s a m e a s that o b s e r v e d i n the s e t s of  Only-  component  observations.  T h u s t h e r e a p p e a r e d to be two b e h a v i o u r a l t y p e s . not m i g r a t e ,  fewer m i g r a t e d only once and m o r e  w o u l d be e x p e c t e d f r o m the m e a n n u m b e r behaviour  M o r e limpets did  migrated many times  of m i g r a t i o n s  than  per individual.  Such  p e r s i s t e d f o r l o n g p e r i o d s of t i m e , a n d c o u l d not be e x p l a i n e d b y  differences in behaviour associated with clump size. observed behaviour  It i s p o s s i b l e t h a t the  pattern resulted f r o m a stimulus distributed randomly  in  space; b e c a u s e the l i m p e t s a r e not d i s t r i b u t e d r a n d o m l y i n space and thus w o u l d not be a f f e c t e d r a n d o m l y by s u c h a s t i m u l u s . sidered unlikely, however; and it was limpets,  homing and non-homing,  This  situation was  con-  c o n c l u d e d that two b e h a v i o u r a l types  of  had been detected.  R e l a t i o n between density and homing  behaviour  A n e x p e r i m e n t w a s p e r f o r m e d to d e t e r m i n e w h e t h e r t h e p e r c e n t a g e n o n - h o m i n g l i m p e t s would i n c r e a s e as the density  increased.  The  of  reasoning  w a s that a n i n c r e a s e i n density i n an e n c l o s e d a r e a would r e s u l t in l e s s favourable local conditions,  w i t h a consequent i n c r e a s e i n the n u m b e r  pets i n t o l e r a n t to l o c a l conditions and thus  In F e b r u a r y ,  1970,  fence.  lim-  non-homing.  the a r e a containing the f i r s t s a m p l e  d e s c r i b e d i n the p r e c e d i n g s e c t i o n was  of  of l i m p e t s  e n c l o s e d b y m e a n s of a p l a s t i c  mesh  A l l A_. d i g i t a l i s w i t h i n t h e e n c l o s u r e w e r e m a r k e d o r r e m a r k e d .  s e t s of o b s e r v a t i o n s  were made  Two  on the m i g r a t i o n b e h a v i o u r at n a t u r a l d e n s i t y .  T h e n l i m p e t s f r o m e l s e w h e r e w e r e a d d e d to the e n c l o s e d a r e a ,  increasing  the  32  T A B L E  X.  The number  of l i m p e t s w h i c h o c c u r r e d i n e a c h c a t e g o r y  migration frequency. IV,  V,  and VI.  the P o i s s o n migrations  of  The data a r e combined f r o m Tables  The combined distribution was  compared  d i s t r i b u t i o n c a l c u l a t e d f r o m the m e a n n u m b e r per limpet.  The last four categories were  to m e e t the r e q u i r e m e n t s  of the C h i - s q u a r e d t e s t f o r  of  Number  migrations  0  limpets  of  observed  Number limpets  goodness  16  6.95  1  8  12.18  2  3  10.65  3  2  6.22  4  4  2.72  5  3  0.95  6  2  0.28  7  2  0.07  Total  40  40.02  Chi-squared  =  33.69,  S i g n i f i c a n t at a l p h a = .001, d.f.  =  3.  of  expected  0  of  grouped  of f i t .  Number  III, with  33  d e n s i t y to t w i c e the p r e v i o u s made.  More  in February,  level,  a n d t w o f u r t h e r s e t s of o b s e r v a t i o n s  l i m p e t s w e r e a d d e d to i n c r e a s e d e n s i t y to f o u r t i m e s the a n d one f u r t h e r s e t of o b s e r v a t i o n s  The percentage  was  density  made.  of l i m p e t s t h a t m i g r a t e d a n d t h e m e a n n u m b e r  of  t i o n s p e r l i m p e t p e r d a y w e r e c a l c u l a t e d f o r e a c h set of o b s e r v a t i o n s , seen in Table XI. ting was  r = .349.  number  The  T h e c o r r e l a t i o n b e t w e e n r e l a t i v e d e n s i t y a n d the r = .242.  a n d the m e a n n u m b e r  A  possible  mean  Neither was  statisti-  at l e a s t w i t h i n the  c r i t i c i s m of t h i s e x p e r i m e n t i s t h a t h o m i n g b e h a v i o u r  was  rather than in several groups  T h e w e a t h e r c h a n g e d d u r i n g the c o u r s e  of the  of  ex-  a n d i t m i g h t be a r g u e d that the w e a t h e r c a u s e d c h a n g e s i n e i t h e r  the h o m i n g b e h a v i o u r changes could have  o r the l o c a l c o n d i t i o n s i n the e x p e r i m e n t a l a r e a .  c o m p e n s a t e d f o r c h a n g e s i n behaviour-, c a u s e d by  d e n s i t y i n the e x p e r i m e n t a l a r e a , was  w i t h the r e s u l t that no change  in  increased  observed.  w e a t h e r b e c a m e p r o g r e s s i v e l y w a r m e r and d r i e r as d e n s i t y i n the was increased,  Such  homing  It i s u n l i k e l y t h a t w e a t h e r h a d t h i s e f f e c t o n l o c a l c o n d i t i o n s .  a n y e f f e c t of w e a t h e r o n h o m i n g b e h a v i o u r  t h e s a m e d i r e c t i o n a s t h a t of i n c r e a s e d d e n s i t y . warmer  migra-  of d e n s i t i e s .  l i m p e t s at the s a m e t i m e .  behaviour  are  nor between relative density  of m i g r a t i o n s p e r l i m p e t p e r day,  o b s e r v e d i n o n e g r o u p of l i m p e t s o v e r t i m e ,  periment,  and  T h i s i n d i c a t e d that t h e r e w a s no r e l a t i o n b e t w e e n r e l a t i v e  d e n s i t y a n d p e r c e n t a g e of l i m p e t s t h a t m i g r a t e d ;  experimental range  migra-  c o r r e l a t i o n between r e l a t i v e density and per cent  of m i g r a t i o n s p e r l i m p e t p e r d a y w a s  cally significant.  were  Since  experiment  would have been in  It i s p o s s i b l e ,  however,  that  and d r i e r w e a t h e r d e c r e a s e d the t e n d e n c y f o r l i m p e t s to m i g r a t e ,  even  though l o c a l conditions a f f e c t e d by d e n s i t y w e r e d e t e r i o r a t i n g .  T h e e f f e c t of w e a t h e r c o u l d be e l i m i n a t e d b y o b s e r v i n g l i m p e t s at different densities within a short period, l i m p e t s at the s a m e t i m e .  There was  or by observing  several groups  of  no t i m e a v a i l a b l e i n w h i c h t h i s e x p e r i -  34  T A B L E XI.  C o m p a r i s o n b e t w e e n r e l a t i v e d e n s i t y a n d p e r c e n t a g e of l i m p e t s t h a t m i g r a t e d d u r i n g t h e p e r i o d of o b s e r v a t i o n , d e n s i t y and the m e a n n u m b e r Relative density was pets b y the n u m b e r  of m i g r a t i o n s  between  c a l c u l a t e d by d i v i d i n g the n u m b e r  i n the e n c l o s u r e on F e b r u a r y  tion coefficients for both comparisons  Total P e r i o d of observation  and  per limpet per 15.  Correla-  were calculated.  Percentage  Mean  Relative  marked limpets  number  density  migrating  Total  number  number  marked  limpets  limpets  133  51  1.00  31.4  0.127  migrations / limpet/day  Feb. Feb.  15-  Apr. May  292  125  100  0.94  28.0  0.146  June June  1 5  290  95  2.18  23.2  0.113  283  117  2.13  28.2  0.162  561  114  4.22  33.3  0.149  19  June  16-  June  19  July July  23-  For  27  the c o r r e l a t i o n b e t w e e n r e l a t i v e d e n s i t y a n d p e r c e n t a g e r  migrating,  = 0.349.  F o r the c o r r e l a t i o n b e t w e e n r e l a t i v e d e n s i t y a n d the m e a n migrations  p e r l i m p e t p e r day,  N e i t h e r i s s i g n i f i c a n t a t a l p h a = .0 5.  r = 0.242.  number  day.  of l i m -  35  ment could have been repeated.  Relation between homing and local environmental  The previous density,  e x p e r i m e n t e x a m i n e d h o m i n g b e h a v i o u r w i t h r e s p e c t to  i n a n a t t e m p t to d e t e r m i n e w h e t h e r the p r o p o r t i o n of h o m i n g  d e c r e a s e d as density i n c r e a s e d .  space.  This  u s e d by the l i m p e t s ,  Two experiments,  one i n 1969 a n d 6ne i n 1970,  In t h e s e e x p e r i m e n t s ,  c l u m p of m a r k e d l i m p e t s w a s r e m o v e d . subsequently and was  w h i c h the a l g a e had not been  proportion  the algae on the r o c k s u r r o u n d i n g The number  of m a r k e d l i m p e t s of  a  which  non-homing, around  removed.  contained s i m i l a r numbers  were located in areas  smaller  of e m i g r a n t s f r o m a c o n t r o l c l u m p  In e a c h of the t w o e x p e r i m e n t s , limpets  a  w e r e p e r f o r m e d to t e s t  e m i g r a t e d f r o m the c l u m p was u s e d as a m e a s u r e  c o m p a r e d w i t h the n u m b e r  was  behaviour.  w h e t h e r a d e c r e a s e i n food a v a i l a b i l i t y r e s u l t s i n a d e c r e a s e i n the of h o m i n g l i m p e t s .  environ-  The same hypothesis  that, i f l o c a l c o n d i t i o n s a r e m a d e u n f a v o u r a b l e f o r l i m p e t s ,  p r o p o r t i o n of t h e l i m p e t s s h o w h o m i n g  would  such as food and  section deals with experiments in which local  mental conditions were manipulated directly. tested;  limpets  It w a s a s s u m e d t h a t i n c r e a s e d d e n s i t y  c a u s e r e d u c t i o n of s o m e r e s o u r c e s shelter  conditions  the e x p e r i m e n t a l a n d c o n t r o l c l u m p s  of l i m p e t s of s i m i l a r s i z e .  The two  of  clumps  of s i m i l a r t o p o g r a p h y w i t h the d i s t a n c e b e t w e e n t h e m  j u s t g r e a t e n o u g h so t h a t l i m p e t s w o u l d not be l i k e l y to m o v e f r o m one to the other.  The algae was  wire brush,  up to 0 . 6 m  r e m o v e d f r o m the r o c k , by s c r u b b i n g the r o c k w i t h a a r o u n d the e x p e r i m e n t a l c l u m p .  r e m o v i n g algae had a possible disadvantage f a c e of t h e r o c k , the  k i l l the l i m p e t s . i n e f f i c i e n t to be  For  of  i n that i t s l i g h t l y a l t e r e d the  but i t w a s the b e s t of a n y m e t h o d t r i e d .  r o c k w i t h a b r u s h to k i l l the a l g a e ,  This method  Alcohol,  sur-  spread  on  t e n d e d to r u n i n t o the l i m p e t c l u m p  B u r n i n g the a l g a e w i t h a p r o p a n e gas t o r c h was too s l o w  and and  practical.  each group,  the n u m b e r  of l i m p e t s r e m a i n i n g i n the c l u m p ,  the  36  n u m b e r of e m i g r a n t s a n d the d i s t a n c e s t h e y t r a v e l l e d w e r e recorded.  subsequently  T h e r e s u l t s of the 1969 a n d I 9 6 0 e x p e r i m e n t s a r e p r e s e n t e d i n  T a b l e s XII and XIII r e s p e c t i v e l y .  I n the f o r m e r e x p e r i m e n t ,  the n u m b e r  of  l i m p e t s w h i c h e m i g r a t e d f r o m the e x p e r i m e n t a l g r o u p w a s o n l y s l i g h t l y h i g h e r t h a n the n u m b e r w h i c h e m i g r a t e d f r o m t h e c o n t r o l g r o u p .  H o w e v e r , the e m i -  g r a n t s f r o m the e x p e r i m e n t a l g r o u p t e n d e d to m i g r a t e m u c h f a r t h e r t h a n t h o s e f r o m the c o n t r o l .  I n the 1970 e x p e r i m e n t ,  the r a t e of e m i g r a t i o n w a s m u c h  h i g h e r i n the e x p e r i m e n t a l g r o u p t h a n the c o n t r o l , a l t h o u g h the  distances  t r a v e l l e d by the e m i g r a n t s w e r e s i m i l a r . T h e 1969 a n d 1970  e x p e r i m e n t s w e r e c a r r i e d out i n s i m i l a r l o c a t i o n s  o n d i f f e r e n t p a r t s of the s h o r e .  O n e d i f f e r e n c e b e t w e e n the e x p e r i m e n t s  t h a t the l i m p e t s t e s t e d i n 1970 w e r e s m a l l e r t h a n t h o s e u s e d i n 1 9 6 9 . l i m p e t s m a y h a v e a g r e a t e r t e n d e n c y to r e m a i n a t the h o m e  was  Larger  site.  It w a s c o n c l u d e d t h a t d e c r e a s e d f o o d a v a i l a b i l i t y r e s u l t s i n a d e c r e a s e i n the p r o p o r t i o n of l i m p e t s w h i c h r e t u r n to t h e i r h o m e s i t e s .  This conclusion  m u s t be m o d i f i e d b y t a k i n g i n t o c o n s i d e r a t i o n the p o s s i b i l i t y t h a t l a r g e r  lim-  p e t s a r e l e s s l i k e l y to m i g r a t e f r o m the h o m e s i t e e v e n d u r i n g u n f a v o u r a b l e local conditions.  T h e p o s s i b l e r e l a t i o n b e t w e e n s i z e a n d t e n d e n c y to m i g r a t e  s h o u l d be i n v e s t i g a t e d f u r t h e r . A n o t h e r f a c t o r w h i c h m i g h t a f f e c t the p r o p o r t i o n of h o m i n g l i m p e t s w a s t h o u g h t to be c r o w d i n g on a r e a s w h e r e l i m p e t s f e e d . would increase  c r o w d i n g o n the f e e d i n g a r e a ;  Increases in l o c a l density  u n d e r the h y p o t h e s i s b e i n g t e s t e d  the p r o p o r t i o n of h o m i n g l i m p e t s w o u l d be e x p e c t e d to d e c r e a s e . T h i s p r e d i c t i o n w a s t e s t e d i n the f o l l o w i n g w a y . clump were marked,  A l l limpets found i n a  t h e n a n e q u a l n u m b e r of l i m p e t s f r o m e l s e w h e r e  a d d e d to the c l u m p . It w a s r e a s o n e d t h a t t h e s e a d d e d l i m p e t s w o u l d  were increase  the d e g r e e of c r o w d i n g o n the f e e d i n g a r e a d u r i n g the f i r s t n i g h t , b u t w o u l d n o t d e c r e a s e the a v a i l a b i l i t y of f o o d i n t h a t t i m e .  If the p r o p o r t i o n of m a r k e d  l i m p e t s that r e t u r n e d to the o r i g i n a l c l u m p b y the f o l l o w i n g d a y w a s  smaller  37  TABLE  XII.  The number  of l i m p e t s r e m a i n i n g ,  the n u m b e r  and the m e d i a n d i s t a n c e t r a v e l l e d b y e m i g r a n t s  of  emigrants  i n the e x p e r i -  m e n t a l (E) a n d c o n t r o l (C) g r o u p s of a n e x p e r i m e n t to m e a s u r e was  response  r e m o v e d f r o m a r o u n d the e x p e r i m e n t a l g r o u p on J u l y  A u g u s t 2,  10,  15 a n d S e p t e m b e r  Number marked Limpets remaining  Date  designed  of l i m p e t s t o r e d u c e d f o o d l e v e l s . A l g a e 1,  1969.  Number  Median  emigrant limpets  distance travelled  (cm)  E  _c  E_  C  E  84  77  -  -  -  -  August 2  83  74  1  3  20.0  16.7  August  10  66  71  6  4  60.0  50.0  August 1 5  62  62  16  10  105.5  38.0  59  58  16  14  160.0  92.1  July  20  September  1  C  30,  38  T A B L E  XIII.  T h e n u m b e r o f l i m p e t s r e m a i n i n g , t h e n u m b e r of e m i g r a n t s a n d the m e d i a n d i s t a n c e t r a v e l l e d by e m i g r a n t s i n the e x p e r i m e n t a l ( E ) a n d c o n t r o l (C) g r o u p s o f a n e x p e r i m e n t d e s i g n e d to m e a s u r e the r e s p o n s e of l i m p e t s to r e d u c e d f o o d l e v e l s . A l g a e was r e m o v e d f r o m a r o u n d the e x p e r i m e n t a l g r o u p on J u n e 4 a n d 5, 1 9 7 0 .  Number  Date  marked  Number  limpets  emigrant  remaining  limpets  E/  _c  June 4  93  111  0  June  84  109  46  90  5  J u n e 16  Median distance travelled  (cm)  E  C  0  0  0  6  0  42.5  0  43  18  54.5  45.0  E  C  39  t h a n t h a t of a s i m i l a r c o n t r o l c l u m p , thesis would be  In June,  hypo-  confirmed.  1969,  w e r e designated as groups.  then the p r e d i c t i o n m a d e f r o m the  four  similar clumps  of l i m p e t s w e r e m a r k e d .  c o n t r o l groups and two w e r e designated as  Two  experimental  L i m p e t s r e m o v e d f r o m e l s e w h e r e w e r e a d d e d to double the  of l i m p e t s i n the e x p e r i m e n t a l c l u m p s .  t h e e f f e c t of r e m o v a l  and  r e p l a c e m e n t on those l i m p e t s that w e r e a d d e d to the e x p e r i m e n t a l g r o u p s  was  tested.  In a d d i t i o n ,  number  A l l the o r i g i n a l l i m p e t s i n one e x p e r i m e n t a l g r o u p  a n d one  control  g r o u p w e r e r e m o v e d f r o m the r o c k a n d r e p l a c e d at the s a m e t i m e .  T h e p o s i t i o n of e a c h m a r k e d l i m p e t w a s  r e c o r d e d the f o l l o w i n g  day.  T h e n u m b e r s. r e m a i n i n g i n e a c h c l u m p a r e p r e s e n t e d i n T a b l e X I V .  Few  l i m p e t s r e m a i n e d i n the c o n t r o l and e x p e r i m e n t a l c l u m p s that w e r e  removed  and replaced,  while a higher  not r e m o v e d and replaced.  percentage  r e m a i n e d i n the two c l u m p s that  T h i s i n d i c a t e d that r e m o v a l and  replacement  c a u s e d a h i g h p r o p o r t i o n of l i m p e t s to m i g r a t e f r o m t h e i r h o m e addition,  T h i s i n d i c a t e d that c r o w d i n g c a u s e d a  i n t h e p r o p o r t i o n of h o m i n g  The experiment was and control groups.  Because  This  slight  r e p e a t e d i n 1970  with three pairs  of e x p e r i m e n t a l  e x p e r i m e n t was i n c o n c l u s i v e f o r the f o l l o w i n g  reason.  of a v e r y  hot  l i m p e t s t h a t a r e p l a c e d d o w n on a n a r e a do n o t m o v e a b o u t to  find shelter sites, turbed limpets,  and thus a r e unable to r e s i s t d e s i c c a t i o n as w e l l as  a l l g r o u p s of l i m p e t s w e r e  the d a y to r e d u c e d e s i c c a t i o n .  During  splashed with seawater  the f o l l o w i n g high tide,  limpets moved away f r o m their original positions, w e l l as the e x p e r i m e n t a l g r o u p s . bance  In  limpets.  L i m p e t s w e r e a d d e d to the e x p e r i m e n t a l g r o u p s on the m o r n i n g day.  sites.  e a c h e x p e r i m e n t a l group had fewer l i m p e t s r e m a i n i n g than the  appropriate control group. decrease  were  caused by splashing  effect made the r e s u l t s  throughout  virtually all  i n the c o n t r o l g r o u p s  T h i s w a s t h o u g h t to r e s u l t f r o m t h e  the g r o u p s throughout the day.  completely inconclusive.  undis-  This  as  distur-  unexpected  The experiment was  not  40  TABLE  XIV.  The initial number  of l i m p e t s i n e a c h of f o u r t r e a t m e n t g r o u p s  of a n e x p e r i m e n t d e s i g n e d to m e a s u r e  of l i m p e t s to  crowding,  aiid the n u m b e r  one day.  L i m p e t s r e m o v e d f r o m e l s e w h e r e w e r e a d d e d to the  experimental groups;  that r e m a i n e d in each group  J u n e 7,  Initial  1969  number  of l i m p e t s  after  d i s t u r b e d groups w e r e r e m o v e d f r o m the  r o c k and r e p l a c e d at the s a m e  Group  response  site.  J u n e 8,  1969  Number  of  limpets  remaining  initial.  Percentage remaining  undisturbed experimental  47  39  82.97  undisturbed control  51  50  98.03  disturbed expe r imental  52  3  5.76  disturbed control  52  7  13.46  41  repeated again because  It w a s decrease  of l a c k of t i m e .  thus not c o n c l u s i v e l y d e m o n s t r a t e d that c r o w d i n g  i n t h e p r o p o r t i o n of h o m i n g  artificial disturbances,  s u c h as  limpets.  splashing  of  in density.  disturbance w e r e thought  The amount  of  the n u m b e r  1970,  open  each 3 0 c m long,  were  E a c h p a r t i a l enclosure was bounded  by the fence.  The fourth side was  on  of the l i m p e t s to l o w f o o d a v a i l a b i l i t y w a s t e s t e d b y p l a c i n g t w e n t y within a partial enclosure b e r of l i m p e t s r e m a i n i n g  f r o m w h i c h the algae had not been  T h e e f f e c t of c r o w d i n g w a s I n o n e of t h e s e ,  high density treatment.  The  numcom-  enclosure  removed.  d e t e r m i n e d i n the r e m a i n i n g two p a r t i a l  forty limpets were placed.  In the other,  This was  c a l l e d the  twenty empty limpet shells were  a n d then twenty limpets w e r e placed (shell treatment). t h e s e t r e a t m e n t s w i t h the c o n t r o l , a m e a s u r e m i g r a t i o n c o u l d be d e t e r m i n e d .  Response  on the f o l l o w i n g day c o u l d be  pared with a control, in which twenty limpets were placed in an  on  limpets  f r o m w h i c h the a l g a e h a d b e e n r e m o v e d . i n the e n c l o s u r e  con-  left open.  T h e two f a c t o r s to be t e s t e d w e r e l o w food a v a i l a b i l i t y and c r o w d i n g .  ments  dis-  w h i c h r e m a i n e d w i t h i n the e n c l o s u r e  t w o s e t s of f o u r a d j o i n i n g p a r t i a l e n c l o s u r e s  structed with plastic m e s h fences.  enclosures.  to  day.  In July,  three sides,  in  by p l a c i n g l i m p e t s i n a p a r t i a l e n c l o s u r e that was  and observing  the f o l l o w i n g  the a m o u n t  Such disturbances  o c c u r n a t u r a l l y a s a r e s u l t of l o c a l i n c r e a s e s  at one end,  sea  result ' in a large decrease  d e s i g n e d to m e a s u r e  c a u s e d b y c r o w d i n g a n d l o w l e v e l s of f o o d .  measured  that  limpets.  A n experiment was  turbance was  however,  the l i m p e t s d u r i n g the day w i t h  w a t e r or r e m o v i n g and r e p l a c i n g the l i m p e t s , the p r o p o r t i o n of h o m i n g  It i s a p p a r e n t ,  causes a  By  comparing  glued e a c h of  o f t h e e f f e c t of c r o w d i n g  on  In a d d i t i o n , the h i g h d e n s i t y a n d s h e l l t r e a t -  c o u l d be c o m p a r e d w i t h e a c h o t h e r .  If t h e n u m b e r  s h e l l t r e a t m e n t w e r e g r e a t e r than the n u m b e r  r e m a i n i n g i n the  r e m a i n i n g in the high  density  42  treatment,  t h i s w o u l d i n d i c a t e t h a t s t a t i o n a r y l i m p e t s do not c a u s e a s  d i s t u r b a n c e to o t h e r l i m p e t s as l i m p e t s w h i c h m o v e  Four successive  much  about.  r e p l i c a t e s w e r e o b t a i n e d ; one f r o m e a c h s e t of e n c l o s u r e s days.  on two  C o m p l e t e l y d i f f e r e n t l i m p e t s w e r e u s e d on the second  The results are shown in Table X V .  trial.  T h e r e was l i t t l e d i f f e r e n c e b e t w e e n the  c o n t r o l m e a n a n d t h e m e a n s of the l o w f o o d a n d h i g h d e n s i t y t r e a t m e n t s ,  but  the s h e l l t r e a t m e n t m e a n w a s h i g h e r t h a n the c o n t r o l .  shells  as  shelter sites,  w h i l e i n the other t r e a t m e n t s they s h e l t e r e d a l o n g the  of the e n c l o s u r e s  Because  L i m p e t s used the  or moved  sides  out.  of the h i g h v a r i a b i l i t y a m o n g  r e p l i c a t e s of s i n g l e t r e a t m e n t s ,  it was not shown that l o w food or c r o w d i n g cause d i s t u r b a n c e s  resulting in  high  e m i g r a t i o n f r o m the e n c l o s u r e s .  T h e e x p e r i m e n t could not be continued b e -  cause  of p r o b l e m s  of l a c k of t i m e a n d b e c a u s e  be concluded,  however,  w i t h the e n c l o s u r e s .  It  could  t h a t l i m p e t s i n s e a r c h of a p l a c e t o s h e l t e r s t o p  they encounter a stationary limpet.  This may partly explain how large  when clumps  of l i m p e t s f o r m i n a n a r e a .  W h e n f o o d a v a i l a b i l i t y i s s u d d e n l y r e d u c e d a r o u n d a c l u m p of l i m p e t s , it appears This  t h a t t h e p r o p o r t i o n of l i m p e t s w h i c h h o m e to the c l u m p  r e s u l t i s in a g r e e m e n t w i t h that p r e d i c t e d by the  decreases.  hypothesis.  E x p e r i m e n t s t h a t e x a m i n e d the e f f e c t of c r o w d i n g o n h o m i n g inconclusive.  were  A n i n t e r e s t i n g r e s u l t i n c i d e n t a l to the e x p e r i m e n t s w a s  l i m p e t s r e s p o n d to a r t i f i c i a l d i s t u r b a n c e s , or being splashed,  such as r e m o v a l and  by moving away f r o m their home  s i m i l a r to that p r e d i c t e d f r o m the h y p o t h e s i s vironmental conditions  site.  This  that  replacement response  w h e n d e t e r i o r a t i o n of l o c a l  is en-  occurs.  Conclusion  A n hypothesis  d e s c r i b i n g l o c a l r e g u l a t i o n of d e n s i t y b y m e a n s of c h a n g e s  43  T A B L E  XV.  The number  of l i m p e t s t h a t r e m a i n e d i n e a c h of f o u r  g r o u p s a f t e r one d a y .  (See t e x t f o r e x p l a n a t i o n . )  treatment  The  number  r e m a i n i n g i n the h i g h d e n s i t y t r e a t m e n t has b e e n d i v i d e d by so that r e s u l t s  can be d i r e c t l y c o m p a r e d .  c a t e s w e r e o b t a i n e d f r o m J u l y 23 t o 2 4 ; J u l y 2 4 t o 2 5,  The f i r s t two  the second two  from  1970.  TreatmentReplicate  Control  Low  food  High  density  Shell  1  17  12  15.5  18  2  15  8  7.5  14  3  11  15  13.0  13  4  4  12  7.0  15  Mean  11.75  11.75  10.75  2,  repli-  15.0  44  in homing behaviour was as a response response  examined.  to f a v o u r a b l e l o c a l c o n d i t i o n s ,  to u n f a v o u r a b l e  states that h o m i n g  and that non-homing  occurs  occurs as  c o n d i t i o n s . ; It f u r t h e r s t a t e s t h a t t h r e s h o l d s  tolerance vary among limpets, are  The hypothesis  a  of  so at a g i v e n d e n s i t y b o t h t y p e s of b e h a v i o u r  found.  Homing  behaviour  was observed in individual A. digitalis.  w e r e o b s e r v e d to h o m e to b o t h h o m e to be two b e h a v i o u r a l t y p e s ,  sites and home  homing and non-homing,  clumps.  Limpets  There  appeared  as p r e d i c t e d f r o m the  hypothesis.  When density was  increased in an enclosed area,  h o m i n g l i m p e t s did not i n c r e a s e . dicted f r o m the hypothesis. artificial means, limpets were  however,  This  t h e p r o p o r t i o n of n o n -  r e s u l t c o n t r a d i c t e d the r e s p o n s e  When local conditions were made unfavourable apparent decreases  i n the p r o p o r t i o n of  homing  observed.  It i s n o t p o s s i b l e t o s t a t e c o n c l u s i v e l y w h e t h e r c h a n g e s i n behaviour  pre-  homing  o c c u r as a r e s u l t of c h a n g e s i n l o c a l c o n d i t i o n s i n the l i m p e t s '  environment.  Further work  s h o u l d b e c a r r i e d out to t e s t t h i s  hypothesis.  by  45  STUDIES OF  SEASONAL MIGRATION  BEHAVIOUR  Introduction  Frank and winter.  ( 1 9 6 5 ) r e p o r t e d t h a t A_. d i g i t a l i s t e n d t o m i g r a t e u p w a r d i n f a l l  H e o b s e r v e d t h a t m i g r a t i o n o c c u r r e d at a l l l e v e l s of the  w h e r e the l i m p e t s w e r e found, areas tion,  and that i n w i n t e r l i m p e t s could be found  w h e r e they w e r e not p r e s e n t d u r i n g the s u m m e r . w h i c h w a s not as g r e a t as the u p w a r d m i g r a t i o n ,  Frank  shore  A  downward  occurred in  s u g g e s t s t h a t t h e d i f f e r e n t i a l d i s t r i b u t i o n of s i z e c l a s s e s ,  l i m p e t s at the u p p e r e n d of the s h o r e a n d s m a l l e r  ones l o w e r ,  in  migraspring.  with larger  is caused  by  the u p w a r d m i g r a t i o n in f a l l and w i n t e r .  A  s i m i l a r p a t t e r n of d i s t r i b u t i o n w a s  ecologically similar limpet,  Patella vulgata.  m i g r a t i o n i n the c o l d e r m o n t h s Both authors  and a l e s s e r  Lewis  a r e a in June,  Lewis  (1954) i n the  also reported vertical  downward  n o t e d t h a t a c o m b i n a t i o n of h o t ,  c a l m seas produced long periods shore.  observed by L e w i s  migration in  dry weather,  spring.  neap tides  of d e s i c c a t i o n a t t h e u p p e r r e g i o n s  and  of the  f o u n d d e a d a n d d y i n g l i m p e t s at the u p p e r l e v e l s of h i s and F r a n k  study  observed a mouse removing and eating limpets  which  w e r e weakened by desiccation.  Although Frank terms  r e c o g n i z e d the i m p o r t a n c e  of t h e v e r t i c a l d i s t r i b u t i o n o f A . d i g i t a l i s ,  p o t e n t i a l i m p o r t a n c e i n r e g u l a t i o n of n u m b e r s . density dependent,  winter,  he d i d n o t i n v e s t i g a t e  densities a higher  is also density dependent.  proportion migrate,  is  mortality  during  After a large upward migration in fall  w h i c h m o r t a l i t y i s r e p o r t e d to be h i g h e s t i n s u m m e r  z o n e of t h e s h o r e ,  (Frank,  1965).  A  p r o p o r t i o n of l i m p e t s w o u l d b e e x p o s e d to s u c h m o r t a l i t y i f t h e u p w a r d  mortality,  its  If the u p w a r d m i g r a t i o n  m a n y l i m p e t s w o u l d be found i n the u p p e r m o s t  tion were small.  in  so t h a t a t l o w d e n s i t i e s o n l y a s m a l l p r o p o r t i o n of l i m p e t s  m i g r a t e and at higher summer  of the u p w a r d m i g r a t i o n  In t h i s w a y m i g r a t i o n ,  combined with subsequent  could have a r e g u l a t o r y effect on the n u m b e r s  in local  and in  smaller migra-  summer populations  46  of l i m p e t s .  This hypothesis  was tested in three stages at B o t a n i c a l B e a c h .  l i m p e t s w e r e o b s e r v e d during the fall,  w i n t e r a n d s p r i n g to d e t e r m i n e  v e r t i c a l migration o c c u r r e d during those periods.  Finally,  m o r t a l i t y was  s t u d i e d d u r i n g the s u m m e r  d e t e r m i n e w h e t h e r i t w a s h i g h e s t a t t h e u p p e r l e v e l s of t h e  O c c u r r e n c e of s e a s o n a l  s t u d y of h o m i n g  period September number  of l i m p e t s ,  behaviour,  1,  1969,  16,  1970.  Figures  on the r e f e r e n c e a r e a s  during this time.  and the  the  number  The decline in number  T h e u p w a r d t e n d e n c y of t h i s m i g r a t i o n i s s e e n i n  in w h i c h the n u m b e r  for  4 a n d 5 s h o w the  during late fall coincides with an i n c r e a s e in number  o u t s i d e the a r e a s .  to  shore.  which had been individually m a r k e d  to F e b r u a r y  s e e n o u t s i d e the r e f e r e n c e a r e a s  6 a n d 7,  of 1970  demonstrated an upward migration during  of m a r k e d l i m p e t s r e m a i n i n g  inside the a r e a s  examined  migration  Two large samples the  whether  The possible r e l a t i o n be-  t w e e n d e n s i t y a n d t h e p r o p o r t i o n of l i m p e t s t h a t m i g r a t e d u p w a r d w a s experimentally.  Marked  seen Figures  of a n i m a l s w h i c h m i g r a t e d i n e a c h d i r e c t i o n i s  plotted for two groups.  The spring migration involved fewer limpets. of the m i g r a t i o n f r o m F e b r u a r y  16 t o J u n e 5,  i n one g r o u p of m a r k e d l i m p e t s .  (Fewer  m i g r a t i o n b e c a u s e the subsequent from reference areas  observations  c a n be s e e n i n F i g u r e  of a n i m a l s t h a t h a d  c o u l d not be f o l l o w e d a c c u r a t e l y . )  of s e a s o n a l  tendency  data w e r e c o l l e c t e d d u r i n g the  movements  migrated f r o m its original position was These  1970,  The downward  8  spring  migrated  No l i m p e t that  o b s e r v e d to r e t u r n t o t h a t p o s i t i o n .  m i g r a t i o n s a r e c o n s i s t e n t w i t h t h o s e of  Frank  (1965).  L i m p e t s that i m m i g r a t e d i n w i n t e r to a r e a s i n w h i c h l i m p e t s h a d viously been m a r k e d was  a p p e a r e d to be s m a l l e r t h a n the m a r k e d l i m p e t s .  s u b s t a n t i a t e d i n one a r e a b y c o m p a r i n g the m e a n l e n g t h s  of t h e t w o  preThis groups  1  — i — ~ i J  F i g u r e 4.  J  The numbers f r o m July,  A  of l i m p e t s  1969,  1  S  1 O  1 N  D  1 J  1  1 F  M  1  r  A  M  1 J  1 — J.  seen i n s i d e and outside the a r e a w h e r e they w e r e  to J u l y ,  1970.  marked,  180  160  H  140  H  9  O  Inside marking  <  <  Outside  marking  area area  120  ioo  H  -t->  S  80  O  u 3  60  40  1  20 <5  <^ 1  AUG  Figure  5.  1  SEP  The number  OCT  T  1  NOV  1  DEC  1  JAN  1  FEB  MAR  of l i m p e t s s e e n i n s i d e a n d o u t s i d e the a r e a w h e r e t h e y w e r e  f r o m August,  1969,  to M a r c h ,  marked,  1970.  oo  Figure  6.  The number  of l i m p e t s w h i c h m i g r a t e d i n e a c h d i r e c t i o n d u r i n g  the p e r i o d f r o m September  1,  1969,  to F e b r u a r y  16,  1970.  D o t s i n the c e n t r e r e p r e s e n t l i m p e t s that d i d not m i g r a t e ; i n t h e u p p e r p a r t of t h e c i r c l e r e p r e s e n t l i m p e t s t h a t upward;* a n d so  on.  dots  migrated  50  Figure  7.  The number  of l i m p e t s w h i c h m i g r a t e d i n e a c h d i r e c t i o n d u r i n g  the p e r i o d S e p t e m b e r  l,*.'l-9"6.9,  to F e b r u a r y  16,  1970.  51  Figure  8.  The number  of l i m p e t s w h i c h m i g r a t e d i n e a c h d i r e c t i o n d u r i n g  the p e r i o d F e b r u a r y  16 t o J u n e 5,  1970.  52  (Table XVI).  T h e m e a n l e n g t h of the i m m i g r a n t l i m p e t s w a s  t h a n t h a t of l i m p e t s p r e v i o u s l y m a r k e d .  This  the f a c t s that l i m p e t s t e n d to be s m a l l e r as  significantly  i s what one w o u l d e x p e c t  one g o e s d o w n the s h o r e  less  from  (Figure  a n d t h a t the w i n t e r m i g r a t i o n t e n d s to be u p w a r d i n d i r e c t i o n ( F i g u r e s  3),  6 and  7).  R e l a t i o n of u p w a r d m i g r a t i o n w i t h d e n s i t y  The hypothesis  s t a t e s t h a t the p r o p o r t i o n of l i m p e t s t h a t m i g r a t e  ward increases  with density.  ment.  1970,  In J u n e ,  cretion, shelf,  which was  A n experiment was  d e s i g n e d to t e s t t h i s  up-  state-  160 l i m p e t s f o u n d o n a n d n e a r a l o w m e t a m o r p h i c  1.5m  across  and projected 0.5m  were marked individually.  Two wedge-shaped  f r o m a flat areas  con-  sandstone  containing most  the l i m p e t s w e r e t h e n p a r t i a l l y e n c l o s e d by m e a n s of p l a s t i c m e s h f e n c e s . fences,  f r o m the top of the c o n c r e t i o n 1.5m  limpets from migrating laterally. d r i l l e d to i n d i c a t e a s e r i e s u s e d to d e s c r i b e the  Within each area,  of h o r i z o n t a l s t r i p s ,  positions  was  prevented  10cm apart  which were numbered  The number  c r e a s e d g r a d u a l l y o v e r the s u m m e r on m i g r a t i o n c o u l d be o b s e r v e d .  Density  was  measured  1969.  9 s h o w s the n u m b e r  w h e n the n u m b e r  1969.  effect  of a l l l i m p e t s w e r e r e c o r d e d at  In a d d i t i o n , the m a r k e d l i m p e t s  22,  was  in-  to d e t e r m i n e w h e t h e r any i m m e d i a t e  The positions  on J u l y 5 and N o v e m b e r  area  of l i m p e t s i n the e x p e r i m e n t a l a r e a  i n t e r v a l s of a p p r o x i m a t e l y t w o w e e k s .  f r o m June through D e c e m b e r ,  and  of m a r k e d l i m p e t s i n t h e o t h e r  i n c r e a s e d b y the a d d i t i o n of l i m p e t s t a k e n f r o m e l s e w h e r e .  Figure  were  a r e a w e r e d e s i g n a t e d as the c o n t r o l  The approximately equal number  used as a test group.  holes  shelf,  The  of t h e l i m p e t s .  L i m p e t s i n one w e d g e - s h a p e d group.  onto the s a n d s t o n e  of  of m a r k e d a n i m a l s  were  remaining in each  T h e r e was little change  until  area  October,  i n the e x p e r i m e n t a l g r o u p b e g a n to d e c l i n e s h a r p l y .  The  number  i n the c o n t r o l g r o u p d e c l i n e d by a c o m p a r a t i v e l y s m a l l a m o u n t .  number  of l i m p e t s r e m a i n i n g w i t h i n e a c h s t r i p i n t h e c o n t r o l a r e a i s s h o w n i n  Figure  10,  and the number  The  i n e a c h s t r i p i n the e x p e r i m e n t a l a r e a i s s h o w n i n  53  T A B L E XVI.  A  c o m p a r i s o n b e t w e e n the m e a n l e n g t h of l i m p e t s t h a t  g r a t e d to a n a r e a d u r i n g f a l l a n d w i n t e r ,  and the m e a n  of l i m p e t s w h i c h r e m a i n e d on the s a m e a r e a f r o m the summer  (residents).  Lengths  were measured  immilength previous  on F e b r u a r y  1970,  Residents  Number  Immigrants  50  60  M e a n l e n g t h (cm)  1.799  1.660  Variance  0.10 3  0 . 0 58  F  of l e n g t h  r a t i o of v a r i a n c e  Student's  1.79  't'  ^ S i g n i f i c a n t a t a l p h a = .0 5;  n. s .  2.59*  d.f. =  108.  16,  Figure  9.  Numbers remaining for  of l i m p e t s i n the e x p e r i m e n t a l a n d c o n t r o l g r o u p s i n the p a r t i a l e n c l o s u r e s  explanation.)  over time.  (See  text  55  UPPER  THREE  STRIPS  20 10 0 JUN  1  JUL  "  A U G  SEP  1  •  OCT  NOV  1  MIDDLE 40  a,  k-A  A - ^  •  30  '  DEC  STRIP  A-  20  u 1  10 0  10  -  -L  JUN  10.  JTjL  '  A U G  1  SEP  1  OCT LOWER  • JUN  Figure  '  A-A-  The number  A U G  FIVE  STRIPS  SEP  OCT  A  NOV  A  DEC  of l i m p e t s r e m a i n i n g i n e a c h p a r t of the c o n t r o l  a r e a during the course explanation.)  NOV  A  *A—A •  J U L  '  of t h e e x p e r i m e n t .  (See t e x t f o r  56  F i g u r e 11.  T h e d e c l i n e w a s g r e a t e s t i n t h o s e s t r i p s w h i c h h a d the h i g h e s t  n u m b e r of l i m p e t s d u r i n g the s u m m e r . It w a s n o t p o s s i b l e t o d e t e r m i n e c o n c l u s i v e l y w h e t h e r t h e c h a n g e i n t h e e x p e r i m e n t a l a r e a w a s a p r o d u c t of e m i g r a t i o n , was made i n D e c e m b e r  m o r t a l i t y or both.  A  search  w i t h i n a r a d i u s of 5 m f r o m t h e c o n c r e t i o n , b u t o n l y  f i v e l i m p e t s f r o m t h e e x p e r i m e n t a l a r e a a n d one f r o m the c o n t r o l w e r e f o u n d . T h i s i n d i c a t e s t h a t t h e r e m a y h a v e b e e n a h i g h e r r a t e of e m i g r a t i o n i n the e x p e r i m e n t a l g r o u p t h a n i n the c o n t r o l .  T h e f i v e l i m p e t s f o u n d do n o t b y t h e m -  s e l v e s a c c o u n t f o r the c h a n g e i n t h e e x p e r i m e n t a l a r e a , b u t o b s e r v a t i o n s e l s e w h e r e s u g g e s t t h a t A . d i g i t a l i s a r e c a p a b l e of m o v i n g m u c h m o r e t h a n during s e v e r a l months.  It was concluded  5m  that e m i g r a t i o n was r e s p o n s i b l e f o r  the d e c l i n e of m a r k e d l i m p e t s i n t h e e x p e r i m e n t a l  group.  G r o w t h r a t e s of m a r k e d l i m p e t s i n the two a r e a s w e r e c o m p a r e d . (Table XVII).  L i m p e t s i n the c o n t r o l g r o u p showed s i g n i f i c a n t l y h i g h e r  g r o w t h d u r i n g the p e r i o d J u l y 5 to D e c e m b e r  10 t h a n t h o s e i n the e x p e r i m e n t a l  g r o u p . H i g h e r d e n s i t y i n the e x p e r i m e n t a l a r e a a p p e a r e d t o be r e s p o n s i b l e for  the d i f f e r e n c e i n g r o w t h r a t e s . R e s u l t s f r o m t h e p r e v i o u s e x p e r i m e n t i n d i c a t e d t h a t t h e p r o p o r t i o n of  l i m p e t s t h a t m i g r a t e d i n f a l l w a s g r e a t e r i n the a r e a w i t h h i g h e r d e n s i t y . F u r t h e r e x p e r i m e n t s w e r e p l a n n e d i n w h i c h l i m p e t s w e r e t o be m a r k e d i n s i m i l a r areas and densities subsequently of a n i m a l s .  manipulated  by a d d i t i o n or r e m o v a l  I t p r o v e d t o be i m p o s s i b l e t o m a r k l i m p e t s d u r i n g t h e f a l l a n d  w i n t e r (see E x p e r i m e n t a l  M e t h o d s ) , so t h e s e e x p e r i m e n t s c o u l d n o t be c a r r i e d  out. C i r c u m s t a n t i a l evidence was obtained,  h o w e v e r , f r o m s i x g r o u p s of  l i m p e t s t h a t h a d p r e v i o u s l y b e e n m a r k e d i n w i d e l y d i f f e r e n t l o c a t i o n s on the beach for different purposes.  D e n s i t i e s i n these a r e a s i n c r e a s e d n a t u r a l l y  d u r i n g the f a l l a n d w i n t e r a s a r e s u l t of i m m i g r a t i o n f r o m t h e a d j o i n i n g - r e g i o n s . The amount by w h i c h d e n s i t y i n c r e a s e d at e a c h l o c a t i o n was c a l c u l a t e d by  40 20 . 0 40 -  A  A  - - ^ A  A  JUN '  . - A — A '  JUL  I ATJG  '  SEP  —  *f  A  I OCT  ***** A  JUN  "TEC  -A-  A-  . -A  • A -  JUL  1  ' NOV  STRIP A B O V E MIDDLE  A  20 0  57  U P P E R SIX STRIPS  SEP  AUG  120  '  .A -A  A •  OCT  '  N  O  DEC  V  MIDDLE STRIP  A'  100  %  A  80 _ tn  60 -  ft  40 -  H W  •A*.  'A.  *-A  * —A. A  20 O  0  JUN '  »{ W PQ  JUL ' AUG  SEP  1  1  OCT  1  N O V TTEC  STRIP BELOW MIDDLE  60 40 -| 20  A  A  - '  A  A - *  0  A—  JUN  A - A 1  JUL '  AUG  A  JUN F i g u r e 11,  JUL  AUG  7JCT—"  SEP  1  NOV—T-TJEC  L O W E R TWO STRIPS  SEP  OCT  NOV  DEC  T h e n u m b e r of l i m p e t s r e m a i n i n g i n e a c h p a r t of t h e e x p e r i m e n t a l a r e a d u r i n g t h e c o u r s e of the e x p e r i m e n t . (See t e x t for explanation.)  58  T A B L E  XVII.  A  c o m p a r i s o n b e t w e e n the g r o w t h of l i m p e t s i n the  experimen-  t a l a n d c o n t r o l g r o u p s of a n e x p e r i m e n t d e s i g n e d to t e s t t h e m i g r a t i o n r e s p o n s e s of l i m p e t s to h i g h d e n s i t y . the e x p e r i m e n t a l g r o u p w a s  The density  made high by adding l i m p e t s .  this c o m p a r i s o n the r e g r e s s i o n s  of a t t a i n e d l e n g t h o n  length for each group are compared.  22,  In  initial  Initial length was  m i n e d on J u l y 5 and attained length on N o v e m b e r  deter-  1969.  Group  Sample  size  Regression F  from of  F  equation  Experimental  Control  31  42  Y=0.28 + 0.85X  comparison  slopes  from comparison of a d j a c e n t m e a n s (X =  1.74)  S i g n i f i c a n t at a l p h a = .01.  Y = 0.42  0.40  ^  n.s.  *  +  of  0.80X  59  d i v i d i n g the t o t a l n u m b e r of a n i m a l s i n A u g u s t , migrate  of a n i m a l s i n F e b r u a r y ,  1969.  grate was  number  (Table XVIII,  Figure  12).  c o m p a r e d w i t h the d e g r e e  T h e p e r c e n t a g e that d i d not  used r a t h e r than the percentage that did m i g r a t e because  w e r e m u c h l e s s e f f e c t i v e at f i n d i n g the l a t t e r . in m i g r a t o r y and n o n - m i g r a t o r y the  by the total  T h e p e r c e n t a g e of m a r k e d l i m p e t s t h a t d i d not  f r o m each location during winter was  density increase  1970,  of mi-  searches  It w a s a s s u m e d t h a t m o r t a l i t y  groups was equal,  so that e a c h g r o u p  was  c o m p l e m e n t of the o t h e r .  T h e c o r r e l a t i o n b e t w e e n d e g r e e of d e n s i t y i n c r e a s e a n d the of l i m p e t s t h a t d i d not m i g r a t e w a s  r = -.573.  c a t e s that a s m a l l p r o p o r t i o n of l i m p e t s i n a r e a s m i g r a t e d d u r i n g the f a l l .  The inverse correlation indiof l o w d e n s i t y  and that the s i x a r e a s  tidal height,  exposure,  It w a s dependent.  A  increase  The c o r r e l a t i o n c o e f f i c i e n t w a s not s i g n i f i c a n t ,  it s h o u l d be n o t e d t h a t a l i m i t e d r a n g e of d e n s i t y i n c r e a s e s w a s t h i s study,  percentage  observed  in  observed varied widely in such factors  as  original density,  and mean l i m p e t length.  concluded that u p w a r d m i g r a t i o n in fall and winter is s m a l l e r p r o p o r t i o n of l i m p e t s m i g r a t e d f r o m a r e a s  density than f r o m a r e a s  of h i g h  but  density  of l o w  density.  Mortality  A c c o r d i n g to the h y p o t h e s i s the  summer  which was being tested,  i s h i g h e s t a t t h e u p p e r l e v e l s of t h e s h o r e .  mortality during  T h e e f f e c t of  summer  m o r t a l i t y i s d e p e n d e n t upon the p r o p o r t i o n of l i m p e t s that m i g r a t e to the u p p e r p a r t s of the s h o r e d u r i n g the p r e c e d i n g f a l l a n d w i n t e r . p o r t i o n of l i m p e t s m i g r a t e , summer  c o m p o n e n t of the h y p o t h e s i s  migrate.  was tested by m e a s u r i n g  survival  l i m p e t s at d i f f e r e n t h e i g h t s o n one p a r t of t h e s h o r e d u r i n g t h e s u m m e r 1970.  pro-  t h e n t h e p r o p o r t i o n of l i m p e t s t h a t d i e d u r i n g t h e  i s h i g h e r t h a n i f o n l y a s m a l l p r o p o r t i o n of l i m p e t s  This  If a h i g h  T h r e e g r o u p s of l i m p e t s ,  of  of  e a c h c o n t a i n i n g 7 5 i n d i v i d u a l s of v a r y i n g  size,  60  T A B L E  XVIII.  Comparison  between density change  and the percentage  of  l i m p e t s t h a t d i d n o t m i g r a t e d u r i n g t h e p e r i o d A u g u s t 11, through February  16,  1970.  Change in density was  1969  calculated  b y d i v i d i n g the t o t a l n u m b e r  of l i m p e t s p r e s e n t i n F e b r u a r y  the total n u m b e r  S i x d i f f e r e n t l o c a t i o n s on the  in August.  beach are compared.  A  correlation coefficient between per  cent r e m a i n i n g and density, change  August,  1969  February,  1970  is calculated.  Density change  Percentage remaining  number  total  number  total  marked  number  marked  number  Area  limpets  limpets  limpets  limpets  1  90  90  52  152  1.69  5-7.78  2  199  199  100  218  1.10  50.25  3  44  56  20  420  7.50  45.45  4  42  82  26  105  1.28  61.91  5  24  26  20  37  1.42  83.33  6  34  41  26  38  0.93  76.49  F o r the c o r r e l a t i o n b e t w e e n d e n s i t y change r e m a i n i n g , the v a l u e of r = - . 5 7 3 .  and  percentage  by  Li  CuO  100  -  80  -  o fl  -a  • r-i T> -(->  OH  60  40  bo  20  -  o  ft — i  1.0  Figure  12.  1 2.0  1 3.0  1 -T— 4.0 4.0 Relative density  6.0  —\—  7.0  The relation between relative density a n d the p e r c e n t a g e of m a r k e d l i m p e t s t h a t d i d not m i g r a t e f r o m the a r e a d u r i n g f a l l a n d w i n t e r . (See t e x t f o r e x p l a n a t i o n . )  62  were marked  individually and m e a s u r e d  on  February  w e r e then r e l e a s e d at three different tidal heights. of e a c h  group  be d e t e r m i n e d .  a n d that the  d u r i n g the night  r e l e a s e v a r i e d f r o m 21 p e r c e n t t o 53 p e r c e n t . B y M a r c h c o u l d be found.  The  groups survival  relative  of i n d i v i d u a l s w i t h i n e a c h g r o u p  M o r t a l i t y in the three groups  o r i g i n a l 225 a n i m a l s  1970.  It w a s h o p e d t h a t  c o u l d be f o l l o w e d t h r o u g h the s u m m e r ,  m o r t a l i t y of the d i f f e r e n t s i z e c l a s s e s  1,  following  31, o n l y 36 of t h e  T h e s e w e r e so w i d e l y d i s p e r s e d  t h e p o i n t s of r e l e a s e t h a t t h e e x p e r i m e n t w a s a b a n d o n e d .  could  from  The high m o r t a l i t y  i m m e d i a t e l y after release was probably caused by l i m p e t s being washed by the h e a v y  A  surf present,  second  levels in May,  but the cause  of t h e l a t e r m o r t a l i t y i s n o t k n o w n .  a t t e m p t w a s m a d e to m e a s u r e  1970.  Six permanent squares  shore at different tidal heights.  largest, square  p a r t of the s h o r e ,  The number  a n d the s q u a r e s (square  6).  of l i m p e t s w i t h i n e a c h s q u a r e  summer  The m e a s u r e d  6 only,  uppermost  square  (square  1)  13 a n d 1 4 .  4 and 5 showed increases  of l i m p e t s  XIX.  square);  d u r i n g the s u m m e r .  l i m p e t s s m a l l e r than 0 . 5 c m w e r e found; 14.  during the  occurred in squares  6 (the l o w e r m o s t  in numbers  and are presented separately in Figure  made  in s i z e t o w a r d the l o w e s t  Declines  and square  was  higher  was  of l i m p e t s f o u n d i n e a c h of t h e m a r k e d s q u a r e s  are seen in F i g u r e s  lowermost  the d e n s i t y w a s  tidal height, o r i g i n a l n u m b e r  shown in Table  a n d 3 (the u p p e r t h r e e s q u a r e s ) , squares  Because  decreased progressively  a n d a r e a of e a c h s q u a r e a r e  The number  the  m o r t a l i t y at different tidal  w e r e m a r k e d o n one p a r t of t h e  r e c o r d e d s e v e r a l t i m e s d u r i n g the s u m m e r . at the l o w e r  away  In  these were counted  1, 2 while square  separately  The d e c l i n e that o c c u r r e d i n the  square a p p e a r e d c o n f i n e d to the s m a l l e r l i m p e t s .  To c h e c k whether m i g r a t i o n f r o m the s q u a r e s w e r e m a r k e d in situ on M a y  1. A l l t h e l i m p e t s i n s q u a r e  50 w e r e m a r k e d i n e a c h o f s q u a r e s c o u l d not be m a r k e d b e c a u s e  occurred,  2,  3, a n d 4,.  some  1 were marked,  Limpets in squares  their shells did not dry.  limpets  B y J u l y 26,  no  5 and  and 6  marked  63  T A B L E XIX.  T h e o r i g i n a l n u m b e r of l i m p e t s , t i d a l h e i g h t a n d a r e a of s i x m a r k e d s q u a r e s , i n w h i c h c e n s u s e s w e r e c a r r i e d out d u r i n g the s u m m e r of 1970 to d e t e r m i n e m o r t a l i t y r a t e s at d i f f e r e n t tidal heights.  Square number  T i d a l height (feet)  Area (m ) 2  O r i g i n a l n u m b e r of limpets  1  14.73  2.21  35  2  14.58  0.83  279  3  12.04  0.85  410  4  10.93  0.62  327  5  10.40  0.45  252  6  9.35  0.21  450  64  I  SQUARE •  MAY F i g u r e 13.  \  A  JUNE  "  1  "  (14.73 feet)  • —-A  JULY  T h e p e r c e n t a g e of l i m p e t s f o u n d i n the u p p e r t h r e e c e n s u s s q u a r e s o v e r the s u m m e r . T i d a l heights are given i n parentheses for each square.  65  100 75 50 25 -  SQUARE  MAY  bJO  JUNE  4  ( 1 0 . 9 3 feet)  i  1  I  JULY  100 75 -  O <D tuD  o  50 SQUARE  25 0  PH  MAY  5 (10.40 feet)  JUNE  JULY  large  100 -  limpets  75 small limpets 50 25 0  F i g u r e 14.  SQUARE MAY  JUNE  5  ( 9.3 5 feet) JULY  T h e p e r c e n t a g e of l i m p e t s r e m a i n i n g i n the t h r e e l o w e r c e n s u s s q u a r e s d u r i n g the s u m m e r . T i d a l h e i g h t s a r e g i v e n i n p a r e n theses for each square.  66  l i m p e t s h a d b e e n f o u n d o u t s i d e s q u a r e s 1 a n d 2; 3,  and  nine  were  found  outside  square  4.  two w e r e found outside  Thus,  declines in number  square i n the  u p p e r t h r e e s q u a r e s d i d n o t s e e m t o b e t h e r e s u l t of m i g r a t i o n f r o m t h e squares.  It w a s  concluded that m o r t a l i t y was  responsible.  C o n d i t i o n s o b s e r v e d d u r i n g the s u m m e r s of 1969 a n d 1970 t h a t d e s i c c a t i o n m a y b e a f a c t o r c o n t r i b u t i n g to s u m m e r 1969.  a f t e r a w e e k of hot,  regions  d r y weather and neap tides,  suggested  mortality.  In J u l y ,  l i m p e t s at the  upper  of t h e s h o r e w e r e e x a m i n e d t o d e t e r m i n e w h e t h e r a n y e f f e c t s o f t h i s  long d r y p e r i o d c o u l d be o b s e r v e d . t h e s i d e of the s h e l l ,  The limpets were given a sharp knock  w h i c h does not u s u a l l y dislodge l i m p e t s .  Those  limpets  which were dislodged were placed in cold sea water and observed after minutes.  15  O f 7 5 l i m p e t s t h u s t e s t e d 54 w e r e n o t d i s l o d g e d f r o m t h e r o c k ,  c a m e a w a y e a s i l y f r o m the r o c k but r e v i v e d i n sea w a t e r , three were dead. dry,  on  and the  18  remaining  A l l 21 l i m p e t s t h a t w e r e d i s l o d g e d a p p e a r e d s h r u n k e n  and  i n d i c a t i n g the e f f e c t of d e s i c c a t i o n .  In s u m m e r periods  of 1970,  the s a m e t e s t was  of h o t w e a t h e r o r n e a p t i d e s .  the r o c k .  c a r r i e d out s e v e r a l t i m e s  No l i m p e t s w e r e e a s i l y d i s l o d g e d  Some were r e m o v e d with a knife, and these appeared  moist.  W h e n the b e a c h was  under  o b s e r v a t i o n i n 1970,  periods  of d e s i c c a t i o n w e r e i n t e r r u p t e d b y s u r f o r r a i n f a l l .  during from  normally  potentially long It i s  possible  t h a t p e r i o d s of d e s i c c a t i o n o c c u r r e d b e t w e e n v i s i t s to the b e a c h .  Groups  of s h e l l s ,  c o n t a i n i n g b i t s of l i m p e t m e a t ,  b e l o w the high tide m a r k .  This  t e r r e s t r i a l or a e r i a l p r e d a t o r s possible predators, were  animals.  found  indicated that they could have been eaten by s i n c e the p r e v i o u s h i g h t i d e .  including mink,  seen on the i n t e r t i d a l  were sometimes  mice,  Although  oyster-catchers and other  several birds,  i t w a s n o t p o s s i b l e to o b s e r v e t h e p r e y of t h e s e  C r o w s a n d s u r f b i r d s w e r e w a t c h e d c a r e f u l l y ; these w e r e not  seen  taking limpets.  It w a s  concluded that m o r t a l i t y was  most  s e v e r e o n the u p p e r p a r t of  67  the s h o r e d u r i n g the s u m m e r  and decreased progressively  at l o w e r  regions,  e x c e p t t h a t l i m p e t s s m a l l e r t h a n 0 . 5 c m d i d not s u r v i v e a s w e l l at the l o w e r m o s t l e v e l as l a r g e r l i m p e t s . l e v e l s of t h e s h o r e a g r e e s (1954).  This  component  T h e o b s e r v a t i o n of h i g h m o r t a l i t y a t t h e  w i t h those r e p o r t e d by F r a n k of the h y p o t h e s i s  A l t h o u g h i t was not n e c e s s a r y the c a u s e of s u m m e r sible.  was  c o n c l u d e d to be  mortality, it appears  the m a j o r  c a u s e of s u m m e r  Lewis  valid.  i n t e s t i n g the h y p o t h e s i s to  demonstrate  that d e s i c c a t i o n is p a r t l y  P r e d a t i o n m a y also be p a r t l y r e s p o n s i b l e .  however,  (1965) a n d  mortality.  upper  respon-  It i s n o t p o s s i b l e to s t a t e , Experiments  in which  p e t s a r e p r o t e c t e d f r o m p r e d a t o r s by c a g e s c o u l d be p e r f o r m e d to  lim-  measure  predation.  Conclusion  A n hypothesis  t h a t d e s c r i b e s r e g u l a t i o n of l o c a l p o p u l a t i o n d e n s i t y  m e a n s of v e r t i c a l m i g r a t i o n a n d s u b s e q u e n t hypothesis  The  It f u r t h e r s t a t e s t h a t m o r t a l i t y d u r i n g  w h i c h i s h i g h e s t o n t h e u p p e r m o s t l e v e l s of t h e s h o r e a n d  s e v e r e at l o w e r l e v e l s ,  is thus density  A n upward m i g r a t i o n was  m i g r a t e d upward was higher was  in areas  and a  of h i g h e r d e n s i t y .  The major  lesser  T h e p r o p o r t i o n of l i m p e t s that Mortality  s h o w n to be h i g h e s t a t t h e u p p e r p a r t of the s h o r e ,  a t l o w e r l e v e l s of t h e s h o r e .  less  dependent.  observed in fall and winter,  downward migration occurred in spring.  summer  examined.  s t a t e s that the p r o p o r t i o n of l i m p e t s that m i g r a t e u p w a r d i n f a l l  and winter is dependent on density. summer,  m o r t a l i t y was  by  cause  of s u m m e r  during  decreasing  m o r t a l i t y was  not  demonstrated.  The hypothesis valid.  w h i c h w a s e x a m i n e d h a s t h u s n o t b e e n s h o w n to be  It a p p e a r s t h a t s e a s o n a l m i g r a t i o n i n t e r a c t s w i t h d e n s i t y a n d m o r t a l i t y  in such a way that l o c a l population density might be regulated.  68  RELATION O F UPWARD MIGRATION WITH  In t h e p r e v i o u s occurs  GROWTH  section it w a s d e m o n s t r a t e d that upward  i n fall and winter,  followed by a l e s s e r  T h e r e i s thus a n e t u p w a r d m o v e m e n t  downward migration in spring.  of l i m p e t s b e t w e e n , s u c c e s s i v e  M o r t a l i t y during the s u m m e r ,  however,  regions  severe at lower levels.  of t h e s h o r e ,  and less  migration  summers.  w a s shown to be highest on the higher It w a s s u g g e s t e d  u p w a r d m i g r a t i o n of l i m p e t s m a y tend to regulate the n u m b e r  that  of l i m p e t s i n a  local population.  B e c a u s e a l i m p e t that m i g r a t e s in s u m m e r  than one that does n o t m i g r a t e ,  be selected against, necessary  upward is exposed to greater  one might expect the behaviour to  a n d to d i s a p p e a r f r o m the s p e c i e s .  to explain why the behaviour is retained.  s u g g e s t s that s o m e t y p e s of b e h a v i o u r ,  mortality  It t h e r e f o r e  (1962)  Wynne-Edwards  which a r e disadvantageous  to the indivi-  dual but b e n e f i c i a l to the species, may-be. selected for; a n d he t e r m e d selection "group  selection".  It m i g h t b e a r g u e d that g r o u p  c a u s e d the r e t e n t i o n of u p w a r d m i g r a t i o n b e h a v i o u r . prevents resources  selection has  Because  such  Such a n argument,  however,  behaviour  would create a new problem;  that of e x p l a i n i n g the m e c h a n i s m b y w h i c h s u c h s e l e c t i o n m i g h t  A b e t t e r a p p r o a c h w a s t h o u g h t to be t h a t of e x p l a i n i n g in terms  such  of l i m p e t s f r o m b e i n g o v e r - e x p l o i t e d , i t i s t h e r e f o r e b e n e -  f i c i a l to the species.  behaviour  becomes  of i n d i v i d u a l s e l e c t i o n .  operate.  migration  F r i t c h m a n (1961) r e p o r t s  that  A . d i g i t a l i s b r e e d i n A p r i l a n d M a y , w h i c h i s b e t w e e n t h e t i m e s of u p w a r d migration and summer migrates  mortality.  It m i g h t b e p o s s i b l e f o r a l i m p e t t h a t  u p w a r d a n d l a t e r dies to l e a v e m o r e  does not m i g r a t e and does not die.  This  o f f s p r i n g than a l i m p e t that  could occur if upward  migration  allowed limpets access to better conditions for growth and fecundity.  The hypothesis l i m p e t s that m i g r a t e  that growth,  a n d hence fecundity,  i s higher i n those  w a s t e s t e d b y c o m p a r i n g g r o w t h r a t e s of m i g r a t o r y a n d  non-migratory limpets.  T w o g r o u p s w e r e m a r k e d a n d m e a s u r e d i n J u l y , 1969.  69  E a c h limpet was  measured  again in February,  to i t s o r i g i n a l p o s i t i o n w a s n o t e d . m i g r a t o r y and n o n - m i g r a t o r y  1970,  and position with  In e a c h g r o u p the g r o w t h r a t e  individuals was  respect  between  compared.  T h e r e w a s no s i g n i f i c a n t d i f f e r e n c e b e t w e e n the m e a n i n i t i a l l e n g t h s m i g r a t o r y and n o n - m i g r a t o r y the m i g r a t o r y migratory  limpets in each group (Table X X ) .  limpets showed a higher  ones (Table XXI).  In e a c h  T h u s the g r o w t h r a t e was  higheir in. l i m p e t s  T h i s f i n d i n g c o u l d be e x p l a i n e d i n one of t w o w a y s . p e r unit a r e a was not m e a s u r e d  shown that density d e c r e a s e s  First,  although the  at d i f f e r e n t t i d a l heights,  it has 2).  on higher  limpets.  (Figure  3).  Stimson  l i m p e t Lottia gigantea defends t e r r i t o r i e s against Acmaea,  density  it has a l s o been shown that l i m p e t s are p r o g r e s s i v e l y  l e v e l s of t h e s h o r e  W i t h i n L o t t i a t e r r i t o r i e s a m a t of a l g a e  larger  (1970) f o u n d t h a t t h e smaller c o u l d be  species,  large  such  Stimson  g e s t e d that L o t t i a r e q u i r e h i g h a l g a l c o n c e n t r a t i o n s i n o r d e r to feed,  sug-  and  m a i n t a i n t e r r i t o r i e s to p r e v e n t s m a l l e r l i m p e t s f r o m g r a z i n g the a l g a e I f s m a l l A_. d i g i t a l i s g r a z e v e r y c l o s e t o t h e r o c k ,  l a r g e r ones r e q u i r e a t h i c k e r a l g a l mat, l i m p e t s m i g h t be due to d e c r e a s e d shore levels.  then higher  (Table XVI)  i m m i g r a t e to a n a r e a d u r i n g f a l l a n d w i n t e r a r e present.  g r o w t h of  migratory  These  two explanations  of h i g h e r  higher  that those l i m p e t s that  s m a l l e r than limpets  U p w a r d m i g r a t i o n of the l a r g e r l i m p e t s f r o m t h a t a r e a m a y  t h e i r c o m p e t i t i o n w i t h the s m a l l e r l i m p e t s ,  close  while  competition with s m a l l e r l i m p e t s at  It h a s b e e n d e m o n s t r a t e d  as  distinguished,  w h i l e a l g a e on the r o c k o u t s i d e the t e r r i t o r i e s was v e r y t h i n .  d o w n to the r o c k .  been  Higher  g r o w t h r a t e s i n m i g r a t o r y l i m p e t s m i g h t s i m p l y r e s u l t f r o m the l o w e r  Second,  that  marked.  with increasing tidal height (Figure  e n c o u n t e r e d by the m i g r a t o r y  group  m e a n i n c r e m e n t of g r o w t h t h a n t h e n o n -  m i g r a t e d d u r i n g f a l l and w i n t e r f r o m the point w h e r e they w e r e  biomass  of  hence allowing better  already reduce  growth.  growth in migratory limpets  were  70  TABLE  XX.  A  c o m p a r i s o n b e t w e e n the m e a n i n i t i a l l e n g t h s of m i g r a t o r y  non-migratory limpets. 1969.  T h e c o m p a r i s o n i s m a d e i n t w o s e p a r a t e g r o u p s of  marked  limpets.  GROUP  1  Non-migratory  M e a n length  (cm)  Variance F  Migratory  42  Number  10  1.768  1.808  0.0460  0.0329  r a t i o of v a r i a n c e  V a l u e of s t u d e n t ' s  't'  GROUP  1.399  n.s.  0.51  n.s.  2  Non-migratory  Mean length  (cm)  Variance F  Migratory  17  Number  r a t i o of v a r i a n c e  V a l u e of s t u d e n t ' s  't'  and  L e n g t h s w e r e d e t e r m i n e d o n J u l y 1,  13  1.422  1.498  0.171  0.0308 1.80  n.s.  1.36  n.s.  71  T A B L E  XXI.  A  comparison  b e t w e e n the m e a n g r o w t h i n c r e m e n t of  and non-migratory m i n e d J u l y 1,  1969-1  The comparison l i m p e t s as  limpets.  The initial lengths were  subsequent  lengths  on F e b r u a r y  i s m a d e i n the s a m e two g r o u p s of  Table  migratory deter16,  1970.  marked  XX.  GROUP  1  Non-migratory  "  Migratory  M e a n g r o w t h i n c r e m e n t (cm)  0.0635  0.1185  V a r i a n c e of i n c r e m e n t  0.00365  -.00221  F  r a t i o of v a r i a n c e s  V a l u e of s t u d e n t ' s  1.65  't'  n.s.  2.7l"  S i g n i f i c a n t a t a l p h a = 0 . 0 5;  d.f. = 28.  GROUP  2  Non-migratory  Mean growth increment  (cm)  Variance F  0.0895  0.0250  0.00562  0.00465 1.65  r a t i o of v a r i a n c e  V a l u e of s t u d e n t ' s  Migratory  4.96"  't'  S i g n i f i c a n t at a l p h a = 0.05;  d.f. = 50.  n.s.  72  examined in an experiment in February, was A  measured  1970.  i n t h r e e g r o u p s of l i m p e t s c o n t a i n e d w i t h i n s i m i l a r  control group contained a given biomass  composed  of l i m p e t s .  A  enclosures.  second group  was  smaller limpets.  the  The t h i r d g r o u p c o n t a i n e d d o u b l e the c o n t r o l b i o m a s s ,  c o m p o s e d p a r t l y of l i m p e t s t h e s a m e s i z e a s t h e c o n t r o l , a n d  comparing  growth  of l i m p e t s s i m i l a r i n s i z e to t h e c o n t r o l , but c o n t a i n e d d o u b l e  control biomass. was  In t h i s e x p e r i m e n t ,  but  partly  T h e e f f e c t of d e n s i t y o n g r o w t h r a t e c o u l d b e e x a m i n e d  t h e g r o w t h r a t e s of t h e f i r s t a n d s e c o n d g r o u p s .  by  The effect on  g r o w t h r a t e of c o m p e t i t i o n w i t h s m a l l e r l i m p e t s c o u l d be e x a m i n e d by  com-  p a r i n g the s e c o n d a n d t h i r d g r o u p s .  In F e b r u a r y ,  1970,  three adjoining plots,  constructed with plastic m e s h fence. the plots was the p l o t s .  each 65cm square,  The natural biomass  d e t e r m i n e d by r e m o v i n g and m e a s u r i n g  T h e l e n g t h of e a c h l i m p e t w a s  m e a n s of a r e g r e s s i o n  were  of l i m p e t s  within  a l l the l i m p e t s found  c o n v e r t e d to b o d y d r y w e i g h t  d e v e l o p e d f o r the p u r p o s e  in  by  f r o m 107 i n d i v i d u a l s .  The  r e l a t i o n b e t w e e n l e n g t h a n d d r y b o d y w e i g h t w a s f o u n d to b e  in d r y w e i g h t (g) The natural biomass  was  with Vernier  i n the p l o t s o r i g i n a l l y ,  - 6 . 6 3 8 + 2.0 5 l e n g t h  f o u n d t o b e 6.87 g r a m s p e r  Limpets from elsewhere, measured  =  2.00  calipers.  plot.  to 2 . 5 0 c m i n l e n g t h ,  These  (cm).  were marked  w e r e l a r g e r than the l i m p e t s  and w e r e a d d e d to the p l o t s to f o r m t h r e e  and  found  treatments.  T h e c o n t r o l t r e a t m e n t c o n s i s t e d of e n o u g h m a r k e d l i m p e t s to e q u a l t h e n a t u r a l plot biomass.  The  second,  called a high density treatment,  c o n s i s t e d of ;  e n o u g h m a r k e d l i m p e t s to e q u a l t w i c e the n a t u r a l p l o t b i o m a s s . called a mixed high density treatment, e q u a l the n a t u r a l plot b i o m a s s limpets  The  third,  c o n s i s t e d of e n o u g h m a r k e d l i m p e t s to  plus an equal biomass  of u n m a r k e d  smaller  ( 1 . 2 5 t o 1.7 5 c m i n l e n g t h ) .  The original density was  maintained in each experiment for five  months,  73  from February animals.  19 t h r o u g h J u l y 2 4 ,  Limpets  1970,  d e s p i t e s o m e e m i g r a t i o n of  t h a t e m i g r a t e d w e r e r e p l a c e d b y l i m p e t s of s i m i l a r  T h e m a r k e d l i m p e t s w e r e r e m o v e d o n J u l y 24 a n d m e a s u r e d calipers. were  The mean growth increments  compared  marked  (Table XXII).  high density plot was  size.  with Vernier  of l i m p e t s i n t h e t h r e e  treatments  T h e m e a n g r o w t h i n c r e m e n t of l i m p e t s i n the  s i g n i f i c a n t l y l o w e r than the c o n t r o l , i n d i c a t i n g that h i g h  density resulted in significantly reduced growth.  The mean increments  of the  h i g h d e n s i t y plot and the m i x e d h i g h d e n s i t y plot w e r e not s i g n i f i c a n t l y d i f f e r e n t , i n d i c a t i n g that c o m p e t i t i o n f o r food w i t h s m a l l e r l i m p e t s had l i t t l e effect on the growth  rate.  It a p p e a r s  that better g r o w t h in m i g r a t o r y l i m p e t s r e s u l t s f r o m  densities encountered tition with smaller  on h i g h e r  l e v e l s of the s h o r e ,  not f r o m r e d u c e d  lower compe-  individuals.  U p w a r d m i g r a t i o n has b e e n s h o w n to be b e n e f i c i a l to i n d i v i d u a l l i m p e t s in terms  of growth,  and possibly  fecundity.  This may  that u p w a r d m i g r a t i o n r e s u l t s in i n c r e a s e d exposure summer; because breeding occurs  in spring,  o c c u r r e d and b e f o r e the s u m m e r m o r t a l i t y  offset the  disadvantage  to m o r t a l i t y d u r i n g  a f t e r the u p w a r d m i g r a t i o n begins.  the has  74  T A B L E XXII.  Comparisons  among  g r o u p s of l i m p e t s .  the  mean  growth  i n c r e m e n t of t h r e e  (See t e x t f o r e x p l a n a t i o n . )  group is c o m p a r e d with the high density group, d e n s i t y g r o u p w i t h the m i x e d h i g h d e n s i t y  The  control  a n d the  high  group.  Mixed  growth  increment Variance  (cm) of  increment  F  r a t i o of  Student's  't'  -.0677  .00208  .00443  3.062  .00259  n.s.  1.24  *  1.997  S i g n i f i c a n t a t a l p h a = .01  density  26  -.0417  .0048  2.13  variance s  high  density  30  25  Number Mean  High  Control  Treatment  (one-tailed);  n.s.  d.f. = 53.  75  DISCUSSION L o c a l p o p u l a t i o n s of A . d i g i t a l i s m a y be r e g u l a t e d b y a m e c h a n i s m i n volving seasonal migration behaviour.  B e c a u s e young l i m p e t s settle at the  l o w e r e n d of the v e r t i c a l d i s t r i b u t i o n of the s p e c i e s , i n c r e a s i n g tidal height, served.  density decreases with  a n d a d i f f e r e n t i a l d i s t r i b u t i o n of s i z e c l a s s e s i s ob-  U p w a r d m i g r a t i o n o c c u r s i n f a l l and winter, and this m i g r a t i o n  a p p e a r s to be d e n s i t y d e p e n d e n t . of the s h o r e ,  If d e n s i t y w e r e to i n c r e a s e a t t h e l o w e r p a r t  as it might f o l l o w i n g a heavy settlement  of y o u n g ,  migration  f r o m t h i s a r e a d u r i n g f a l l a n d w i n t e r w o u l d t e n d to d e c r e a s e d e n s i t y .  Density  o n t h e n e x t h i g h e s t l e v e l w o u l d i n c r e a s e a s a r e s u l t of the m i g r a t i o n f r o m b e l o w , l i m p e t s a l r e a d y p r e s e n t i n that a r e a w o u l d m i g r a t e ,  a n d so o n .  Density  a t a l l l e v e l s m i g h t b e a d j u s t e d to l e v e l s s u i t a b l e f o r t h e h a b i t a t b y m e a n s of density dependent m i g r a t i o n . L i m p e t s a t the u p p e r e n d of the s h o r e a r e e x p o s e d t o h i g h m o r t a l i t y i n summer.  T h e f a c t o r t h a t l i m i t s t h e u p p e r d i s t r i b u t i o n of t h i s s p e c i e s  to be d e s i c c a t i o n ( F r a n k ,  1965).  D a v i e s , (1969) h a s s u g g e s t e d t h a t  d e t e r m i n e s t h e u p p e r l i m i t of t h e s i m i l a r l i m p e t ,  Patella vulgata.  which produce desiccation are most severe during summer; l i m p e t s m i g r a t e to the u p p e r m o s t r e g i o n s , m o r t a l i t y d u r i n g the f o l l o w i n g s u m m e r . the h i g h e s t r e g i o n s of the s h o r e ,  appears  desiccation Conditions  i n winter  some  i n w h i c h t h e y a r e e x p o s e d to h i g h  M o r t a l i t y i s h i g h e s t i n s u m m e r on  and d e c r e a s e s with d e c r e a s i n g tidal height.  B e c a u s e the r a t e a t w h i c h l i m p e t s m i g r a t e to t h e u p p e r r e g i o n s i s d e n s i t y dependent,  m o r t a l i t y d u r i n g s u m m e r i s density dependent.  T h i s m a y h e l p to  regulate local populations. Several factors  m a y explain why upward m i g r a t i o n occurs.  f o u n d i n t h i s s t u d y that l i m p e t s s e t t l e b e l o w l o w e r h i g h w a t e r  (LHW).  A l t h o u g h i t w a s n o t s h o w n t h a t L H W i s a c r i t i c a l t i d e l e v e l (Doty, small limpets,  It w a s  1946) f o r  D a v i e s (1969) h a s s h o w n t h a t s m a l l e r P_. v u l g a t a a r e l e s s  c a p a b l e of w i t h s t a n d i n g d e s i c c a t i o n t h a n l a r g e r o n e s .  Upward migration may  76  allow limpets,  as they grow,  to e x p l o i t h a b i t a t s that w e r e p r e v i o u s l y  unsuit-  able.  Second,  it has been shown that m i g r a t o r y l i m p e t s g r o w at a faster  rate than n o n - m i g r a t o r y l i m p e t s .  It i s s u g g e s t e d t h a t t h i s r e s u l t s f r o m l o w e r  d e n s i t i e s e n c o u n t e r e d b y m i g r a t o r y l i m p e t s on h i g h e r l e v e l s of the Higher  fecundity may accompany  migration, is  higher g r o w t h rate, w h i c h would m e a n that  although it r e s u l t s i n h i g h e r m o r t a l i t y during the f o l l o w i n g  selectively advantageous  late spring, summer  shore.  rather than disadvantageous.  Breeding  occurs  mortality occurs.  m i g r a t i o n thus appears  to be a n a d a p t a t i o n w h i c h r e s u l t s  t h e e x p l o i t a t i o n of m o r e f a v o u r a b l e h a b i t a t . conditions are most favourable for growth, cation is present. behaviour  It o c c u r s d u r i n g a t i m e  A l t h o u g h it has been d e m o n s t r a t e d that seasonal  m a y b e c a p a b l e of r e g u l a t i n g l o c a l p o p u l a t i o n s ,  when  migration  further work  h o m i n g behaviour was also e x a m i n e d i n this study.  It w a s  Homing  and both homing and non-homing  migrate away f r o m home  reduced directly.  observed  limpets  i n c r e a s e d within an enclosed area,  L i m p e t s did, however,  availability was  was  when desiccation is a potentially dangerous factor.  are  demonstrated.  food  contradictory.  This  e x p l a i n w h y t h e p r o p o r t i o n of h o m i n g l i m p e t s d i d n o t d e c r e a s e . i s r e q u i r e d to t e s t t h i s h y p o t h e s i s  were  but this did not  clumps when  The evidence thus appears  in  would  Other evidence indicates that l i m p e t s a r e conservative i n m o v e m e n t summer,  in  behavioural types  p r e d i c t e d t h a t the p r o p o r t i o n of n o n - h o m i n g  increase when density was  must  occurs.  A n h y p o t h e s i s w h i c h d e s c r i b e d r e g u l a t i o n b y m e a n s of c h a n g e s  marked individuals,  in  and when, little danger f r o m d e s i c -  be c a r r i e d out to d e t e r m i n e w h e t h e r s u c h r e g u l a t i o n a c t u a l l y  occur.  in  b e t w e e n t h e t i m e of m i g r a t i o n a n d g r o w t h a n d the t i m e at w h i c h  Seasonal  found.  summer,  during may  Further  work  a n d to d e t e r m i n e w h y t w o t y p e s of b e h a v i o u r  77  LITERATURE  Brock,  CITED  V . E . and R . H . Riffenburgh. I960. Fish, schooling, a possible factor i n r e d u c i n g p r e d a t i o n . J . d u C o n s e i l I n t . E x p l . M e r . , 25: 3 0 7 - 3 1 7 .  C a s t e n h o l z , R i c h a r d W . 1961. T h e e f f e c t of g r a z i n g o n m a r i n e l i t t o r a l diatom populations. Ecology, 42:783-794. Connell, J . 1963. T e r r i t o r i a l behaviour and dispersion i n some invertebrates. R e s . P o p . 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T h e b e h a v i o u r of l i m p e t s w i t h p a r t i c u l a r r e f e r e n c e  to the h o m i n g i n s t i n c t . Wynne-Edwards, behaviour. 653pp.  V.C.  1962.  J . A n i m . Behav.,  7: 3 8 7 - 3 9 5 .  A n i m a l d i s p e r s i o n i n r e l a t i o n to s o c i a l  Oliver and Boyd,  Ltd.  Edinburgh and London,  xi  +  79 ACKNOWLEDGMENTS I w i s h to t h a n k m y s u p e r v i s o r , and c r i t i c i s m during this  D r . J. Robin Harger,  for his advice  study.  I n a d d i t i o n , I a m m o s t g r a t e f u l to D r . J o h n S t i m s o n ,  who p r o v i d e d  the i n i t i a l s t i m u l u s w h i c h r e s u l t e d i n t h i s p a r t i c u l a r s t u d y , a n d who r e m a i n e d a n i n v a l u a b l e s o u r c e of e n t h u s i a s m a n d a d v i c e d u r i n g the s t u d y . I a m grateful,  also,  to D r . N o r m a n J . W i l i m o v s k y , w h o s e  gracious  p e r m i s s i o n e n a b l e d m e to w o r k at the P o r t R e n f r e w B i o l o g i c a l S t a t i o n ; a n d to S t e p h e n R . H e i z e r , K h o o H o n g W o o , a n d J o h n H i m m e l m a n , w i t h w h o m I s h a r e d the B o t a n i c a l B e a c h  experience.  T h i s s t u d y w a s s u p p o r t e d b y a g r a n t f r o m the N a t i o n a l C o u n c i l of C a n a d a to D r . H a r g e r .  Research  

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