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Eye of the harbour seal, Phoca vitulina Jamieson, Glen Stewart 1970

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THE EYE OF THE HARBOUR SEAL, PHOCA V I T U L I N A  by  GLEN STEWART JAMIESON B.Sc. ( A g r . ) ,  M c G i l l U n i v e r s i t y , . 1967  A THESIS SUBMITTED I N P A R T I A L FULFILMENT  OF  THE REQUIREMENTS FOR THE DEGREE OF MASTER OF  SCIENCE  i n the Department of Zoology  We a c c e p t t h i s required  t h e s i s as conforming t o the  standard  THE UNIVERSITY OF B R I T I S H COLUMBIA October,  1970  In p r e s e n t i n g t h i s  thesis  an a d v a n c e d d e g r e e a t the L i b r a r y I further for  agree  the U n i v e r s i t y  make  it  freely  this  written  thesis for  It  ^-p-p/a-tyx/ ' / Columbia  the  requirements  B r i t i s h Columbia,  is understood  permission.  of  I agree  r e f e r e n c e and copying of  this  that  not  copying or  for  that  study. thesis  by t h e Head o f my D e p a r t m e n t  financial gain shall  The U n i v e r s i t y o f B r i t i s h V a n c o u v e r 8 , Canada  Date  of  that permission for extensive  representatives.  Department o f  fulfilment  available for  s c h o l a r l y p u r p o s e s may be g r a n t e d  by h i s of  shall  in p a r t i a l  or  publication  be a l l o w e d w i t h o u t  my  -  Abstract The  Eye  —  of the Harbour S e a l , Phoca  vitulina By  Glen  The eye,  s t r u c t u r a l and  as r e p r e s e n t e d by  Stewart  Jamieson  f u n c t i o n a l o r g a n i z a t i o n of the the harbour  seal,  more c o m p l e x t h a n p r e v i o u s l y r e a l i z e d .  Phoca v i t u l i n a , The  s e a l i s -similar to t h a t of the harp s e a l , zonation of the inner n u c l e a r cells, retinal  and  has  i s the t h i c k e s t of a l l the r e t i n a l c e l l s o f t y p e s B and 1:23  respectively.  T h i s r a t i o and and  light-sensitive retina.  w i t h a r a t i o of  photoreceptor  and  Refractive observations obtained  and h y p e r m e t r o p i c  w i t h no  The  density along  folding  through  is  in  r e t i n o s c o p y show astigmatic in a i r ,  discernable astigmatism  in  is horizontal;  water.  current  t h e o r i e s p r e d i c t t h a t the s t e n o p e i c p u p i l would thus i n a i r . The  roughly  discussed.  i s v e r y m y o p i c and  a x i s of l e a s t corneal curvature  astigmatism  layer  Photoreceptor  existence of r e t i n a l  m a r i n e mammals i s a l s o q u e s t i o n e d  s e a l eye  different  e x t e n s i v e tapetum, i n d i c a t e s a h i g h l y The  t h a t the harbour  little  large horizontal  outer nuclear  layers.  C were observed  with a well-developed  The  harbour  i n t h a t i t shows  layer, possesses  the r e t i n a .  i s much  r e t i n a of the  s i m i l a r n u c l e a r d e n s i t i e s w i t h i n the  l a y e r s throughout  pinniped  correct for  f u n c t i o n a l s i g n i f i c a n c e of the  i n t h e o r d e r o f 7 t o 13 d i o p t e r s , h a s n o t b e e n  astigmatism,whic  adequately  determined  t o d a t e , however.  These o b s e r v a t i o n s a r e i n agreement  w i t h those r e c e n t l y r e p o r t e d f o r the harp  seal.  B e h a v i o u r a l s t u d i e s have a l s o c o n f i r m e d the h i g h of the harbour  s e a l eye t o l i g h t ,  observations.  Two  a two-bar  as s u g g e s t e d by the  captive harbour  p a t t e r n over a one-bar  pattern  i n b o t h a i r and  f o r t h e p r e s e n c e o f a gap.  c a r r y out t h i s  t a s k w e r e m e a s u r e d a s t h e gap  gradually reduced  histological  s e a l s were t r a i n e d t o  discriminating  p a t t e r n was  sensitivity  in size.  select water,  Their a b i l i t i e s  The  i n the light  to  two-bar  intensity 2  o f t h e w h i t e p a r t s o f t h e s t i m u l u s c a r d s was Under t h e s e c o n d i t i o n s , s m a l l a s 1 mm  harbour  seal  t h e h a r b o u r . s e a l c a n d e t e c t a gap  a t a d i s t a n c e o f 1.7  These s t u d i e s f a i l  284-369 candela/m  m i n b o t h a i r and  as  water.  to p r o v i d e v a l u e s of the v i s u a l a c u i t y of  i n the s t r i c t  of the pattern u t i l i z e d .  sense, however, owing  light.  the  to the p h y s i c s  R a t h e r , t h e y i n d i c a t e a measure o f  a b s o l u t e s e n s i t i v i t y o f t h e eye t o  .  the  Acknowledgments:  Appreciation  i s e x p r e s s e d t o D r . A. Q. M c C o r m i c k , D e p t . o f  O p h t h a l m o l o g y , U . B „ C ; D r s . D. T e n n a n t a n d B. J e r v i s , optometrists;  a n d D r . G.G.E. S c u d d e r , D e p t . o f Z o o l o g y , U.B.C.,  f o r many h e l p f u l d i s c u s s i o n s . Laboratory  their  Allan Mitchell,  o f t h e D e p a r t m e n t o f Z o o l o g y , U.B.C., a r e t h a n k e d  technical assistance.  The s t a f f o f t h e V a n c o u v e r  Aquarium i s a l s o thanked f o r t h e i r and  Peter  o f the Pathology  o f t h e D e p a r t m e n t o f O p h t h a l m o l o g y , U.B.C., a n d M i s s  Joan Meredith, for  Vancouver  Chappell  Particular supervisor,  who h e l p e d  assistance,  as a r e Jane  Crocker  i n t e s t i n g the seals.  t h a n k s a r e e x p r e s s e d t o D r . H.' D. F i s h e r ,  a n d t o my w i f e ,  Public  Dorothy,  fortheir  my  unrelenting  support. This research  was s u p p o r t e d  A-2091.  i n p a r t b y NRC r e s e a r c h  grant '  i i  c  TABLE OF  L i s t of Figures List  CONTENTS  . . . . . . . . . .  o f Tables  iv . v i i  Introduction  1  M a t e r i a l s and Methods  6  M o r p h o l o g y and H i s t o l o g y Refraction  6 7  Visual Discriminations Results  11  . . .  19.  Morphology and H i s t o l o g y Refraction  19 29  Visual Discriminations  31  . . . . .  Discussion  .  38  Summary  .  63  G l o s s a r y o f terms .  65  L i s t of References  66  L i s t of Figures  Figure 1.  Apparatus used i n the underwater of t h e harbour s e a l eye.  refraction  2.  Measuring the r e f r a c t i o n of the harbour eye i n a i r .  3.  Photo o f t h e tanks a t t h e Vancouver P u b l i c Aquarium where t h e b e h a v i o u r a l a s p e c t s o f t h i s s t u d y were done.  13  4.  Apparatus used i n t e s t i n g the harbour s e a l ' s underwater d i s c r i m i n a t i o n o f one- and two-bar p a t t e r n s p r e s e n t e d s i m u l t a n e o u s l y . The r e l a t i o n s h i p s o f t h e t a n k s a r e shown i n b o t h d o r s a l (A) a n d l a t e r a l (B) v i e w s . F o r t e s t i n g i n a i r , t h e w a t e r was d r a i n e d f r o m t h e s t i m u l u s p r e s e n t a t i o n t a n k (P) a n d t h e s t i m u l u s c a r d s w e r e p r e s e n t e d a t t h e l e v e l o f t h e upper s e t o f windows ( W 2 ) ; t h e w a t e r l e v e l i n t h e t e s t i n g t a n k (T) r e m a i n e d t h e same b u t t h e s e a l s w e r e t r a i n e d t o swim a t t h e s u r f a c e . One s e a l a t a t i m e was l e t f r o m t h e h o l d i n g t a n k (H) i n t o t h e t e s t i n g t a n k b y means o f a s l i d i n g p a r t i t i o n .  14  seal  9 9  5. a - 5 . b - 5 . c . Photos showing d e t a i l s o f t h e b e h a v i o u r a l d i s c r i m i n a t i o n t a n k s and t h e p r e s e n t a t i o n o f a t y p i c a l problem. A. Photo o f t h e s t i m u l u s c a r d s , w i t h t h e p o s i t i v e card a t the reader's r i g h t . In a n o r m a l t r i a l , t h e c a r d s would n o t be h e l d a s h i g h , s o t h a t t h e s e a l s w o u l d n o t be a b l e to see the c a r d s . B. A c o r r e c t r e s p o n s e b y t h e s e a l , a s i t c h o s e t h e r i g h t - h a n d l e v e r . - The s e a l ' s a p p r o a c h was made u n d e r w a t e r , w i t h t h e c a r d s v i e w e d t h r o u g h t h e l o w e r window. C. Photo showing d e t a i l s o f t h e s t i m u l u s p r e s e n t a t i o n tank. The s t i m u l u s c a r d s w e r e presented j u s t to the fore o f the bucket, which contained the h e r r i n g rewards.  15  6. a - 6 . b - 6 . c - 6 . d . Photos o f h i s t o l o g i c a l s e c t i o n s of p i n n i p e d eyes. A. H a r b o u r s e a l . (Phoca v i t u l i n a ) B. N o r t h e r n f u r s e a l . ( C a l l o r h i n u s u r s i n u s )  20  iv  C. D. 7.  Harp s e a l . (Pagophilus groenlandicus) K i l l e r whale. (Grampus o r c a )  Close-up photo of the e x t e n s i v e f i b e r network c h a r a c t e r i s t i c about the p e r i p h e r y of the harbour seal i r i s .  21  8. a - 8 . b - 8 . c . P u p i l shapes under d i f f e r e n t l i g h t intensitites. A. V e r y h i g h l i g h t i n t e n s i t y . This i s as s m a l l as t h e p u p i l may be made. B. H i g h l i g h t i n t e n s i t y . The p u p i l i s more t e a r - s h a p e d t h a n i n A. C. E f f e c t o f a t r o p i n e . T h i s shows t h e maximum s i z e o b t a i n a b l e , and represents t h e p u p i l s i z e f o u n d u n d e r v e r y low l i g h t intensities.  22  9.  24  V e r t i c a l c r o s s s e c t i o n of the the r e t i n a . Toluidine blue.  c e n t r a l area  of  10.  V e r t i c a l s e c t i o n through the o p t i c nerve f i b e r and g a n g l i o n l a y e r s . L a r g e r a d i a l f i b e r s o f M u l l e r can be s e e n v e r t i c a l l y t r a n s v e r s i n g t h e s e layers. A r r o w s p o i n t t o g a n g l i o n c e l l s and a small blood v e s s e l . Toluidine blue.  24  11.  Vertical H & E.  24  12.  V e r t i c a l s e c t i o n through the i n n e r n u c l e a r l a y e r s h o w i n g l a r g e h o r i z o n t a l c e l l s (h) , a m a c r i n e c e l l s (a) , b i p o l a r c e l l s (b) and n e u r o g l i a l c e l l s (n). H o r i z o n t a l c e l l a x i s c y l i n d e r s a r e marked by a r r o w s . Toluidine blue.  24  13.  V e r t i c a l s e c t i o n through the inner n u c l e a r l a y e r s h o w i n g t h e s y n a p s e s b e t w e e n b i p o l a r c e l l s (b) and a p o l y s y n a p t i c p e d i c l e ( a r r o w ) . T o l u i d i n e blue.  24  14.  V e r t i c a l s e c t i o n t h r o u g h t h e o u t e r p l e x i f o r m and o u t e r n u c l e a r l a y e r s s h o w i n g o l i g o s y n a p t i c (o) and p o l y s y n a p t i c (p) t e r m i n a l p e d i c l e s . A r r o w p o i n t s to cone-type photoreceptor nucleus. Toluidine blue.  26  15.  V e r t i c a l s e c t i o n through the outer l i m i t i n g membrane and l a y e r o f r o d s and c o n e s , s h o w i n g photoreceptor i n n e r ( r i s ) and o u t e r ( r o s ) s e g m e n t s . Arros p o i n t s to cone-type photoreceptor nucleus. Toluidine blue.  26  s e c t i o n through a giant ganglion  v  cell.  16.  V e r t i c a l s e c t i o n through the pigment e p i t h e l i u m . Arrows'point t o c y t o p l a s m i c p r o c e s s e s e x t e n d i n g towards t h e p h o t o r e c e p t o r o u t e r segments. Toluidine blue.  26  17.  H o r i z o n t a l s e c t i o n ( s l i g h t l y oblique) through t h e r e g i o n o f t h e o u t e r l i m i t i n g membrane (points) . Arrows p o i n t t o photoreceptor n u c l e i . Azur I I .  26  18.  V e r t i c a l s e c t i o n t h r o u g h pigment e p i t h e l i u m and tapetum. Arrows p o i n t t oc h o r i o - c a p i l l a r i e s t r a n s v e r s i n g t h e tapetum. Toluidine blue.  26  19.  21.  a n d 20. V e r t i c a l s e c t i o n through the r e t i n a showing t h e s l i g h t form o f r e t i n a l f o l d i n g . Only t h e o u t e r n u c l e a r l a y e r and l a y e r o f r o d s . a n d c o n e s a r e f o l d e d . H & E. V e r t i c a l s e c t i o n through the r e t i n a showing t h e complex form o f r e t i n a l f o l d i n g . A l l t h e r e t i n a l l a y e r s a r e f o l d e d . H & E.  28  29  22. a-22.b. P e r f o r m a n c e o f t h e two h a r b o u r s e a l s during conditioning. Each day's t r i a l s were b r o k e n down i n t o b l o c k s o f t w e n t y t r i a l s e a c h to b e t t e r describe performance behaviour. A. The p e r f o r m a n c e o f t h e f o u r - y e a r o l d female, Blondie. B. The p e r f o r m a n c e o f t h e t w o - y e a r o l d male, Scar.  32  23.  A comparison o f the a b i l i t y o f the four-year o l d female harbour s e a l to d i s c r i m i n a t e small d i f f e r e n c e s i n t h e gap s i z e o f t h e t w o - b a r p a t t e r n i n b o t h a i r and water. A weighed l i n e a r r e g r e s s i o n was p e r f o r m e d t o d e t e r m i n e the d i f f e r e n c e t h r e s h o l d which i s d e f i n e d as t h e g a p s i z e c o r r e s p o n d i n g t o t h e 7 5 % (60 a r c s i n %) p e r f o r m a n c e l e v e l .  36  24.  A comparison o f t h e a b i l i t y o f the two-year o l d male harbour s e a l t o d i s c r i m i n a t e s m a l l d i f f e r e n c e s i n t h e gap s i z e o f t h e two-bar p a t t e r n i n b o t h a i r and w a t e r . A w e i g h t e d l i n e a r r e g r e s s i o n was performed t o determine the d i f f e r e n c e threshold t h i c h i s d e f i n e d a s t h e gap s i z e c o r r e s p o n d i n g t o t h e 7 5 % (60 a r c s i n %) p e r f o r m a n c e l e v e l .  37  vi  L i s t of  Tables  Table R e t r a c t i v e measurements o f the h a r b o u r s e a l as o b t a i n e d i n t h i s s t u d y and by J o h n s o n (1893) . H a r p s e a l m e a s u r e m e n t s w e r e t a k e n by P i g g i n s (1970). T h e s e v a l u e s w e r e o b t a i n e d i n a i r by means o f r e t i n o s - c o p y , w i t h t h o s e v a l u e s o b t a i n e d in water noted.  30  R e s p o n s e s o f b o t h s e a l s i n b o t h a i r and w a t e r for f i n a l testings. Values are given i n % c o r r e c t f o r e a c h gap s i z e .  35  Estimated r e s o l u t i o n thresholds f o r both seals i n a i r and w a t e r . Values i n parentheses r e f e r the 95% c o n f i d e n c e l i m i t s .  57 to  A c o m p a r i s o n o f v i s u a l a c u i t y t h r e s h o l d s (V.A.T.) and r e t i n a l s t r u c t u r e f o r a number o f a n i m a l s f r o m various sources.  v i i  60  1  Introduction  Present evolutionary evolved  i n w a t e r and  structure: The  theory  hence the  d i c t a t e s t h a t the eye  was  i t ' s basic plan being  initially  established  vertebrates an  i n the f i s h e s .  p h y s i c a l p r o p e r t i e s of water which i n f l u e n c e the  vision  - properties  e f f e c t s and  finally  s u c h as  friction  same p r o p e r t i e s  - are  the  of  light,  in a i r .  T h u s , when t h e  they were faced  A few  q u a l i t a t i v e changes were r e q u i r e d  index of  t h e medium.  required  t o p r o t e c t and  surface involved  i n the  eye,  New  features  cornea.  The  for  aerial  i n the  adnexa were  also  important r e f r a c t i v e  A l l the above  modifications (Wall,  \ secondary adaptation  i s not  adaptation  to water w i t h  s o w e l l d o c u m e n t e d and i s obviously  general  may  species  which are  be  completely  aerially-adapted This  secondary  cetaceans, which i n  aquatic.  a m p h i b i o u s , t h o u g h , t h a t an  I t i s i n those adaptation  somehow t h e y m u s t see w e l l i n b o t h a i r  Such forms a r e  of view since  an  understood.  p r e s e n t i n the  considered  might a r i s e since water.  influence  i n t h i s c h a n g e o f m e d i a a r e q u i t e w e l l known  1942) .  the  to the d i f f e r e n c e i n r e f r a c t i v e  m o i s t e n t h e now  the  scattering  primarily with a quantitative  vision.  however, m o s t l y r e l a t i n g  and  vertebrates  f o r most o f the p h y s i c a l p r o p e r t i e s w h i c h  vision,  eye  e s s e n t i a l l y exaggerations of  that occur  took to land,  absorption  disparity  eye  aquatic  of  s u c h a way  i n t e r e s t f r o m an of  l i f e may  evolutionary  represent  stages i n  problem and point the  2 I  !  land t o water There  transition.  is little  1  i n f o r m a t i o n on v i s i o n  i n a m p h i b i o u s mammals,  s u c h a s p i n n i p e d s , w h i c h b r e e d on l a n d b u t spend most o f t h e i r lives  i n water.  habitat,  B e i n g o n l y s e c o n d a r i l y a d a p t e d t o an a q u a t i c  one w o u l d assume t h a t t h e v i s u a l d i f f i c u l t i e s o f  a m p h i b i o u s mammals, i f a n y , l i e p r i m a r i l y u n d e r w a t e r . problem o f aquatic v i s i o n  The  f o r t h e t e r r e s t r i a l e y e i s one o f  d e v e l o p i n g enough accommodation t o n e u t r a l i z e t h e h y p e r m e t r o p i a w h i c h r e s u l t s when t h e c o r n e a i s i n c o n t a c t w i t h However, r a t h e r  than u s i n g  i n t r a - o c u l a r muscles  c u r v a t u r e o f t h e l e n s , as does t h e o t t e r  to increase the  (Walls,  have a c h i e v e d a moderate degree o f hypermetropia (Piggins, of /  1970) b y means o f a s p h e r i c a l  lens.  water.  1942), p i n n i p e d s underwater  The c o m b i n a t i o n  a stenopeic p u p i l arranged along the axis of greatest  curvature  and a s e n s i t i v e r e t i n a  accommodation i n a i r unnecessary air,  t h e eyes a r e v e r y myopic  1893;  Piggins,  1970).  corneal  i s t h o u g h t t o make e x t e n s i v e  (Walls,  1942), a l t h o u g h i n  and q u i t e a s t i g m a t i c  In w a t e r , t h i s myopia  (Johnson,  and a s t i g m a t i s m  d i s a p p e a r owing t o t h e s i m i l a r i t y o f t h e r e f r a c t i v e  indices  of t h e c o r n e a and w a t e r . To d a t e , m o s t o f t h e e m p h a s i s of p i n n i p e d s h a s been d i r e c t e d p i n n i p e d s employ a c t i v e sonar et  al.,  on t h e s e n s o r y  capabilities  towards a c o u s t i c s and whether (eg. P o u l t e r ,  1966; Schusterman  1967; S h a v e r a n d P o u l t e r > 1 9 6 7 ) , w i t h r e l a t i v e l y  emphasis  being devoted t o the study o f the other senses,  vision.  P r e v i o u s r e s e a r c h on t h e h a r b o u r s e a l ,  little  particularly  P_. v i t u l i n a  3  ( F e i n s t e i n and R i c e , Zalophus  1966), and t h e C a l i f o r n i a  californianus  (Schusterman  et al.,  sea l i o n ,  1965), has been  concerned w i t h the a b i l i t i e s o f these animals t o d i s c r i m i n a t e between t a r g e t s o f v a r i o u s s i z e s  i n water.  Their  size-  d i s c r i m i n a t i o n t e s t s h a v e shown t h a t t h e s e s p e c i e s c a n make f i n e d i s c r i m i n a t i o n s and t h a t b o t h s p e c i e s p e r f o r m equally well.  almost  No t e s t s h a v e b e e n made t o d a t e , h o w e v e r , on  the r e l a t i v e a b i l i t y  o f p h o c i d s t o see above and below  a l t h o u g h Schusterman  and B a l l i e t have r e p o r t e d t h e v i s u a l  a c u i t y o f the harbour (1970a) air  s e a l and  Steller  sea l i o n  and t h e v i s u a l a c u i t y o f t h e C a l i f o r n i a  water,  underwater sea l i o n i n  (1970b) . A c c o u n t s on t h e s t r u c t u r a l o r g a n i z a t i o n o f t h e p i n n i p e d  eye a r e a l s o few and f r a g m e n t a r y . (1865)  and Johnson  devoted  on t h e h a r b o u r  s t u d i e s by W i l s o n  s e a l were  primarily  t o t h e d e s c r i p t i o n o f t h e d i o p t r i c mechanism and  contributed remained  (1893)  Initial  little  f o rPutter  to the understanding of the r e t i n a . (1903)  It  t o d e s c r i b e t h e g r o s s morphology  t h e r e t i n a , w h i c h he d i d f o r s e v e r a l p i n n i p e d s p e c i e s . owing  to the h i s t o l o g i c a l  level.  He d i d d e s c r i b e t h e t a p e t u m  gross c h a r a c t e r i s t i c s of the d i f f e r e n t r e t i n a l a l s o confirmed Johnson's absence  However,  techniques a v a i l a b l e to him a t that  t i m e , he was u n a b l e t o d e s c r i b e i n d e t a i l t h e r e t i n a l at the c e l l u l a r  of  (1901)  earlier  organization  and t h e  layers.  He  s t a t e m e n t on t h e  o f an a r e a c e n t r a l i s , w h i c h d i s c l a i m e d C h i e v i t z ' s  r e p o r t o f the e x i s t e n c e o f such a f e a t u r e .  (1889)  4  The  most c o m p l e t e a c c o u n t  pinniped retina Pagophilus  i s b y Nagy and  groenlandicus.  t o d a t e on Ronald  the s t r u c t u r e of  (1970) on  These a u t h o r s  the harp  also reported  a b s e n c e o f an a r e a c e n t r a l i s and w e r e a b l e t o o b s e r v e photoreceptors,  although  cones were noted.  However, as t h e y were u n a b l e  apparent  in this  species.  l a c k of zonation i n the  particularly To d a t e ,  emphasized. t h e r e has  l o c a l pinnipeds,  i s hardy.  The  only  rod of  to d i s c e r n  any  t h a t cones  huge h o r i z o n t a l c e l l s  inner nuclear  concerted  i n both  and  l a y e r were a l s o  Thus, t h i s  attempt  i n pinnipeds  a i r and w a t e r w i t h e y e  the harbour  i s p e r h a p s t h e most c o n v e n i e n t and  the  '• • -  b e e n no  to r e l a t e v i s u a l a b i l i t y Among t h e  seal,  terminal pedicles characteristic  f u r t h e r morphological d i f f e r e n c e s , they concluded are not present  the  seal  f o r study.  s p e c i e s was  (Phoca I t has  structure.  vitulina) l a r g e eyes  chosen f o r a study  of  t h e v i s u a l a d a p t a t i o n s r e q u i r e d f o r an a m p h i b i o u s h a b i t .  The  o b j e c t i v e s were: 1. a h i s t o l o g i c a l and m o r p h o l o g i c a l s e a l eye w i t h p a r t i c u l a r of the 2.  harbour  the s t r u c t u r a l o r g a n i z a t i o n  retina. a r e f r a c t i v e study of the harbour  and w a t e r t o s e e  s e a l eye  i n both a i r  i f t h e e y e s a r e a s a s t i g m a t i c and m y o p i c  c l a i m e d by J o h n s o n 3-  e m p h a s i s on  study of the  (1893).  a b e h a v i o u r a l i n v e s t i g a t i o n of the harbour  discriminatory abilities of measuring v i s u a l  as  i n both  acuity.  seal's  a i r and w a t e r w i t h t h e  intent  5  With  this  i n f o r m a t i o n , t h e d e g r e e and  seal's visual and  discussed.  nature of the  harbour  a d a p t a t i o n s t o an a m p h i b i o u s h a b i t w i l l be  evaluated  6  M a t e r i a l s and  1.  Methods  M o r p h o l o g y and H i s t o l o g y : The  s t r u c t u r a l aspects of the harbour  a t the l e v e l o f the l i g h t microscope. s e a l s caught formalin. death,  Eyes o b t a i n e d  embedded w h o l e i n B i o l o i d , A r b o r , M i c h . ) , and  v e r t i c a l plane, i e . p a r a l l e l photoreceptors.  S t a i n i n g was  eosin, periodic acid-Schiff In a d d i t i o n  following  (Haver-Lockhart  of the  56-58° C. mp  (Will  s e c t i o n e d a t 5-8  the  after  a l l s i g n s o f b o d y movement  Kansas C i t y )  centimeter.  Barb-Euthol  i n t o the e x t r a d u r a l  Large  the  cut into  squares  s e c t i o n s were  n e c e s s a r y as w i t h s m a l l e r s e c t i o n s t h e r e t i n a d e t a c h e d the c h o r o i d i n  p o s t f i x a t i o n and became l o s t  Owing t o t h e t h i n n e s s o f t h e r e t i n a l exposed  of the f i x a t i v e  to the f i x a t i o n , arose.  ( S a b a t i n i et. a_l. , 1963) buffer  (Culling,  1962),  no  F i x a t i o n was o f ph = 7.4  vein.  open around  t h e p o s t e r i o r h e m i s p h e r e was  o f a p p r o x i m a t e l y one  directly  and  thionin.  i n j e c t i o n o f a l e t h a l dose o f  Lab.,  o r a t e r m i n a l i s and  solution.  Scientific  with H a r r i s haemotoxylin  These eyes i m m e d i a t e l y a f t e r removal were s l i t  from  animal's  u i n the  to the o r i e n t a t i o n of  (PAS) , o r  10%  t o t h e a b o v e m e t h o d , some e y e s w e r e r e m o v e d  f r o m a n i m a l s a few m i n u t e s ceased  from  i n the S t r a i t of G e o r g i a were f i x e d whole i n  E y e s w e r e f i x e d w i t h i n a few m i n u t e s  I n c o r p . , Ann  had  s e a l eye w e r e s t u d i e d  found itself  i n the  layer, which  was  cases o f poor p e n e t r a t i o n with glutaraldehyde i n Sorensen's  f o l l o w e d by p o s t f i x a t i o n  phosphate i n osmium  7  tetraoxide,  dehydration  (Luft,  1961).  S e c t i o n i n g was done w i t h g l a s s k n i v e s on t h e S o r v a l l  ultra-  m i c r o t o m e , MT-1, planes.  a n d e m b e d d i n g i n Epon  a t 0.5-1 u i n b o t h t h e v e r t i c a l a n d h o r i z o n t a l  S t a i n i n g was w i t h a l k a l i n e  t o l u i d i n e blue or a l k a l i n e  Azur I I . Cell  types  w i t h Polyak  i n t h e r e t i n a were i d e n t i f i e d  i n accordance  (1941) a n d c e l l n u m b e r s w e r e d e t e r m i n e d w i t h t h e M  equation  P = A  a s d e s c r i b e d by A b e r c r o m b i e  (1946) ,  L + M w h e r e P = t h e a v e r a g e number o f n u c l e a r  points per s e c t i o n ,  A = t h e c r u d e c o u n t o f n u c l e a r numbers s e e n i n s e c t i o n , M = the  t h i c k n e s s o f t h e s e c t i o n i n u, and L = t h e a v e r a g e  o f t h e n u c l e i i n u.  Five c e l l  counts  length  (A) f o r e a c h c e l l  type  w e r e made f r o m p h o t o g r a p h s o f t h e c e n t r a l r e g i o n o f t h e r e t i n a . Eyes were a l s o observed the  study  in living  i n d i v i d u a l s throughout  and p u p i l shapes w i t h d i f f e r e n t  light  intensities  were noted. 2.  Refraction: The f o u r s e a l s c o n s i d e r e d  f r o m one t o f o u r y e a r s  in this  study  ranged  and w e r e a l l c a p t i v e a n i m a l s  i n age t h a t had  b e e n h e l d a t e i t h e r t h e U n i v e r s i t y o f B.C. o r t h e V a n c o u v e r P u b l i c A q u a r i u m f o r a t l e a s t one y e a r . were taken and  b y two o p t o m e t r i s t s  an o p h t h a l m o l o g i s t  the experience myself.  Retinoscopy  measurements  ( D r s . D. Tenn'ant and B. J e r v i s )  (Dr. A. M c C o r m i c k ) , a s I h a d n e i t h e r  nor the equipment t o undertake these  studies  The e y e s w e r e r e f r a c t e d u n d e r c y c l o p l e g i a , w i t h  3 d r o p s o f 1% a t r o p i n e s u l p h a t e  s o l u t i o n ( A l c o n o f Canada  8  Ltd.,  Toronto)  Instilled  i n each eye.  Cycloplegia  typically  t o o k a t l e a s t f o r t y m i n u t e s t o c o m p l e t e a n d was j u d g e d b y t h e absence o f a d i r e c t p u p i l r e f l e x t o l i g h t . attempt a t r e f r a c t i o n , be  obtained  I felt  accomplished with Lab.,  t h a t more a c c u r a t e  i f t h e s e a l was f i r s t  were q u i t e a c t i v e and d i f f i c u l t  A f t e r our f i r s t  anaesthetized,  to handle.  results  could  as a few  Anaesthesia  2.5% sodium t h i o p e n t a l s o l u t i o n  was  (Abbott  M o n t r e a l ) w h i c h was i n j e c t e d i n t r a v e n o u s l y i n t o t h e  extradural vein.  The s e a l was m a i n t a i n e d  of complete anaesthesia action  j u s t on t h e v e r g e  so t h a t b r e a t h i n g , w h i c h i s a v o l u n t a r y  i n s e a l s , c o u l d be maintained.  runs the r i s k o f k i l l i n g  the animal  Complete i fmaintained  anaesthesia f o r more  t h a n 5-10 m i n u t e s . R e f r a c t i o n was c a r r i e d o u t w i t h b o t h a s l i t and  a round, or p o i n t , r e t i n o s c o p e ,  t h e two i n s t r u m e n t s An with  being  retinoscope  t h e o n l y d i f f e r e n c e between  t h e s h a p e o f t h e l i g h t beam p r o d u c e d .  a t t e m p t was a l s o made t o r e f r a c t t h e e y e s u n d e r w a t e r ,  the animal's  methacrylate)  head immersed i n a p l e x i g l a s s  container  f i l l e d w i t h water  (methyl  ( F i g . 1) .  This  apparatus,  made o f 0.6 cm p l e x i g l a s s ,  container,  15 cm b y 18.5 cm b y 60 cm, m o u n t e d a t an a n g l e o f  40°.,  The s e a l ' s h e a d c o u l d b e p l a c e d  consisted of a rectangular  i n t h i s c o n t a i n e r and  p o s i t i o n e d s o t h a t i t was p o s s i b l e t o o b s e r v e t h e e y e t h r o u g h the p l e x i g l a s s w a l l . In c o n j u n c t i o n w i t h  these  r e t i n o s c o p y measurements, a  k e r a t o m e t e r was u s e d t o m e a s u r e t h e c u r v a t u r e o f t h e a n t e r i o r  Figure  1.  the harbour  Figure in a i r .  2.  Apparatus used  i n the underwater  refraction  seal.  Measuring the r e f r a c t i o n  of the harbour  seal  of  10  s u r f a c e o f the cornea. an  Its principle  i s the r e f l e c t i o n of  image o f f t h e a n t e r i o r c o r n e a l s u r f a c e , w h i c h i s t h e n  against a standard found this  measurement b u i l t  viewed  into the instrument.  I  I was u n a b l e t o o b t a i n e v e n a n a p p r o x i m a t e m e a s u r e w i t h technique,  however, as due t o t h e extreme f l a t n e s s o f  the cornea,  i t s m e a s u r e o f c u r v a t u r e was w e l l o u t s i d e t h e  calibration  o f t h e k e r a t o m e t e r w h i c h was c a l i b r a t e d f o r human  use. was  By u s e o f a P l a c i d o ' s d i s c , observed.  white  though, c o r n e a l  T h i s d i s c c o n s i s t s o f a l t e r n a t i n g b l a c k and  c o n c e n t r i c r i n g s a n d when h e l d  viewed through a hole  i n i t s center,  surface o f the cornea r e s u l t Corneal  i n f r o n t o f t h e eye and elliptical  i fcorneal  irregularity  a plastic  contact  p l a s t e r m o u l d on a l i v e a n i m a l  failed,  were f o r c e d a p a r t  this  l e n s , w h i c h c o u l d be measured An i n i t i a l  t o the extent  complete c o r n e a l surface,  animals  exists.  The p l a n was t o make a p l a s t e r m o u l d a n d  through u l t r a s o n i c techniques.  distorted  i m a g e s on t h e  m o u l d s o f t h e e y e w e r e t h e n made t o m e a s u r e  refractive error. from t h i s ,  astigmatism  f o r when t h e e y e l i d s  r e q u i r e d f o r a mould  the pressure  t h e c o r n e a l shape.  a t t e m p t t o make a  exerted  of^the  on t h e g l o b e  Two a t t e m p t s w e r e t h e n made o n  w h i c h had j u s t been k i l l e d w i t h B a r b - E u t h o l .  After  r e m o v a l o f t h e e y e l i d s , a p r e l i m i n a r y m o u l d o f t h e e y e was made w i t h  "Ophthalmic M o l d i t e "  fast-hardening mixture hard-boiled stone,  egg w h i t e .  a very  (Obrig Lab., I n c . ) , a s o f t ,  which hardens t o the consistency A p o s i t i v e mould o f c o e c a l  of a  dental  f i n e p l a s t e r , was t h e n made f r o m t h e " M o l d i t e "  11  m o u l d a s s o o n a s i t was r e m o v e d f r o m t h e c o r n e a . n e c e s s a r y a s t h e " M o l d i t e " m o u l d soon u n d e r g o e s d i s t o r t i o n as t h e water  i n i t evaporates.  T h i s was  '  s h r i n k a g e and  From t h e p l a s t e r  m o u l d , a c r u d e c o n t a c t l e n s was l a t e r made a n d m e a s u r e d b y the  Plastic  C o n t a c t Lens  Co. (Canada) L t d .  F u n d u s c o p i c e x a m i n a t i o n s w e r e a l s o made on two o f t h e s e a l s e x a m i n e d a s shown i n F i g . 2 t o d e t e r m i n e f o l d s were v i s i b l e 3.  Visual  i f retinal  i n the l i v i n g eye.  Discriminations:  B e h a v i o u r a l experiments were c a r r i e d o u t i n t h e r e s e a r c h compound o f t h e V a n c o u v e r  P u b l i c Aquarium  S a l t w a t e r was f o u n d e s s e n t i a l  i n S t a n l e y Park.  t o m a i n t a i n t h e eyes  i n perfect  c o n d i t i o n a s many o f t h e a n i m a l s m a i n t a i n e d i n f r e s h w a t e r sooner o r l a t e r  showed a w h i t e n i n g o f t h e c o r n e a .  This  condition  was  f o u n d t o b e u s u a l l y d u e t o an i n v a s i o n o f l e u c o c y t e s i n t o  the  cornea as a r e s u l t o f c o r n e a l u l c e r s .  in  f r e s h w a t e r seem t o b e more s u s c e p t a b l e t o t h e s e u l c e r s i s  not presently  clear.  A t o t a l o f seven a two-year being to  Why s e a l s m a i n t a i n e d  o l d male,  chosen  s e a l s were i n i t i a l l y and B l o n d i e ,  as t h e g a p . i n t h e two-bar with  2.44  Each  s e a l was r e q u i r e d  p a t t e r n and a one-bar  p a t t e r n was g r a d u a l l y r e d u c e d  t h e p r e s e n c e o f a gap b e i n g t h e c r i t i c a l All  t e s t s were u n d e r t a k e n  m filled  w i t h seawater  Scar,  a four-year o l d female,  a s t h e most m a n a g e a b l e .  d i s c r i m i n a t e between a two-bar  selected, with  i n a plywood  pattern i n size,  parameter.  t a n k 1.22 m b y  t o a d e p t h o f 51 cm.  The s t i m u l u s  12  c a r d s were p r e s e n t e d  i n a s m a l l e r t a n k 0.86 m b y 0.90  adjacent  to the t e s t i n g  windows,  1.27 cm b y 25.4 cm b y 55.9 cm, w e r e b u i l t  tanks  tank  ( F i g . 3 and 4 ) .  m  Plexiglass into  both  a t t h e same l e v e l p r o v i d i n g a c l e a r v i e w b e t w e e n t h e  two t a n k s .  Two  tanks were used, a l o n g w i t h  of the seawater,  to ensure t h a t suspended d e b r i s  tank would not i n t e r f e r e w i t h primarily of f i s h were e n c l o s e d  flushing  i n the larger  the view; t h i s d e b r i s  s c a l e s shed from t h e r e w a r d s .  consisted  The  tanks  i n a h u t t o remove a n y e x t e r n a l s t i m u l i and t o  allow c o n t r o l of l i g h t i n g of the stimulus The w a l l s o f t h e t e s t i n g The l i g h t  continuous  cards.  tank were a l i g h t  i n t e n s i t y of the white  gloss  blue.  parts of the stimulus  cards  2 was 284-369 c a n d e l a / m . a f l a t black  The w a l l s o f t h e s m a l l e r  t o remove t h e p o s s i b i l i t y o f g l a r e .  were i l l u m i n a t e d i n d e p e n d e n t l y interior 40 w,  o f t h e h u t was  cool white  with a "Photovolt  with  illuminated with  f l o u r e s c e n t tubes.  three,  not  considered  animals before  to affect  the r e s u l t s ,  The e x p e r i m e n t e r s t o o d b e h i n d  stimulus  cards  c a r d s were w h i t e  four-foot,  cards  was c o n t r o l l e d  This range o f i l l u m i n a t i o n  commencement o f t h e d a y ' s  both stimulus  and t h e  200", and t h e  was  s i n c e i n a l l cases,  w e r e a d a p t e d t o an i l l u m i n a t i o n o f t h i s  presented  patterns  M e a s u r e m e n t s w e r e made  U n i v e r s a l P h o t o m e t e r , Mod.  means o f a dimmer s w i t c h .  Both  75 w f l o o d l a m p s  i n t e n s i t y o f l i g h t d i r e c t e d onto the stimulus by  tank were  the  magnitude  testing. t h e s m a l l e r t a n k and  simultaneously  plexiglass,  0.32  (Fig. 5).  The  cm b y 30.5 cm b y  13  F i g u r e 3.  Photo o f the tanks a t the Vancouver  Aquarium where t h e b e h a v i o u r a l a s p e c t s o f t h i s were done.  Public study  14  o s  to  s  1  m.  w. w.  Fig.  4.  H  T  P Apparatus  o n e - and t w o - b a r  used i n t e s t i n g  patterns  the harbour  seal's  presented simultaneously.  a r e shown i n b o t h d o r s a l (A)  and l a t e r a l  (B)  presented at  the l e v e l of  testing  (T)  One  s e a l at  means o f E,  tank  a t i m e was l e t  sliding  experimenter;  underwater);  PD,  For  (P)  testing in air,  (l^L,);  the water  stimulus cards;  point  of  W^,  (H)  into  the  l o w e r s e t of windows  discrimination for  the  water  t e s t i n g tank  seal.  (for  the  the  partitions. S,  tanks  the  level in  t h e s e a l s were t r a i n e d t o swim a t  from the h o l d i n g tank  the  and t h e s t i m u l u s c a r d s were  the upper s e t of windows  r e m a i n e d t h e same b u t  d i s c r i m i n a t i o n of  The r e l a t i o n s h i p s o f  views.  was d r a i n e d f r o m t h e s t i m u l u s p r e s e n t a t i o n t a n k  underwater  testing  surface.  (T)  by  Figures  5.a-5.b-5.c.  discrimination A.  tanks  Photo of the  Photos showing d e t a i l s of the and  the p r e s e n t a t i o n of a t y p i c a l  stimulus  at the reader's  right.  B.  In a normal t r i a l ,  with  The  the cards  so t h a t t h e  the  seal,  The  as  cards  s e a l would  i t chose the  s e a l ' s a p p r o a c h was  viewed  through the  Photo showing d e t a i l s of the tank.  the  be  cards.  A c o r r e c t r e s p o n s e by hand l e v e r .  C.  the  problem.  cards, w i t h the p o s i t i v e card  w o u l d n o t be h e l d as h i g h , u n a b l e t o see  behavioural  stimulus  f o r e of the bucket,  made u n d e r w a t e r ,  lower  stimulus  window.  presentation  cards were p r e s e n t e d which contained  right-  j u s t to  the  the h e r r i n g rewards.  16  61.0  cm,  20.3  cm  w i t h e i t h e r one l o n g and  the one-bar 5.08  cm  w e r e o f 2.54  (negative)  black bars.  cm ACS  o f t h e gap  bar  (positive) pattern.  The  any  p a i r o f p a t t e r n s was  21 cm.  By  entrance  m  i n t o one  (Fig. 4). was  of the bar  the corresponding  the use  of  Any  with  i t s corresponding  in  the s m a l l e r tank.  the  s e a l was  a wrong response.  was  An  and  was  t h e r i n g i n g o f an  raise a lever with  a n n o u n c e d by  tank  w h i l e the cards,  t r a i n e d t o swim f o r w a r d  of alleyways,  i n c o r r e c t r e s p o n s e was  1/3  the next  t r i a l was  Both s e a l s were f i r s t  and  t h e amount o f f o o d  of h e r r i n g responses).  of the  after trial  testing card  in position bell,  a f t e r making a  choice  A c o r r e c t response i n t o the  a two  (Clupea minute  testing pallasii). "time-  r e q u i r e d to wait  two  presented.  t r a i n e d to push the  m a k i n g an u n d e r w a t e r a p p r o a c h .  a  A  electric  of a h e r r i n g  s e a l was  of  the  a two-bar  i t s nose.  f o l l o w e d by  o u t " p e r i o d d u r i n g which the  daily  and  a 5 second f l a s h of l i g h t  rewarded w i t h  minutes before  tanks,  one-bar c a r d , were p l a c e d On  two-  vertical  change o f a l l e y w a y s  c o n s i s t e d o f t h e s e a l w a i t i n g a t t h e f a r end from the experimenter  of  was  choice of t a r g e t s at  considered  tank  were  In the c a s e  t a r g e t s In b o t h  s e a l s w e r e f o r c e d t o make t h e i r 1.7  on  bars  d i s t a n c e between the c e n t e r s  s e p e r a t i n g t h e two  distance of  The  tape.  p a t t e r n , the width  p l u s the width  partitions  o r two  Sixty  levers after  t r i a l s were  presented  f e d t o e a c h s e a l d a i l y was  2.7  kg  ( i n c l u d e d i n t h i s are the rewards f o r c o r r e c t T h i s amount o f f o o d was  such t h a t both  s e a l s were  17  able  to maintain  healthy  a p p e t i t e and remain  p o s i t i v e c a r d was r a n d o m l y a l t e r n a t e d b e t w e e n t h e  r i g h t and l e f t  sides  i n order  to prevent lever  presented sizes.  c o n s i s t e d o f f r o m two t o t h r e e  with This  the r a t i o biased  A smaller  i n favour  gap s i z e was o n l y  o f t h e l a r g e r gap  introduced  p r e s e n t l y used.  discriminatory abilities  when t h e d a t a  P i n a l measurements o f t h e i r  i n b o t h a i r and w a t e r were t a k e n i n  cards were presented  the upper s e t o f windows  (VI^ i n F i g . 4) .  600  (Table 1 ) .  t e s t i n g i n a i r , t h e w a t e r was d r a i n e d  t a n k and t h e s t i m u l u s  1)  t o see t h e s m a l l e s t  t r i a l s w i t h gap s i z e s b e t w e e n 0.05 - 0.3 cm  from the s m a l l e r  at the l e v e l of The w a t e r l e v e l i n  t e s t i n g t a n k r e m a i n e d t h e same a n d t h e s e a l s w e r e t r a i n e d  t o swim on t h e w a t e r  surface.  I t w i l l be n o t i c e d  that  i n T a b l e 1, some o f t h e gap  s i z e s mentioned a r e n o t included above. the  (Table  f i r s t h a l f o f 1969 and w e r e b a s e d on a p p r o x i m a t e l y  For  such  i n making a c o r r e c t response-  showed e v i d e n c e t h a t t h e s e a l was a b l e s i z e being  Each  gap s i z e s r a n d o m l y  ensured t h a t a m a j o r i t y o f t h e t r i a l s were  t h a t t h e s e a l s h a d no d i f f i c u l t y  the  ( i n cm):  1.27, 0.63, 0.32, 0.25, 0.21, 0.13, 0.09 and 0.05.  day's t r i a l s  the  preference.  f o l l o w i n g i n i t i a l s e r i e s o f gap s i z e s w e r e u s e d  2.54,  gap  i n a generally  condition.  The  The  their  This  frequent  was t h e r e f o r e  i n the i n i t i a l s e r i e s mentioned  i s a r e s u l t o f shrinkage  o f t h e ACS t a p e d u e t o  immersions of the stimulus  cards  i n water.  It  found n e c e s s a r y t o r e m e a s u r e t h e gaps a f t e r e v e r y  18  30  trials  occured. the  and n o t e any changes i n t h e i r High periods  s i z e s w h i c h may h a v e  o f h u m i d i t y were a l s o found t o a f f e c t  tape, making d a i l y measurements d u r i n g  also essential.  aerial  testing  A l l m e a s u r e m e n t s w e r e made b y e y e w i t h t h e ths  use  o f a r u l e r c a l i b r a t e d i n 1/50  o f an i n c h .  Each o f  t h e s e m e a s u r e m e n t s was f u r t h e r d i v i d e d on t h e b a s i s t h e b o r d e r o f t h e gap f e l l  and a l t h o u g h  i ti s r e a l i z e d that  a f e w e r r o r s w e r e no d o u b t made, i t i s f e l t trials  they would c a n c e l  out.  below which the s e a l s could i s n o t claimed, level  but rather  o f where  t h a t o v e r 600  Furthermore, a d e f i n i t e value  n o t s e e and above w h i c h t h e y simply  that their  i s w i t h i n a c e r t a i n range o f values.  lower  could  threshold  19  Results  1.  M o r p h o l o g y and The  are  Histology  basic morphological features  shown i n F i g . 6.  absolute  s i z e and  Briefly,  i n the  circular  and  anterior  chamber w h i c h h a s  of the  i t has  v e r t i c a l diameter of heavily vascularized, of  the  (Fig.  c o r n e a by 7).  portion  of  an  the  large, both  cornea  slightly  i s not  The  iris  mm.  The  The  s c l e r a i s thickened  slightly  the  eye  out  and  The  in addition,  virtually  contains  the whole of  area c e n t r a l i s could P u p i l s i z e and influenced  by  pinhole  the  at  choroid  inner  the  be  the  extensive  vitreous  shape under  the and  posterior  of p r o p o r t i o n  to  the  terrestrial and  pigmented  tapetum which l i n e s  cavity.  l i g h t present,  u p p e r m o s t end  elliptical  the  No  fovea  nor  distinguished.  amount o f  light  to  network  i n the  s h a p e i n a i r a p p e a r t o be  under h i g h  is  aspect  is heavily vascularized  an  lens  that of  o t h e r c o m p o n e n t l a y e r s when c o m p a r e d t o a t y p i c a l mammalian e y e .  the  i s very muscular  i s anchored to the  i s a l s o not  globe  of p r o p o r t i o n  means o f a v e r y p r o f u s e f i b e r  but  in  arched over  2-2.5 out  a  eye  i s roughly  e q u a t o r i a l diameter 37%  eye.  and  The  a depth of  although large,  eye;  eyes are  adult.  flattened, being only  s p h e r i c a l and  the harbour s e a l  i n r e l a t i v e s i z e t o the body, w i t h  d i a m e t e r o f 39-40 mm  rest  the  of  low  c l o s i n g down t o  of a v e r t i c a l l y  i n t e n s i t i e s and light  o p e n i n g up  primarily  constricted t o an  oval  i n t e n s i t i e s (Fig. 8).  a  slit  or No  F i g u r e s 6.a-6.b-6.c-6.d. of p i n n i p e d eyes.  S c a l e - 1 cm.  A. H a r b o u r s e a l . " B. N o r t h e r n  Photos o f h i s t o l o g i c a l s e c t i o n s  (Phoca  fur seal.  C. H a r p s e a l .  vitulina)  (Callorhinus ursinus)  (Pagophilus  D. K i l l e r w h a l e .  (Grampus  groenlandicus) orca)  20  Figure  7.  Close-up of the extensive  c h a r a c t e r i s t i c around seal  iris  (I) .  the periphery  Arrow p o i n t s t o  fiber  network  of the harbour  pupil.  F i g u r e 8.a-8.b-8.c.  - P u p i l shapes under d i f f e r e n t  light  intensities. A. V e r y h i g h  light  intensity.  This  i s as s m a l l  a s t h e p u p i l may be made. B. H i g h l i g h t shaped than  intensity.  The p u p i l  i s more t e a r -  i n A.  C. E f f e c t o f a t r o p i n e .  This  shows t h e maximum  s i z e o b t a i n a b l e , and r e p r e s e n t s found under v e r y  low l i g h t  the p u p i l  intensities.  size  23  pupil  slit  that  i s f u n c t i o n a l along  formed i n b r i g h t In w a t e r , s i z e and  retina  s u c h as w o u l d e x i s t on  the p u p i l i s the  same-' s i z e as  the v e r t i c a l l y  giant by  little  cell.  cells  around these c e l l s  layer  zonation  a m a c r i n e and  plexiform  pedicles inner  regions  of  their  and  by  extensive  dendritic  branching.  the very layer  extensive  the  absence  large horizontal  ( F i g . 12).  Interspersed  a x i s c y l i n d e r s are The  bipolar  and  the  inward  the  located predominately l a y e r and  may  be  into  the  terminal the  ganglion  i n the  outer  identified  in  oval nuclei.  c e l l s h e r e i d e n t i f i e d as a m a c r i n e c e l l s ,  s i n g l e processes extending i n t o the  the  cells  into  12 b y ' t h e i r o u t w a r d e x t e n d i n g p r o c e s s e s and  only broad,  occasional identified  l a y e r , where they synapse w i t h  Other o c c a s i o n a l  The  processes which extend outward  inner nuclear  around  easily  i s d i s t i n g u i s h e d by and  is  be  neuroglial cells.  b i p o l a r s are the  layer  interspersed  l a y e r , where t h e y synapse w i t h  plexiform The  seal  c e l l s may  of the p h o t o r e c e p t o r c e l l s ,  cells.  Fig.  and  c h a r a c t e r i z e d by  outer  the harbour  ( F i g . 10)  c e l l s which extend throughout t h i s  are  of  i n s i z e except f o r the  ( F i g . 11)  inner nuclear  distinctive  bipolar,  also primarily  Deep t o t h e n e r v e f i b e r  These l a t t e r  t h e i r huge b o d i e s  o f any  pupil  t r a n s v e r s i n g r a d i a l f i b e r s of Muller.  c e l l s vary  ganglion  The  features  s e e n i n F i g . 9.  a s i n g l e layer of ganglion  ganglion  day.  light intensity.  gross h i s t o l o g i c a l  are  is  a sunny  in a i r , with  h e n c e s h a p e i n w a t e r a p p e a r i n g t o be  i n f l u e n c e d by The  light,  i t s e n t i r e length  inner  with  plexiform  Explanation  Abbreviations  used  of Figures  9-21  in Figures:  nf  o p t i c nerve f i b e r  g  ganglion  ip  inner plexiform  in  inner nuclear  op  outer  on  , outer  cell  layer  layer layer  layer  plexiform nuclear  layer  layer  r  layer of photoreceptors  ris  receptor  inner  segments  ros  receptor  outer  segments  pe  pigment  epithelium  t  tapetum  b  bipolar  h  horizontal  Mf  r a d i a l f i b e r s of Muller  a  amacrine  n  neuroglial cell  bv  blood  p  polysynaptic  o  oligosynaptac  cell cell  cell  vessel pedicle pedicle  Vertical  cross  retina.  Toluidine  Vertical  section  and  ganglion  Muller  c a n be  layers.  section  Large r a d i a l  to ganglion  Toluidine  and  Vertical  ganglion  small  cell.  layer  (h) , a m a c r i n e  cells  (b) a n d n e u r o g l i a l c e l l s  a x i s c y l i n d e r s a r e m a r k e d by  (n) . arrows.  blue. section  through the inner  nuclear  showing the synapses between b i p o l a r c e l l s and a . p o l y s y n a p t i c blue.  these  c e l l s and a  through the inner nuclear  (a), b i p o l a r c e l l s cell  of  eosin.  showing l a r g e h o r i z o n t a l c e l l s  Toluidine  fibers  blue.  through a giant  section  Horizontal  fiber  seen v e r t i c a l l y t r a n s v e r s i n g  Arrows p o i n t  Haemotoxylin Vertical  through the o p t i c nerve  section  the  blue.  layers.  blood vessel. Vertical  of the c e n t r a l area of  pedicle  (arrow).  layer (b)  Toluidine  25  l a y e r , w e r e o b s e r v e d on t h e i n n e r a s p e c t s layer.  o f the inner  nuclear  The s m a l l , d a r k l y - s t a i n e d n u c l e i s u r r o u n d e d b y  cytoplasm i n the inner regions  of this  layer  little  ( F i g . 13) a r e t h e  nuclei of neuroglial cells. Two v a r i e t i e s o f p h o t o r e c e p t o r in the outer and  p e d i c l e can be d i s t i n g u i s h e d  plexiform layer, rod-like oligosynaptic pedicles  cone-like polysynaptic  pedicles  (Fig. 14).  p e d i c l e s a r e most numerous, w i t h a r a t i o They a r e a l s o much s m a l l e r  f i l a m e n t s found  polysynaptic the  T h e r e i s no o u t e r  layers,  outer being  i n contrast  i n polysynaptic  fibre  10-12 c e l l s d e e p . l a y e r was o n l y  two r e l a t i v e l y  thinned  Regional  v a r i a t i o n i nthe  observed  i n the outer  out.  The  peripheral  photoreceptor  w i t h most o f t h e n u c l e i  l a r g e clumps o f chromatic  i n from the outer  c o n t a i n many s m a l l e r adjacent  The  layer i s the t h i c k e s t o f a l l the r e t i n a l  n u c l e i a p p e a r t o b e o f two t y p e s ,  located  pedicles.  short,  l a y e r o f Henle.  a r e a s where i t g r a d u a l l y  and  t o t h e many  l a y e r than a r e the o l i g o s y n a p t i c p e d i c l e s .  nuclear  thickness of this  containing  23:1.  pedicles are also situated slightly closer to  inner.nuclear  The  of approximately  than t h e p o l y s y n a p t i c p e d i c l e s and  c o n t a i n o n l y one b a s a l f i l a m e n t converging  The o l i g o s y n a p t i c  limiting  membrane.  material  A few, however,  c l u m p s o f c h r o m a t i n and a r e d i r e c t l y  to. t h i s membrane  ( F i g . 14 a n d 15) .  These l a t t e r  cone-  t y p e n u c l e i a r e a l s o o f t e n a s s o c i a t e d w i t h a l a r g e gap i n t h e outer  l i m i t i n g membrane.  Fig.  14  Vertical outer and  s e c t i o n through the outer  nuclear  l a y e r s s h o w i n g o l i g o s y n a p t i c (o)  polysynaptic  points  p l e x i f o r m and  (p) t e r m i n a l p e d i c l e s .  to cone-type photoreceptor  Arrow  nucleus.  Toluidine  blue. Fig.  15  Vertical and  points  ( r i s ) and o u t e r  (ros) segments.  to cone-type photoreceptor  Toluidine 16  Vertical  17  to fine  point Fig.  18  of outer  cytoplasmic  tapetum.  outer  processes  extending  segments.  ( s l i g h t l y oblique)  l i m i t i n g membrane  to photoreceptor  Vertical  epithelium.  blue.  Horizontal section region  Arrow  nucleus.  s e c t i o n through pigment  towards the photoreceptor Toluidine  photoreceptor  blue.  Arrows p o i n t  Fig.  l i m i t i n g membrane  l a y e r o f r o d s and cones, showing  inner  Fig.  s e c t i o n through the outer  nuclei.  through  (points).  Arrows  Azur I I .  s e c t i o n t h r o u g h pigment e p i t h e l i u m and Arrows p o i n t  to  t r a n s v e r s i n g t h e tapetum.  chorio-capillaries Toluidine  blue.  27  The stained darker  photoreceptor  l a y e r shows two  ( F i g . 15) , a l i g h t l y s t a i n i n g outer  photoreceptor morphology  can  be  ( F i g s . 15  s t a i n i n g inner  segment l a y e r .  17).  photoreceptor  cell  a unicellular  l a y e r , from the  Closely associated with  No  p a r t of the r e t i n a ,  well-developed  and  extensive  i s composed o f  20-22 l a y e r s o f  c o u l d be  observed.  there also e x i s t s a  tapetum c e l l u l o s u m flattened cells  towards the p e r i p h e r y .  penetrated  by  e p i t h e l i u m and  laterally  ( F i g . 18). lying  The  a s l i g h t one  ( F i g . 19 and  20)  and  photoreceptor  but  inner  observed  not  choroid pigment  the  i n t h e s e e y e s , was  much more i n t r i c a t e and  The  second  of  the  whole  m o s t common t y p e o f f o l d i n g  layers of  nuclear  the r e t i n a .  This  seven harbour s e a l eyes  o n l y absent i n the  c o m p l e x and  observed i n three  i n the  i n v o l v i n g the outer  i n f o u r of the  of the r e t i n a .  l a y e r , was  from the  " r e t i n a l f o l d i n g " were observed  was  regions  the  thinning  i n the r e g i o n between the  s e c t i o n s embedded i n p a r a f f i n .  e x a m i n e d and  in  It  tapetum.  types of  f o l d i n g was  very  layer i s regularly  many s m a l l c a p i l l a r i e s a r i s i n g  which transverse  Two  This  16),  the  s u p e r f i c i a l c h o r o i d deep t o the pigment e p i t h e l i u m , gradually  the  cytoplasmic  segments o f  pigment granules  segment  (Fig.  c e l l s of which f i n e  a  of inner  l a y e r i s the pigment e p i t h e l i u m  cells.  Although not  type  the b a s i s of  p r o c e s s e s e x t e n d down b e t w e e n t h e o u t e r photoreceptor  z o n e s when  segment l a y e r and  O n l y one  d i s t i n g u i s h e d on and  distinct  peripheral  t y p e o f f o l d i n g ( F i g . 21) , i n v o l v i n g the whole  retinal  seven harbour s e a l s  studied 'i  Fig.  19 and  20  Vertical  sections  showing the s l i g h t  Fig.  21  the  outer nuclear  are  folded.  Vertical  through the r e t i n a  form o f r e t i n a l l a y e r and  folding.  l a y e r o f r o d s and  H a e m o t o x y l i n and  Only cones  eosin.  s e c t i o n through the r e t i n a showing the  complex form of r e t i n a l layers are folded.  folding.  A l l the  H a e m o t o x y l i n and  retinal  eosin.  28  29  ( t h e s e t h r e e a l s o showed t h e f i r s t This f o l d i n g  i s much more l o c a l i z e d  f o l d i n g mentioned, proportion  type of r e t i n a l than the f i r s t  a n d was o n l y o b s e r v e d  usually  shown  this  folding  s h o u l d be c l e a r l y  t h r o u g h f u n d u s c o p i c e x a m i n a t i o n . . S i n c e none was i t was n o t p o s s i b l e t o c o n f i r m t h a t t h i s  detail  i n Epon.  observed,  type o f f o l d i n g  occurs  t y p e was o b s e r v e d  i n those  sections  I t i s not p o s s i b l e t h e r e f o r e to describe i n  the r e t i n a l  o r g a n i z a t i o n about  these  folds.  The p e r i p h e r a l t e r m i n a t i o n o f t h e s e n s o r y r e t i n a , t e r m i n a l i s , was o b s e r v e d muscles.  visible  phenomenon.  No f o l d i n g o f e i t h e r embedded  through  i n the c e n t r a l or middle r e t i n a l r e g i o n s ;  Owing t o i t s magnitude,  as a n a t u r a l  type o f  t o o c c u r on a s m a l l  ( 1 0 % a t t h e most) o f t h e r e t i n a  sectioning,  folding).  the ora  t o be a t t h e b a s e o f t h e c i l i a r y  T h e r e was t h u s no p a r s p l a i n a a s o c c u r s i n t h e human  eye. 2.  Refraction R e f r a c t i v e measurements o f t h e f o u r harbour  b y r e t i n o s c o p y a r e g i v e n i n T a b l e 1. controlling  are of  were s t i l l  correctly.  encountered  A l l three people  i n measuring  refraction  i n humans,  i n t h e measurement  b u t none o f them c o u l d s t a t e w i t h  the values f o r the harbour  the tapetum,  the o p t i c a l  t h a t took t h e s e measurements  p r o f e s s i o n a l d o c t o r s and w e l l - s k i l l e d  certainity  difficulties in  t h e s e a l s w e r e o v e r c o m e by means o f a n a e s t h e t i c s ,  but problems axes  Initial  s e a l s examined  seal.  a l a y e r n o t p r e s e n t i n man,  The e f f e c t o f  in reflecting  back  30  Seal  Refraction Axis Vertical (Diopters)  Refraction Axis Hor i z o n t a l (Diopters)  Astigmatism (Diopters)  Instrument  Operator  Su s i .  -7.0  (B)  -7.0  (B)  0.0  Slit  1  Orphie  -5.0  (B)  -5.0  (B)  0.0  Slit  1  1.5  (B)  2.0  (B)  0.5  Slit  1  -3.5 -2.5  (R) (L)  3.0 (R) 4.5. (L)  6.5 7.0  Round  2  3.0  (L)  5.5  (L)  2.5  Slit  2  8.0  (B)  1. 0 (B)  7.0  Slit  3  B l o n d i e -11.5  (B)  1.5  (B)  13. 0  Round  2  (B) *  1.5  (B)*  13.0*  Round  2  4.0  (B)  0.0  Slit  1  9. 0  -  Scar  -11.5  4. 0 (B)  Orphie (Water)  -  -4. 0  -13. 0  Mean o f -12. 5 (-16 .5 -8.0 12 e y e s t o -11 .5) (harp s eal) ,  V  f ° 5.25 12 e y e s (harp seal)(Water)  M  f  n  Table in  l.:  this  4.70 (9 t o 3. 75)  (11.5 t o -3.5)  6.0  (12 t o 5)  Piggins  (9 t o 3. 75)  0.0  (2 t o 0)  Piggins  R e f r a c t i v e measurements o f t h e harbour  s t u d y and b y J o h n s o n  taken by P i g g i n s (1970).  (1893).  s e a l as o b t a i n e d  Harp s e a l measurements were  These v a l u e s were o b t a i n e d i n a i r by  means o f r e t i n o s c o p y , w i t h t h o s e v a l u e s o b t a i n e d i n w a t e r B = i n both anaesthesia.  eyes,  Johnson  R = right  eye, L = l e f t  eye, * = w i t h o u t  noted.  31  the r e t i n o s c o p y upward t i l t , not  light,  made a l l t h e m e a s u r e m e n t s q u e s t i o n a b l e ;  possible to definitely  were a l o n g  of contact  studied,  s t a t e t h a t measurements  obtained  a t o b t a i n i n g a c c u r a t e measurements by t h e  lenses f a i l e d ,  i t was n o t e d  f o r when t h e l e n s e s w e r e  t h a t d i m p l i n g o f the cornea  had  T h i s was p r e s u m a b l y a s a r e s u l t o f a d r o p i n b l o o d within  i t was  the o p t i c axis.  Both attempts use  c o m b i n e d w i t h t h e l a r g e e y e and i t s  t h e eye owing t o death.  a l s o f o u n d t o be t o o f l a t  Furthermore,  later occured.  pressure  t h e l e n s e s were  t o be m e a s u r e d b y c o n v e n t i o n a l  commercial u l t r a s o n i c equipment, which i s c a l i b r a t e d f o r human e y e m e a s u r e m e n t .  Owing t o a s h o r t a g e  i n measuring c o r n e a l c u r v a t u r e by t h i s  of seals,  technique  efforts  were n o t  pursued. Corneal  astigmatism  was o b s e r v e d  means o f a P l a c i d o ' s d i s c . effect  i n the v e r t i c a l ,  corneal curvature  3.  This astigmatism  which  however, by  had i t s d i s t o r t i o n  indicates the radius of least  i n the h o r i z o n t a l .  are obtainable w i t h t h i s  to occur,  technique  No q u a n t i t a t i v e v a l u e s  however.  Visualdiscriminations: .The  g r e a t e s t problem encountered i n t h i s phase o f the  s t u d y was t h e l o n g p e r i o d o f t i m e r e q u i r e d . t o t r a i n  the seals  to respond t o the c o r r e c t stimulus card  As  mentioned e a r l i e r , as b e i n g seals.  (Fig. 22).  B l o n d i e a n d S c a r w e r e t h e two s e a l s c h o s e n  t h e most m a n a g e a b l e f r o m s e v e n i n i t i a l l y However, e v e n t h e s e  two w e r e v e r y  slow  selected  learners.  F i g u r e s 22.A-22.B.  P e r f o r m a n c e s o f t h e two h a r b o u r  during conditioning.  Each day's  i n t o blocks, of twenty t r i a l s  seals  t r i a l s w e r e b r o k e n down  each to b e t t e r  describe  performance behaviour. A. The p e r f o r m a n c e o f t h e f o u r - y e a r o l d f e m a l e , B l o n d i e B. The p e r f o r m a n c e o f t h e t w o - y e a r o l d male,. S c a r . Dotted l i n e = stage at introduction of the b a f f l e s i n t o the tank; Star = s i m p l i f i c a t i o n of the problem f r o m a t w o - b a r ( p o s i t i v e ) : o n e - b a r ( n e g a t i v e ) one , t c a one c a r d one ( o n l y t h e t w o - b a r ( p o s i t i v e ) c a r d was p r e s e n t e d ) .  33  Blondie,  the  f e m a l e , was  t r i a l s before block's blocks  trials o f 20  problem. three  she was  each),  c a r d and  u s e d as but  30  was  r e q u i r e d t o s o l v e was  a  p o s i t i v e stimulus  card,  stimulus  blank  card.  cards,  a  This combination  l a t e r dropped.  responses a f t e r  t h e p r o b l e m was S c a r was  940  a  was  Scar,  (negative) c a r d one,  Schusterman  and  yet  still  to l e a r n the  two-bar  e v e n when t h e p r o b l e m was  presented,  with  the  solve However,  card).  comparable similar  seal. the  same number  (positive):  simplified  ( o n l y t h e -two-bar  of  required a  one-bar  after  three 600  (positive) card  the only requirement being  t h e same s i d e as  from  part.  p r o b l e m as B l o n d i e r e q u i r e d t o l e a r n t h e  c a r d one  increasing  (1968) a l s o e n c o u n t e r e d  the male, r e q u i r e d a p p r o x i m a t e l y  t o a one  l e v e r on  after  required to  i n t r a i n i n g on my  the  improvement  i n c o r r e c t response  i n h i s t r a i n i n g of the harbour  ( F i g . 22)  sudden  occurred  l a r g e number o f t r i a l s  trained after Blondie  difficulties  in training  T h i s s u g g e s t s t h a t p e r h a p s one  inexperience  number o f t r i a l s .  The  trials  p e r i o d f o l l o w i n g an  the reasons f o r the  was  black  seconds to 3 minutes.  trials  negative  three  f o r s o l v i n g the  long p e r i o d of time r e q u i r e d  t h i s aspect  "time-out"  trials  criterion  into  one  some g e n e r a l i z a t i o n e x p e r i m e n t s w e r e i n i t i a l l y p l a n n e d ,  iri B l o n d i e ' s the  two  a blank  owing to the  seals,  the  1540  to a l l of  t r i a l s were s u b d i v i d e d  c o n s i s t i n g o f one  t w o - b a r p a t t e r n , and  required  to respond c o r r e c t l y  p r o b l e m w h i c h she was  c a r d one,  white  able  ( t h e d a y ' s 60  trials  The  t r a i n e d f i r s t and  to push  the  Once t h e p r o b l e m  was  l e a r n t , h o w e v e r , b o t h s e a l s showed c o n s i d e r a b l e . r e t e n t i o n o f  of  34  the problem  and w o u l d  p e r f o r m c o r r e c t l y even  after  a two month  lapse i n testing. The for  r e s p o n s e s o f B l o n d i e and S c a r  the f i n a l  i n b o t h a i r and water  s e t o f m e a s u r e m e n t s , i e . f o r gap s i z e s  0.05--3.0 cm, i s g i v e n i n T a b l e 2 a n d F i g s . r e s p e c t i v e l y . Weighted  criterion  i s applied  23 a n d 24  r e g r e s s i o n s were performed  f o r b o t h a i r and w a t e r ,  between  on t h e d a t a  a n d when t h e s t a n d a r d p s y c h o p h y s i c a l  to the data  (thed i f f e r e n c e  threshold  b e i n g d e f i n e d a s t h e gap s i z e c o r r e s p o n d i n g t o t h e 7 5 % performance  level),  i t i s found t h a t both s e a l s c o u l d  resolve  a v e r t i c a l gap b e t w e e n two b l a c k b a r s a s s m a l l a s 1 mm when a t a d i s t a n c e o f 1.7 m.  There  i s no s i g n i f i c a n t  (p = 0.05) b e t w e e n t h e v a l u e s i n a i r and w a t e r and b e t w e e n s e a l s  in air.  b e t w e e n the' two s e a l s  There  i n water.  difference for either  i s a significant  seal  difference  35  Table  2:  Responses  testings.  Gap (mm)  Values  of  are  both given  Size  seals in  in  both  % correct  a i r for  and each  water gap  of  Air  •  100  30  85  108  3.57  90  100  1. 66  3.34  84  98  1.54  3 . 07  -  1.42  2 . 83  1.29  2.57  1. 22  2.16  4.35  2. 03  4.  1.  79  10  Water  n  -  f i n a l  i  size.  Blondie  (min. arc)  •  for  Scar n  Air  n  Water  -  100  20  -  -  -  98  90  -  -  100  100  -  -  100  -  -  96  24  -  -  96  100  58  -  -  -  • -  99  101  -  -  83  -  -  99  144  -  -  95  2.43  -  -  -  -  -  1.16  2.31  85  60  97  105  .1.08  2 . 15  -  -  94  1.02  2.05  83  0 . 97  1 . 95  80  0 . 90  1.80  0 . 76  n  59 141  92  22  95  60  9/  130  17  -  -  69  48  90  51  88  120  83  24  20  -  -  75  20  -  -  -  81  47  -'  -  62  21  1.54  64  24  75  44  58  30  57  99  0.71  1.41  -  -  74  19  -  -  -  0.64  ,1.28  59  55  70  33  63  70  62  26  0.5 7  1 . 13  - .  71  17  -  -  63  30  0.51  1.02  -  58  50  -  .  ,.; . 46  '  119  40  654  '  .-  660  660  -  -  -  600  FOUR-YEAR  OLD  FEMALE  1.5  0.5  GAP  Fig.  SIZE mm.  '23.A comparison of the a b i l i t y  f e m a l e ha.rbour s e a l t o d i s c r i m i n a t e gap  s i z e of the two-bar p a t t e r n . i n  weighted l i n e a r regression difference  threshold  was  of the four-year small  differences  i n the  b o t h a i r and w a t e r . . A  performed  which i s defined  corresponding to the 7 5 % ( 6 0  old  to determine  a s t h e gap  the  size  a r c s i n tt) p e r f o r m a n c e  level.  37  TWO-YEAR  i  OLD  1  MALE  *  0.5  i  i  1.5  1  GAP  Fig.2h. A comparison harbour  of the two-bar regression  pattern  a r c s i n 7%)  i  small  o f the two-year differences  o l d male  i n the gap s i z e  t o determine the d i f f e r e n c e  as the gap s i z e c o r r e s p o n d i n g t o the  performance  i  i n both a i r and water. A weighted l i n e a r  was p e r f o r m e d  which i s defined  i  2  SIZE mm.  o f the a b i l i t y  seal to discriminate  i  level.  threshold  75%(60  38  Discussion  The and  g e n e r a l aim of t h i s  degree of the harbour  amphibious h a b i t . and h i s t o l o g i c a l  I will  i s to determine  Walls'  first  d i s c u s s the gross  i s essential before  be  flattened  to  anterior  similar  i n a b s o l u t e s i z e and  i s roughly spherical  confirms  Briefly,  i n shape.  The  which  t y p i c a l l y has  i t i s not  the  lens in  "very deep" as  i s not out of p r o p o r t i o n to the r e s t  large, i s not  i s o n l y 0.37,  four-hour ungulates (0.50)  (Walls, 1942).  p o i n t s out,  lens i s e s s e n t i a l l y  "nocturnal" in size.  e q u a t o r i a l diameter  t h e eye  than  chamber i s r e l a t i v e l y d e e p , a t l e a s t when c o m p a r e d  and  and  is  (1893).  The  its  size  cornea  s t a t e d by J o h n s o n .  t h e eye  the  in relative  I t i s n o t more c o n v e x a s s t a t e d by J o h n s o n  contact w i t h the cornea;  although  the  seal i s  t o t h a t o f o t h e r p i n n i p e d s and  t h e a q u a t i c t e l e o s t eye,  direct  of  i n i t s c e n t e r r e g i o n s , w h e r e i t i s much f l a t t e r  i n humans. The  morphological  i t s functional  (1942) d e s c r i p t i o n o f t h e p i n n i p e d e y e .  t h e b o d y , and  an  assessed.  e y e b a l l i s large, both to  nature  seal's v i s u a l adaptations to  T h i s s t u d y shows t h a t t h e eye o f t h e h a r b o u r morphologically  the  f e a t u r e s , a s an a d e q u a t e u n d e r s t a n d i n g  s t r u c t u r e o f t h e eye c a p a b i l i t i e s may  study  relative  s p h e r i c a l i n shape  this  to t h a t of the  of twenty-  that of the n o c t u r n a l c a r n i v o r e s  T h i s i n c r e a s e i n c u r v a t u r e , as  seems t h u s t o be  of  I mean t h a t  to the v e r t i c a l diameter  a v a l u e much c l o s e r (0.40) t h a n  By  again  an a d a p t a t i o n t o t h e  Walls  'loss'  of  39-  the cornea further  u n d e r w a t e r and  supported  The  sclera  by  not  t o n o c t u r n a l i t y , an  the d i u r n a l h a b i t of  it  i s thickened  nerve.  slightly  as  supporting as  sclera  i n the r e g i o n surrounding  i n cetaceans  s t r u c t u r e owing to the  the harbour greater  s e a l , has  (1942) c l a i m t h a t  large absolute  the  a thicker  to f i n e  Lagenorhynchus  sclera.  by Nagy and i n the  Ronald  cetaceans,  especially  inner nuclear  study  similar  to  B o t h s p e c i e s show l i t t l e  have s i m i l a r n u c l e a r d e n s i t i e s w i t h i n the  retinal  l a y e r s throughout the r e t i n a .  q u i t e h i g h and  to b i p o l a r c e l l s  The  degree of  to g a n g l i o n c e l l s  s i m i l a r b e t w e e n t h e two  laterally  conducting  network e x i s t s ,  the f u n c t i o n a l importance of such a network  v  that described  retinal  different summation i s also  species, roughly  e x t e n s i v e h o r i z o n t a l c e l l network a l s o suggests  well-developed  the  zonation  l a y e r as t h e t h i c k e s t o f a l l t h e  and  The  shows t h a t  t h a t o f t h e h a r p s e a l as  layers,  cells  resulting  layer, possess large h o r i z o n t a l c e l l s ,  have the outer n u c l e a r  of receptor  w i t h the  the  necessitates this difference.  structure, this  (1970).  than  However, p e r h a p s  r e t i n a of the harbour s e a l i s h i s t o l o g i c a l l y of other pinnipeds,  s i z e of  a smaller absolute eyeball size  in corneal stress,  With regard  the o p t i c  a  swimming s p e e d s a t t a i n e d by  increase  cetaceans;  f u n c t i o n s p r i m a r i l y as  the P a c i f i c w h i t e - s i d e d d o l p h i n ,  o b l i q u i d e n s , w h i c h has  proportion  i s the case i n  T h i s w o u l d seem t o q u e s t i o n W a l l s '  the t h i c k e n e d  eye,  pinnipeds.  i n the harbour s e a l i s not out of  i n t h i c k n e s s t o t h e r e s t o f t h e eye,  hypothesis  100:10:1.  that a although  is s t i l l speculative.  40  In t h e i n n e r n u c l e a r l a y e r , n e u r o g l i a l c e l l s were i n i t i a l l y the methods o f G o l g i and E h r l i c h methods were n o t used i n t h i s elements  identified  amacrine and  and c l a s s i f i e d by  ( P o l y a k , 1941) .  study,  Since  identification  i s u n c e r t a i n , owing t o t h e i r  appearances. and  the b i p o l a r ,  similar  these  of these  morphological  The s h a p e , s i z e and p o s i t i o n o f t h e n u c l e i ,  the appearances o f the c e l l u l a r  processes,  a r e n o t sure  criteria. I n c o n t r a s t t o Nagy a n d R o n a l d felt in  t h a t cone-type  the c l a s s i c a l  c o n t e x t as d e s c r i b e d by Polyak  of photoreceptors  (1941)  (1941)  a new m o r p h o l o g i c a l  o f t y p e A, r e c e p t o r s  Pedler  classification  involving three basic v a r i e t i e s :  The p r e s e n c e  insensitive,  i t is  r e c e p t o r s a r e present, although n o t perhaps  (1969) h a s r e c e n t l y p r o p o s e d  B a n d C.  (1970), however,  T y p e s A,  relatively  polysynaptic c e l l s representative of Polyak's  "cones",  was n o t c o n c l u s i v e l y d e m o n s t r a t e d ,  a s i t was  i m p o s s i b l e t o d i s c e r n any d e f i n i t e m o r p h o l o g i c a l d i f f e r e n c e s between t h e i n n e r segments o f t h e p h o t o r e c e p t o r s (Fig.  15) o r h o r i z o n t a l  some e v i d e n c e  indicating  ( F i g . 17) s e c t i o n . that this c e l l  in vertical  However, t h e r e i s  t y p e may i n f a c t b e  p r e s e n t on t h e b a s i s o f t h e l a r g e gaps i n t h e o u t e r membrane w h i c h nuclei  are c h a r a c t e r i s t i c a l l y  associated with  cone-type  (Fig.. 15)  In l i e u o f t h e i n a b i l i t y  t o confirm the presence  A r e c e p t o r s , type B r e c e p t o r s , r e l a t i v e l y cells,  limiting  a r e suggested  o f type  sensitive, polysynaptic  as t h e c o n e - l i k e type observed  i n the r e t i n a  41  owing t o the presence o f p o l y s y n a p t i c p e d i c l e s . possessing  o l i g o s y n a p t i c p e d i c l e s are  c e l l s o f the receptors  type  termed  type  "rods" by Polyak  Those  C receptors, sensitive These  (1941).  comprise the m a j o r i t y o f the p h o t o r e c e p t o r s  i n the harbour s e a l r e t i n a . a l s o appear t o be p r e s e n t  Type B a n d C p h o t o r e c e p t o r s  distinct  a photoreceptor  o n l y one m o r p h o l o g i c a l  composition  t h e i m p l i c a t i o n s o f such  i n the phocid  retina  sparse  numbers t h a t a r e p r e s e n t .  undergone by the ganglion  cells  sensitive receptors  likely  The e x t e n s i v e  Those r e c e p t o r s p o s s e s s i n g  low l i g h t  (Pedler,  intensities  groups  such as would  ice, or a t night.  connections  information processing  1 9 6 9 ) as these  direction.  summation  p o l y s y n a p t i c p e d i c l e s may i n t u r n  connected t o h o r i z o n t a l c e l l s , a high  owing t o  through the b i p o l a r and  e x i s t a t g r e a t depths, beneath snow-covered  impart  eye,  creates highly sensitive receptor  to d e a l w i t h the r e l a t i v e l y  be  are n o t  The p r e s e n c e o f t y p e B r e c e p t o r s d o e s n o t n e c e s s a r i l y  g i v e them b e t t e r v i s u a l a c u i t y t h a n a n a l l r o d the  terminals  i n n e r segment v a r i e t y .  Lacking neurophysiological data,  clear.  would  i n the h a r p s e a l on the b a s i s o f  They a l s o r e p o r t e d  1970).  latter  found  the presence o f b o t h r o d - l i k e and c o n e - l i k e r e c e p t o r (Nagy a n d R o n a l d ,  cells  cells  which  capability  conduct impulses  might  t o the  retina  i na l a t e r a l  Such a s y s t e m w o u l d c e r t a i n l y b e a d v a n t a g e o u s t o  a predator  r e q u i r e d t o hunt a fast-moving  dimensional  environment under low l i g h t  R e t i n a l f o l d i n g h a s been r e p o r t e d  prey  i na three-  intensities. i n the cetacean  eye by  42 1  Mann  (1946) on t h e b a s i s o f h i s o b s e r v a t i o n s  whale  (Physter macrocephalus)  and t h e f i n w h a l e  p h y s a l u s ) , and m o s t r e c e n t l y , b y beluga  (Delphinapterus  and t h e o t t e r b y Mann  layer of rods  i n the  The r e t i n a l  folding  a n d c o n e s and was s p e c u l a t e d He t h e o r i z e d  (H. a m p h i b u s ) , reported  l a y e r and  to function i n  t h a t s i n c e movements a r e  seen i n t h e p e r i p h e r a l r e g i o n s o f t h e r e t i n a  centrally,  the less the r e t i n a  i n c r e a s e d summation  than  i s adapted f o r acute  vision,  t o d e t e c t movement.  Thus,  r e s u l t i n g from u n d u l a t i o n s  of the  t h e b e t t e r - w i l l be i t s a b i l i t y the  (1967)  involved only the outer nuclear  movement-detection. better  Pilleri  photoreceptor  l a y e r , w h i c h i n c r e a s e s t h e number o f r e c e p t o r s  per  of r e t i n a l  u n i t area  increase  1  (Balaenoptera  l e u c a s ) , the hippopotamus  (Lutra l u t r a ) .  (1946)  on t h e s p e r m  surface without  i n t h e number o f n e r v e f i b e r s  a  corresponding  i n the nerve  fiber  l a y e r , would, d e c r e a s e v i s u a l a c u i t y a n d s o a i d i n movementdetection.  This  s i t u a t i o n with regard  factors  i s n o t b o r n e o u t , however,  (1942),  p s y c h i c f a c t o r s support  top h y s i o l o g i c a l  although  according  to Walls  b e t t e r movement-detection i n  t h e p e r i p h e r y , w h e r e movements h a v e a s a l i e n c y a n d a t t e n t i o n value q u i t e out of proportion t o the c l a r i t y with which are a c t u a l l y d i s c r i m i n a t e d . that r e t i n a l speculates. The  folding  I t seems u n l i k e l y ,  they  therefore,  f u n c t i o n s i n m o v e m e n t - d e t e c t i o n a s Mann •  f o l d i n g s r e p o r t e d by P i l l e r i  (1967)  l a y e r s o f the r e t i n a w i t h t h e e x c e p t i o n  involved a l l the  o f the pigment  epithelium  43  As  t h r e e mammalian o r d e r s w e r e i n v o l v e d , h e c o n c l u d e d  f o l d i n g was an a d a p t a t i o n that  i t f u n c t i o n e d somehow i n d i v i n g ,  adaptation eye  to aquatic l i f e ,  pockets  This  suggested  t h i s perhaps being  t o changes i n water p r e s s u r e  volumes.  and  that  an  to allow f o r f l u c t u a t i n  i s u n l i k e l y a l s o - t h e e y e c o n t a i n s no a i r  whose c o m p r e s s i o n w o u l d c a u s e  This r a i s e s the question f o l d i n g a s a " r e a l " phenonom  distortion.  of the existence of r e t i n a l i n t h e mammalian e y e , b e . i t  a d a p t e d t o an a q u a t i c , a m p h i b i o u s o r t e r r e s t r i a l mode o f life If  as i t i s hard  i t i s "real",  (Cain,  1964).  m i g h t b e an  t o s e e how s u c h a f e a t u r e m i g h t f u n c t i o n .  i t must h a v e some f u n c t i o n a l s i g n i f i c a n c e  Is there,  therefore, a p o s s i b i l i t y  artifact?  The a p p e a r a n c e o f t h e f o l d s on h i s t o l o g i c a l supports  their  "reai"  s t a t u s , as l i t t l e  appears t o have o c c u r r e d . between f o l d s a r e s t i l l This  i s especially  which j u s t  Often,  i n the f i r s t  this observation  s e a l eye.  through  This  cells  type o f f o l d i n g ,  and p h o t o r e c e p t o r  s h o u l d be q u a l i f i e d .  involving a l l the r e t i n a l  a l m o s t c e r t a i n l y an a r t i f a c t , o b s e r v e any f o l d i n g  shearing of the r e t i n a  connected t o the pigment e p i t h e l i u m .  evident  second type o f f o l d i n g ,  examination  the photoreceptor  involves the outer nuclear  Nevertheless,  living  that i t  layers.  The  layers, i s  owing t o our i n a b i l i t y t o  funduscopic  examination  of the  second type, because o f i t s magnitude,  s h o u l d be c l e a r l y v i s i b l e  i f present.  44  T h a t f o l d i n g o f a n y n a t u r e was p r e v e n t e d a n i m a l and was an a r t i f a c t likely  f o r the f o l l o w i n g reasons.  t h e o u t e r l i m i t i n g membrane cells  i n the prepared  i n the l i v i n g  s e c t i o n s seems  The p r i m a r y  i s to orient  function of  the photoreceptor  i n t h e same m e r i d i a n a s t h e l i g h t p a s s i n g t h r o u g h t h e  retina.  This i s necessary  degree o f v i s u a l a c u i t y , even o b l i q u e l y o r i e n t e d retina,  then the c e l l s  i n order to provide a reasonable  f o r i f they were p e r p e n d i c u l a r o r t o the l i g h t p a s s i n g through the  c o u l d be s t i m u l a t e d o v e r a r e l a t i v e l y  l a r g e r e t i n a l a r e a as opposed t o j u s t t h e diameter segment p o r t i o n o f t h e p h o t o r e c e p t o r c e l l .  of the  Also the e f f e c t  o f h a v i n g o n l y p a r t s o f t h e r e t i n a w i t h r e c e p t o r s i n t h e same m e r i d i a n as t h e l i g h t p a t h  i s the formation of a l t e r n a t i n g  r e g i o n s o f a c u t e and f u z z y v i s i o n , not  a s i t u a t i o n which  seem a d v a n t a g e o u s i n a p r e d a t o r .  to suggest folding), An  a function  type o f f o l d i n g  difficult (gross  a s s u m i n g t h a t i t i s n o t an a r t i f a c t .  e q u a l l y important c o n s i d e r a t i o n i s the problem o f  retinal nutrition the r e t i n a layer  f o r t h e second  I t i s e v e n more  would  c r e a t e d by t h e f o l d i n g .  superficial  The r e g i o n o f  t o and i n c l u d i n g t h e o u t e r p l e x i f o r m  i s t r a n s v e r s e d by b l o o d v e s s e l s o r i g i n a t i n g from t h e  o p t i c n e r v e r e g i o n o f t h e r e t i n a and r e c e i v e s i t s n u t r i t i o n from  these v e s s e l s .  The o u t e r n u c l e a r and p h o t o r e c e p t o r  l a y e r s however, r e c e i v e t h e i r n u t r i t i o n from  through  diffusion  t h e c h o r o i d a l b l o o d v e s s e l s t r a n s v e r s i n g between t h e  pigment e p i t h e l i u m and tapetum  (Fig. 18).  D i s t a n c e from  these  45  vessels by  i s therefore a c r i t i c a l factor.  folding,  too g r e a t  e s p e c i a l l y of the  second type,  to ensure adequate n u t r i t i o n  However, t h e s e o b j e c t i o n s a r e b a s e d p r i m a r i l y on function  an  t o i t , w h i c h as  sufficient  criterion.  have proven f a l s e . before  Cain  Too  this question  may  f u l l y resolved.  obtained  occured. no  Unfortunately,  folds.  their  Their  this The  as  my  than the w i d t h  tapetum c e l l u l o s u m Walls  terminating  to Walls  (1942),  among mammals. uncertain, suggested  specific  i s not  such  conclusions  required  Confirmation  exact  of  in  cellular relationships  i f localized  shearing  Epon-embedded s e c t i o n s  has showed  necessarily rule  out  the d i s t a n c e between f o l d s  was  of the r e t i n a l  sections  cut  technique--  i n d i c a t e d by  cavity,  " r e a l " phemenon  i f f o l d s are observed  absence here does not  e x i s t e n c e , h o w e v e r , as  often greater with  o b s e r v e d t o see  1970).  t o a s s o c i a t e any  i s therefore  s e c t i o n s embedded i n Epon, f o r t h e t h e n be  (McCormick,  i n the past  study  created  w o u l d seem j u s t  (1964) p o i n t s o u t ,  often  be  distance  t o f o l d i n g as a  inability  Further  i t s p r e s e n c e c o u l d be  could  The  i n t h e h a r b o u r s e a l i s as  (1942) and  lines  The  although  According  t h e most e x t e n s i v e  mechanism of t a p e t a l r e f l e c t i o n s e v e r a l have been s u g g e s t e d .  i n t e r f e r e n c e e f f e c t s but  showing t h a t t h i s  the whole v i t r e o u s  j u s t . b e f o r e the r e t i n a does.  t h i s gives pinnipeds  is likely,  s o l e mechanism i s u n l i k e l y .  Pedlar  shows t h a t T h e r e may  extensive  (1963),  is  tapetum still  Walls  has  although  i n t e r f e r e n c e as i n a d d i t i o n be  a  a pigment,  46  although  none h a s  been i s o l a t e d  (1956) h a v e i s o l a t e d but of  no  connections  W e i t z e l ejt a l .  z i n c c y s t e i n e from the  the  c o l o u r of the  tapetum.  states that this  v i s u a l power o f c a r n i v o r e s (1970) p o i n t o u t , when one  i s r e l a t e d to the  i n the dark.  considers  the e x t e n s i v e knowledge r e l a t i n g  conversion  l i g h t energy i n t o n e u r a l  of  If a pigment i s indeed f u n c t i o n as  of r i b o f l a v i n a c t i v a t e d by will  present,  suggested f o r r i b o f l a v i n  erassicaudatus)  re-emitted  retina i s 520  mu  (Pirie,  t h e eye, as  and  be  of the r i b o f l a v i n .  mu,  photochemical  impulses.  i n the  lemur  mu.  same  (Galago  The  fluoresence  and  i s maximally  maximum  These w a v e l e n g t h s  a b s o r b e d by  l i g h t o f 520  the  subserve the  1959) .  445  by  to  i t may  at n e u t r a l i t y ,  l i g h t o f 370  penetrate  Ronald  h o w e v e r , t h a t t h i s w o u l d seem u n l i k e l y ,  e x c i t a t i o n as a c c l o m p l i s h e d  the  t a p e t u m and  owing to the  then  fluorescense  Thus, i f a p i g m e n t i s p r e s e n t ,  i t would  advantageous f o r i t to r e f l e c t p a r t i c u l a r wavelengths  l i g h t w h i c h a r e more s u i t a b l e t o t h e o p t i m u m of  (1967)  increased  Nagy and  photoreceptor  be  Heller  colour  shown t h a t t h i s compound p o s s e s s e s p h o t o e l e c t r i c p r o p e r t i e s ,  h o w e v e r , and  be  s e a l tapetum,  h a v e b e e n shown t o e x i s t b e t w e e n t h e  t h i s m e t a l and  has  to date.  the v i s u a l c e l l s ,  s t i m u l u s r e c e i v e d by  for i t could  sensitivity  therefore increase  the photoreceptor  of  the  cells.  Refraction: jyxy r e f r a c t i v e r e s u l t s those  r e p o r t e d by  Johnson  (Table  1) a g r e e b a s i c a l l y  (1893) and  Piggins  with  (1970) f o r  the  47  harbour  s e a l and harp  s e a l r e s p e c t i v e l y ; b y means o f  retinoscopy, both noted i n a i r , and i n w a t e r , and  c o n s i d e r a b l e myopia and a s t i g m a t i s m  P i g g i n s found moderate  n e g l i g i b l e astigmatism.  Hypermetropia,  hypermetropia or far-sightedness,  results  i f t h e e y e b a l l i s t o o s h o r t , so t h a t t h e accommodation  process  i s inadequate  to p u l l  t h e image f o r w a r d o n t o t h e  r e t i n a and t h e s h a r p  image l i e s b e h i n d  or  i s t h e o p p o s i t e , where t h e e y e b a l l i s  near-sightedness,  t o o e l o n g a t e d a n d t h e image l i e s vitreous cavity.  The t h i r d  i s when t h e r e t i n a l line,  i e . i nthe  and l a s t r e f r a c t i v e e r r o r ,  always  i n the cornea  have l e n t i c u l a r  i n i t s curvature.  This i s  astigmatism or astigmatism r e s u l t i n g  seals i s especially  partly  i n humans, b u t i t i s p o s s i b l e t o  the back o f t h e eye i t s e l f .  from  The l o c a t i o n o f t h e a s t i g m a t i s m  critical,  i t be removed under w a t e r ,  refractive  astigmatism,  image o f a p o i n t i s n o t a p o i n t b u t a  o w i n g t o one o f t h e r e f r a c t i n g s u r f a c e s b e i n g  almost  will  Myopia,  too f a r forward,  c y l i n d r i c a l as w e l l as s p h e r i c a l  in  t h e eye.  as o n l y i f i t i s c o r n e a l owing t o t h e s i m i l a r i t y i n  i n d i c e s between t h e cornea  and water.  T h i s i s why  c o n s i d e r a b l e e f f o r t was made t o q u a n t i f y t h e c o r n e a l a s t i g m a t i s m i n d e p e n d e n t l y o f any w h i c h eye  components.  eye  as a whole, and cannot  of  m i g h t be i n h e r e n t i n t h e o t h e r  Retinoscopy  measures t h e r e f r a c t i o n o f t h e  be u s e d t o d e t e r m i n e  the r e f r a c t i o n  a n y one r e f r a c t i n g s u r f a c e . With r e s p e c t t o Johnson's  results,  (1893) a n d P i g g i n s ' (1970)  owing t o t h e d i f f i c u l t i e s  encountered  by v e r y  competent  48  doctors  i n t h e t a k i n g o f my m e a s u r e m e n t s , t h e a c c u r a c y o f  their results  seems q u e s t i o n a b l e , a s n e i t h e r h a s h a d h i s  r e s u l t s confirmed.  I found  considerable variation  i n values  t a k e n b y d i f f e r e n t d o c t o r s on t h e same s e a l on t h e same d a y and  e v e n b y t h e same d o c t o r w i t h d i f f e r e n t  round r e t i n o s c o p e versus  a slit  retinoscope)  s e a l on t h e same d a y ( s e e T a b l e 1 , S c a r ) . these d i f f i c u l t i e s  instruments  on t h e same  The r e a s o n s f o r  are not certain, but I feel  problems were t h e presence  o f a tapetum,  the upward and h o r i z o n t a l l y  (a  the major  t h e l a r g e e y e , and  (15°) d i r e c t e d o p t i c a x i s , a l l  f e a t u r e s n o t c h a r a c t e r i s t i c o f t h e human e y e w i t h w h i c h t h e y were most e x p e r i e n c e d .  T h e s e f e a t u r e s c o m b i n e d made i t  v e r y d i f f i c u l t t o be s u r e  t h a t c o r r e c t measurements  the o p t i c a l a x i s were taken. no  e f f e c t on eye r e f r a c t i o n ,  refraction  Anaesthesia  appeared t o have  a s we w e r e a b l e t o t a k e t h e  o f one o f t h e s e a l s  v a l u e s were s i m i l a r  along  (Blondie) without  i t ; these  t o l a t e r o n e s t a k e n w h i l e s h e was  anaesth  One f u r t h e r s p e c u l a t i o n a s t o why we h a d d i f f i c u l t y i n determining  the harbour  of the animal's  seal's refraction  nervous state.  i s the influence  I t has been c o n c l u s i v e l y  d e m o n s t r a t e d t h a t among humans, e s p e c i a l l y  children,  t e n s i o n can and does change t h e r e f r a c t i v e  s t a t e o f t h e eye  significantly,  o f t e n up t o 3 d i o p t e r s  (Radler, 1955).  nervous  This  c h a n g e i s t y p i c a l l y one o f i n c r e a s e d m y o p i a , and a p p e a r s t o be r e l a t e d  to the d i f f i c u l t y  i s no r e a s o n  of the task presented.  There  t o suppose t h a t a s i m i l a r r e a c t i o n t o s t r e s s  49  could not occur place while  i n other  animals,  t h e s e a l s were b e i n g  and i f such changes tested, this could  account f o r a part o f the v a r i a t i o n involved  t y i n g the animal  observed.  took  perhaps  Refraction  down and. s h i n i n g a l i g h t  into i t s  e y e s a n d was d e f i n i t e l y b o t h e r s o m e t o a t l e a s t some o f t h e seals. With a P l a c i d o ' s d i s c , Johnson's  (1893) a n d P i g g i n s  of least corneal curvature therefore, as and  I was, h o w e v e r , a b l e t o c o n f i r m  questions  1  i n the h o r i z o n t a l .  Orphie) and a s t i g m a t i s m 1, O p e r a t o r  (Table  1, O p e r a t o r  3, S c a r ) .  suggests  as a l l o t h e r  t h a t t h e l e n s and p o s t e r i o r c u r v a t u r e  v a l u e s are m e a n i n g f u l ,  species.  It i s felt  f o r when c o m b i n e d w i t h  Johnson's measurements f o r t h e h a r b o u r s e a l , provide  they  a t l e a s t an i n d i c a t i v e r e f r a c t i v e v a l u e If this  measurements w i l l  i s indeed confirm  seems p a r t l y c o n f i r m e d  Susie  i n the h o r i z o n t a l are  o f t h e eye a r e n o t s i g n i f i c a n t l y a s t i g m a t i c . that these  1,  Only those measurements showing  t h e r e f o r e c o n s i s t e n t w i t h my o b s e r v a t i o n s , to date  observation  with this axis i n the v e r t i c a l  the a x i s o f l e a s t c o r n e a l curvature  evidence  This  some o f t h e r e t i n o s c o p y m e a s u r e m e n t s ,  some showed n o a s t i g m a t i s m  (Table  (1970) o b s e r v a t i o n o f t h e a x i s  the case,  for this  and o n l y f u r t h e r  i t , then W a l l s '  (1942) t h e o r i z i n g  i n that the astigmatism  e l i m i n a t e d by t h e v e r t i c a l l y  should  constricted pupil.  w i l l be The power  o f a c y l i n d e r i s d i r e c t e d a t 90° t o i t s a x i s o f l e a s t ie.  the horizontal.  However,  the function of  curvature  astigmatism  50  in  the  study be  dioptric of  the  mechanism  bearded  interesting,  as  contrast  to  of  corneal  least  v e r t i c a l ,  if  This  a  of  pupil  f u l l y  ever on  d u l l  behavioural or  than  would  under  widened  astigmatism  diopter  of  myopia  astigmatism) deviation  pronounced In  be  would  then from  a  the  the  the this  the- o p e r a t o r  the taking  be  in  under  the  as  value seal  horizontal, cannot  stenopeic  be  of  as  more  oval  intensity.  light  intensities of  eliminated the  meridian  considered  the by  degree  correct  would  is  light  resulting  aperture  the  my  effects  is  v e r t i c a l  does  rather  of  a  bright  often,  low  the  be  performance  day,  actual  entirely  harbour  a  axis  requires  very  and  in  Its to  the  sunny  under  now  a l l .  conditions  mean  the  at  to  adopts  poorer  effect.  water,  on  1942).  however,  a  the  pupil  not  If  for  in  on  intensities,  hypermetropia  of  present  refractiv  would  expected  condition  depending  pupil.  corneal  any  the  l i k e l y  the  of  and  light  be  approach  occur  under  astigmatism  diopters  as  A  horizontal  (Walls,  Only  days  thus  high  an  this  corneal now  that  approach  shape,  almost  thus is  results.  such  is  pinnipeds would  explained. barbatus,  astigmatism,  experiments,  e l l i p t i c a l  Vision  so  aperture  though,  case  the  not  s l i t  astigmatism  aperature,  pinhole  pupil  other  indeed  s t i l l  Erignathus  curvature  conditions  the  seal,  i t s  of  removal  stenopeic of  that  is  in  of  (0.6  and  8.5  9 diopters ,  lightly, l i k e l y  and  have  . refractive them  was  results unable  to  are  questionable  detect  as  astigmatism  a  51  when t e s t i n g  i n a i r , astigmatism d e f i n i t e l y  by u s e o f a P l a c i d o ' s d i s c . hypermetropia  However, t h e y  w i t h no a s t i g m a t i s m ,  shown t o e x i s t imply moderate  r e s u l t s which  r e a s o n a b l e when c o m p a r e d t o t h o s e o f t h e h a r p 1970)  i n Table  1.  technique, although again,  be  expected.  As t h e s e a r e t h e o n l y underwater  species of phocids  of Johnson phocids  (1893) a n d W a l l s  its refractive  reasons  these  These f i n d i n g s a l s o agree w i t h t h e i n f e r e n c e s  underwater,  (1942) t h a t t h e a s t i g m a t i s m i n for i ti s largely  index  removed  c o r n e a l a s t i g m a t i s m s h o u l d be l o s t w i t h (assuming  i s t h e same a s o t h e r mammals  to that of water).  would thus  refractions  in refraction  the r e d u c t i o n i n e f f e c t i v e n e s s o f the cornea  similar  should  i t would appear t h a t  at least are similar  i s r e s i d e n t i n the cornea,  in water;  (Piggins,  individual variation  c a r r i e d o u t t o d a t e on p i n n i p e d s ,  underwater.  seal  B o t h methods o f measurement a r e comparable  in  two  seem  l i k e l y be p o o r e r  mentioned e a r l i e r ,  i n being  V i s i o n under low l i g h t i n a i r than water  that  intensities  f o r t h e same  as t h e a s t i g m a t i s m would o n l y be  e f f e c t i v e and d e t r i m e n t a l i n a i r . The  q u e s t i o n i s now r a i s e d a s t o why s e a l s  a s t i g m a t i s m a t a l l a n d how t h e y p u t up w i t h s u c h degree as 9 d i o p t e r s . no  1.0 D.,  i t i s found throughout  cetaceans  an e x t r e m e  Among t h e c a r n i v o r e s and h i g h e r  s i g n i f i c a n t astigmatism  although  possess  i s the rule  t o a s m a l l degree,  the orders.  i s i t present  Only  (Johnson,  mammals,  1901),  i e . b e t w e e n 0.5 -  among t h e p i n n i p e d s and  t o s u c h an e x t r e m e d e g r e e .  Matthiesen  52  (Johnson,  1 9 0 1 ) , who h a s r e f r a c t e d s e v e r a l A t l a n t i c  reports a corneal astigmatism  o f between 4 - 4 . 5  among a l l t h e s p e c i e s h e o b s e r v e d . . w o u l d be o r i e n t e d d i f f e r e n t l y as  from t h a t o f the pinnipeds,  hypermetropic.  ( 2-5 d i o p t e r s )  the edentates  Johnson's  excluding considerably  i s characteristically  A s l i g h t degree o f hypermetropia  i s t y p i c a l o f the higher  hypermetropia rodents,  shape.  i n which r e f r a c t i o n v a r i e s  in a l l directions of error, vision  diopter)  diopters  (1901) a l s o show t h a t among mammals,  the domestic s p e c i e s ,  eyes  Presumably, however, i t  their p u p i l i s of a considerably different  observations  cetaceans'  mammals, w h i l e  (under one higher  i s found i n w i l d species o f  and t h e m a r s u p i a l s .  t h e r e f o r e unique, along w i t h the primates,  Pinnipeds are i n possessing  m y o p i a a s a common a n d p e r s i s t e n t c h a r a c t e r . Unfortunately, irregularities t h e s i s confirms  no a c c e p t a b l e  explanation  f o r these  i s presently a v a i l a b l e , f o r although t h e i r presence,  i t i s impossible  t o draw  c o n c l u s i o n s on t h e f u n c t i o n a l s i g n i f i c a n c e o f t h e s e f u r t h e r study.  correct  t h e d e t r i m e n t a l e f f e c t s imposed by a s t i g m a t i s m  primary  errors are s t i l l  function of the s l i t  c u t down t h e i n t e n s e under p h o t o p i c  light  conditions.  when t h e p u p i l e n l a r g e s that these  pupil will  characters  without  myopia, b u t these  A stenopaic  this  functionally  not required.  and  In f a c t , t h e  p u p i l i s presumably simply t o  encountered by t h e r o d - r i c h r e t i n a I t i s under low l i g h t  intensities,  and c e a s e s t o b e f u n c t i o n a l l y  e r r o r s become s e r i o u s .  stenopaic,  Visual  Discriminations:  When t h e to the data  standard  psychophysical  criterion  (the d i f f e r e n c e t h r e s h o l d b e i n g  is applied  defined  v i s u a l angle c o r r e s p o n d i n g to the  75% p e r f o r m a n c e  it  resolve  i s found t h a t both s e a l s could  b e t w e e n two  black  l i n e s as  1.7  m.  Further,  was  the  same i n a i r and  t h e r e was  s m a l l as  f o r each s e a l the i n water  1 mm  i n water.  owing to the  and  This d e v i a t i o n  24)  (1968) h a s  two  is felt  been measured w i t h  summarized the  a black  the  therefore against  represent  than morphologic  their  patterns  used, i n  different  the p a t t e r n  chosen.  states that  "when t h e  oriented  Lit for  subject  is  the presence or absence of illuminated  b a c k g r o u n d becomes e q u i v a l e n t  i n the  sensitivity  of  the  experiment reported  t o t h e d e t e c t i o n o f an  a black background rather  of a separation  stimulus  t a s k o f d e t e c t i n g an  measuring the a b s o l u t e If both seals  to  types of experiments p o s s i b l e  to d i s c r i m i n a t e only  test stimulus..., against  seals  i t i s always n e c e s s a r y to ask what p h y s i o l o g i c a l  m e a s u r i n g v i s u a l a c u i t y and required  although  seals personality differences.  v i s u a l d i s c r i m i n a t i o n s t u d i e s , and  c a p a c i t y has  of  visual discrimination ( F i g . 23  Owing t o t h e w i d e v a r i e t y o f  characteristics,  gap  at a distance  i n d i v i d u a l b e h a v i o u r d i f f e r e n c e s , however, r a t h e r differences,  the  level),  a vertical  a s i g n i f i c a n t d i f f e r e n c e between the  discriminations  as  between the b a r s ,  eye  to that  the  target of  to l i g h t . "  on h e r e i n  were  illuminated  gap  than to the d i s c r i m i n a t i o n t a r g e t s i z e would not  be  54  critical, The  as  t h e r e s o l u t i o n o f t a r g e t d e t a i l was  s m a l l n e s s o f an  object  t h a t can  be  adequate measure o f v i s u a l a c u i t y . acuity  i s defined  as  the  capacity  absolute  than v i s u a l a c u i t y  strict  involved.  i s not  located eye  i n the  to  an  i f visual  of v i s u a l systems  s e n s i t i v i t y of the i n the  detected  Therefore,  d i s c r i m i n a t e f i n e d e t a i l s of o b j e c t s niew, the  not  light  to field  of  rather  s e n s e w o u l d seem t o h a v e  been measured. In t h e  evaluation  sensitivity  to  light  of  the harbour s e a l eye's  i n b o t h a i r and  water,  the  the d i f f e r e n t p h y s i c a l p r o p e r t i e s between the be  ignored.  l i g h t are animal  For  example, the  greater  i n water.  i s away f r o m t h e  the  l o s s of object  the  seals  threshold the  in this  and  a result,  test stimulus,  the  of water.  e f f e c t s of  two  media  cannot  s c a t t e r i n g of further  greater  the  will  to a i r .  be  Since  study were d i s c r i m i n a t i n g at a presumably distance the  less i n water;  eye  i n b o t h a i r and  would t h e r e f o r e  i n t h i s medium t h e  u n i t area would decrease at a greater  square of  the  l i g h t i n water r e l a t i v e  s e n s i t i v i t y of  e x p e c t e d t o be per  As  l e v e l at a standard  absolute  absorption  absolute  the d i s t a n c e  owing to the  H o w e v e r , t h i s was  indicate s i m i l a r values  not  the  be  light intensity  r a t e than  exaggerated  water,  the  properties  case - the r e s u l t s  i n both media.  There are  two  possible 2  explanations: was with  great 1.2  e i t h e r the  enough t o h i d e m being  too  range of any  i l l u m i n a t i o n , 85  d i f f e r e n c e which might  short a distance  to  candela/m , exist,  significantly  55  i n f l u e n c e the r e s u l t s , size,  thereby  (1893) h a s  o r t h e r e was  c o u n t e r - a c t i n g any  suggested t h a t the  the p u p i l opens t o the f u l l ,  an  increase  scattering effects.  latter  occurs,  the  However, no  s e a l eye  significant  i n w a t e r has  possibility,  that this  i s not  been o b s e r v e d .  technique  noted, however, t h a t w i t h  this property  stating  that in  needed i n  change i n p u p i l s i z e  was  case.  testing  two  this  of  first  designed  the  increased  d i f f e r e n c e between the  The  not  t h i s v a r i a t i o n , w o u l d t h u s seem t o be be  Johnson  i . e . , a complete c i r c l e ,  w a t e r , as a c o r r e c t i o n f o r a s t i g m a t i s m medium.  in pupil  to It  detect should  distances  m e d i a c o u l d become  significant. A wide v a r i e t y of t e s t o b j e c t s are a v a i l a b l e f o r measuring v i s u a l a c u i t y , but have d i s c u s s e d  four d i f f e r e n t  r e s o l u t i o n , and  visual acuity is specified of  the v i s u a l angle  a p p e a r s t o be  one  or absence of the o b j e c t definition t a s k as one  As  tasks:  detection,  In each  case,  i n terms of the r e c i p r o c a l v a l u e some d e t a i l o f t h e  discussed  above, the  test  object  task  in  o f d e t e c t i o n , where o n l y the i s asked but  owing to the  broad  o f r e s o l u t i o n , where d i s c r i m i n a t i o n o f a  (1933) h a v e d i s c u s s e d  i s required.  the ambiguity  this  presence  of v i s u a l a c u i t y , i t i s p o s s i b l e to consider  between elements of the p a t t e r n Purdy  L i t (1968)  classification  localization.  formed by  which i s d i s c r i m i n a t e d . study  (1965) and  v i s u a l a c u i t y i n terms of a  system which i d e n t i f i e s recognition,  both Riggs  Wilcox  associated  this  separation and with  56  i  t h i s double l i n e  target  in detail,  and  aspects r e l a t i n g  to the  separation  threshold,  visible two  threshold"  stimulus  i s the  l i n e s of  "minimum v i o i b l c  i n f i n i t e s i m a l width,  the  lines  i s so g r e a t  i n t h e i r width would not  o f a gap  b e t w e e n them.  of the  values not  as  latter,  the  defines  "minimum s e p a r a b l e " , objects  harbour s e a l i n b o t h media  air,  i t has  the  (Johnson, pinhole  1 8 9 3 ) ; an  aperture  like  its slit  p u p i l ever f u l l y  c o n d i t i o n and  s u c h as  i n the  experiments,  o v a l or  elliptical  shape.  intensities,  t h a t u n d e r low  a  such  approach  light  the p u p i l adopts a r a t h e r I f Johnson's  the  However, i t h a s  as  t h i s narrow s l i t  the  pupil  c h a r a c t e r i s t i c s of  light  does the  for  the harbour s e a l i n  i s thus thought to r e s u l t .  a b r i g h t day,  separate,  is astigmatic with  vertical,  approach to the  s e e n as  3).  v i s u a l a c u i t y of eye  as  distance  been o b s e r v e d t h a t o n l y under h i g h o c c u r on  these  of v i s u a l a c u i t y .  for this property  (see T a b l e  being  measure  this difference is  f o r them t o be  b e e n shown t h a t t h e  a x i s o f most c u r v a t u r e  my  i . e . the minimal angular  i n order  further  the r e s o l u t i o n t h r e s h o l d  i s p o s s i b l e to c a l c u l a t e values  In c o n s i d e r i n g  that a  p o s s i b l e to consider  d i s t i n g u i s h e d i n the b r o a d d e f i n i t i o n  b e t w e e n two it  i t is s t i l l  i f one  the  improve the d i s c r i m i n a t i o n  a m e a s u r e o f v i s u a l a c u i t y as  Therefore,  between  threshold  With these d e f i n i t i o n s ,  but  "minimum  whereas  is that resolution  increase  was  the  two  least perceptible distance  threshold"  where the b r e a d t h o f  distinguish  intensities, more  (1893) m e a s u r e m e n t s  57  Table air  III:  and  Estimated  water.  confidence  resolution  Values  in  thresholds  parentheses  refer  for to  both the  seals  95%  limits.  Resolution  Thresholds  t, .  (min.  of  old  male  n j . ^ d f  M  ,  arc)  Air Two-year  in  Water  1.64  (3.35-0.06  2 . 13  • (2. 9 6 - 1 . 30)  0.65  2.13  ( 4 . 1 1 - 0 . 15)  1.32  (2.48-0.16)  0.69  Seals Four-year t  <^>  df  old  female  0.44  2.20  t  P  =  . . .05  «»-df  1.960  58  of  the degree of c o r n e a l  astigmatism  (9 d i o p t e r s )  for  t h e h a r b o u r s e a l t h e n t h i s c a n n o t be  and  any  widening of  short distance  the b a s i s of current  the  poorer  corneal  by  t h e now  Since  my  r e s u l t s showed no  eye  and  widened  T h u s , my  of  the  better  eye  I suggest the  in this  width,  test  i n water,  would not  difference  to l i g h t , not  on  be  as  entirely  i n a i r and rather  not  oval  that  the  that  same i n a i r as t h a t u n d e r low  moderate d i s t a n c e s ,  (and  the  absolute  v i s u a l a c u i t y , was  n e c e s s a r i l y imply  opposite,  water  measured.  the v i s u a l  i n water. light  v i s u a l a c u i t y would  be  in water.  Further  Their  Therefore,  i n a i r than  i t seems m o s t l i k e l y  e x p e r i m e n t s do  i n t e n s i t i e s and  m.  pupil.  a c u i t y of the harbour s e a l i s the Instead,  1.7  t h e p u p i l i n a i r was  hence a s t i g m a t i c ) ,  sensitivity  of  astigmatism  eliminated  discriminations,  lightly,  t h e o r i e s , v i s u a l a c u i t y under the  l i k e l y be  e f f e c t s of the  considered  correct  the p u p i l would have a pronounced e f f e c t  i n a i r , even a t the  c o n d i t i o n s would  are  support that v i s u a l a c u i t y per  study  i s provided  patterns  by  were g r a t i n g s  se was  S c h u s t e r m a n and c o n s i s t i n g of  Balliet  of.8.5'  vitulina  i n water, values  obtained  in this  thesis  and.8.1' were o b t a i n e d considerably (2').  be  i t i s not  i n my for  noted,  t h a t s i n c e a l l t h e s e m e a s u r e m e n t s w e r e made on individuals,  (1970a).  only  in  patterns.  Phoca  d i f f e r e n t from  I t should  measured  lines varying  thereby c o r r e c t i n g the problems inherent  Threshold values  not  those  however, a  few  p o s s i b l e to d e c i d e the degree to w h i c h  59  these values a r e c h a r a c t e r i s t i c o f the species. These v a l u e s , t h e r e f o r e , a r e n o t as s u i t a b l e as a measure o f v i s u a l a c u i t y might have been i n t h e e s t i m a t i o n o f t h e e f f e c t i v e n e s s o f t h e d i o p t r i c mechanism i n b o t h H o w e v e r , t h e y do s u p p o r t  my h i s t o l o g i c a l o b s e r v a t i o n s a n d  i n d i c a t e t h a t the harbour visual  system;  species  s e a l possesses  comparable w i t h other  i n the l i t e r a t u r e .  different definitions,  f o rp a r a l l e l  a s p o i n t e d o u t b y L i t (1968) , a n d  purposes.  lines  F o r example,  (from v a r i o u s s o u r c e s ) , b u t as he  the values a r e meaningless  The  comparative  s e n s e may n o t  thesis.  amphibious p i n n i p e d eye as r e p r e s e n t e d by t h e harbour  i s significantly different I t possesses  a spherical  from the t e r r e s t r i a l lens f o rv i s i o n  a complex d i o p t r i c mechanism f o r a i r .  commonly f o u n d that although  i n t e r r e s t r i a l mammals.  mammalian  i n water,  with  The most u n i q u e  f e a t u r e s a r e h i g h myopia and a s t i g m a t i s m ,  possess  used  f o r accurate  Indeed, v i s u a l a c u i t y i n t h e s t r i c t  have been measured, as i n t h i s  eye.  (Table 4 ) .  t o s p e c i f y t h e p a t t e r n s , d i s t a n c e s and l i g h t i n g  each case,  seal  values  (1942) p r e s e n t s a t a b l e o f t h e v i s u a , l a c u i t i e s o f a f e w  animals  in  that  D i f f e r e n t w o r k e r s may u s e  o f t e n d i f f e r e n t e x p e r i m e n t a l methods  fails  sensitive  I t must b e b o r n e i n m i n d , h o w e v e r ,  these values are not e a s i l y  Walls  a very  i t i s c o m p a r a b l e t o t h a t o f many n o c t u r n a l  (Table 4 ) .  published  media.  characters not  I t s h o u l d be noted  t h e e y e s o f many s m a l l n o c t u r n a l mammals a l s o  spherical  lenses, this  i s an a d a p t a t i o n t o n o c t u r n a l i t y .  60  Table  4.  retinal  s t r u c t u r e f o r a number o f a n i m a l s  ("Walls, 4  A comparison of v i s u a l a c u i t y thresholds  1942;  2  = M c C o r m i c k , 1970;  =Schusterman & B a l l i e t , 7  1961;  =Spong, 1968  N = n o c t u r n a l ; D/N Species  5  =Nagy & R o n a l d ,  = 24 h r . h a b i t ;  # tested  1, 2  Pattern  V.A.T. (min.)  2.7  P.L.  0.440. 83 4.0  Str iae o w l monkey 4 (Aotus t r i v i r g a t u s 3 Str iae Tree shrew 4 (Tupaia g l i s ) 3 Bush baby 4 Striae (Gagalo c r a s s i c a u d a t u s ) 3 Lemur 4 Striae (Lemur c a t t a ) Harbour s e a l i n 1 Striae water4 (Phoca v i t u l i n a ) Harbour s e a l i n a i r 2 and w a t e r (Phoca v i t u l i n a ) '  P.L.  P.L.  S.A.  2  P.L.  l  only)).  D =  = parallel  sources. 1968; 6  ^Pilleri,  diurnal;  lines  15 4.5  D  3.4  D  10  N 2.5  13  0.5  8  8.3  10  2.0  10  4.5  2.5  6.0  D  D/N  D/N D/N  7.5  Pacific white-sided 1 2 P.L. dolphin' (Lagenorhynchus o b l i q u i d e n s )  D N  10  6.0  and  Outer Rows Gangnuclei of lion Inner c e l l s nuclei  4.0  P.L.  2  1970;  1. 0  5 Harp s e a l (Pagophilus groenladicus) 6 Beaver (Castor canadensis) K i l l e r whale (Grampus o r c a )  from v a r i o u s  =0rdy & Samorajski,  (behavioural aspects  Domestic pigeon (Columba l i v i a ) Human, a d u l t  1970a;  3  (V.A.T.)  N 4.5  1  D/N D/N  61  In  these  animals,  to  eye s i z e ,  the lens i s always very  thereby  p l a c i n g t h e o p t i c a l c e n t e r o f t h e eye  deep w i t h i n t h e eye. net  result  large i n proportion  This  i s not the case i n pinnipeds.  The  i n n o c t u r n a l mammals i s a s m a l l b u t b r i g h t image  e q u a l l y good i n t h e p e r i p h e r y o f t h e r e t i n a as i n t h e c e n t e r . This  f e a t u r e i n s m a l l mammals i s a l s o t y p i c a l l y  w i t h a broad cornea field  to maintain  a wide-angled  isdifficult  t o compare t h e a m p h i b i o u s p i n n i p e d eye  t h e a q u a t i c mammalian e y e , f o r a s i n a m p h i b i o u s mammals,  relatively  little  i s known a b o u t t h e i r v i s u a l s y s t e m s a n d  capacities.  Gross eye morphology i s well-documented  1903;  1942) b u t l i t t l e  Walls,  o r g a n i z a t i o n o f t h e eye. reported  The c e t a c e a n  tapetum lucidum,  (Slijper,  eye, however, i s  the c i l i a r y  horizontally  (Mann, 1 9 4 6 ) .  muscles a r e well-developed  almost non-existant  i n mysticetes.  has a greater r e f r a c t i v e  mammals, w h i l e t h e c o r n e a ,  According  f e a t u r e o f the cetacean l e n s and r e t i n a and  index  conducting  Morphologically, i n odontocetes  but are  than  that of t e r r e s t r i a l  a q u e o u s humor, and p r o b a b l y t h e  t o Mann  index as sea water  (1946),  however, t h e main  eye i s t h a t t h e d i s t a n c e between t h e  i s greater  s m a l l e r i n t h e lower  those o f  The l e n s i s s p h e r i c a l  a i t r e o u s humor h a v e t h e same r e f r a c t i v e ( S l i j p e r , ' 1962).  than  1962), as w e l l as cones, a  and a w e l l - d e v e l o p e d  network w i t h i n the r e t i n a  (Putter,  i s known a b o u t t h e s t r u c t u r a l  t o c o n t a i n many r o d s w h i c h a r e b i g g e r  t e r r e s t r i a l mammals  and  visual  ( W a l l s , 1942) . It  to  i n order  associated  i n t h e upper p o r t i o n o f t h e eye  p o r t i o n o f the eye.  The e y e i s t h u s  62  divided of the eye,  i n t o two eye  the  and  parts, with  water  images r e c e i v e d  long-axis.  w a t e r he  terms  This  not  v i s u a l a p p a r a t u s w o u l d be  observations and  that  the  i t i s h a r d t o see  eye,  in that  has  many b a s i c eye  However, such a s t a t e i s d i f f i c u l t  similarities.  has  passed the  i n the  and  to assess,  transition  as  are  opposed t o  the  lens  turtle,  and  the o t t e r ) ,  ( t h e m e t h o d e m p l o y e d by  required, techniques.  The  the  extent  of t h i s a d a p t a t i o n  e l e c t r o n m i c r o s c o p y and  exact n a t u r e of the  for further  study.  aquatic  i n water.  exactly  wringing cormorant, though. are  now  physiological  v i s u a l pigments  a pigment i s present i n the  suggested subjects  "halfway  uncertain.  the  w o u l d seem a m a j o r a c h i e v e m e n t ,  utilizing  whether or not  the  It  an  "efficient"  p r e s e n c e o f a s p h e r i c a l l e n s , as  s t u d i e s on  such a  fragmentary  The  Further  the  i s indeed a q u a t i c a l l y adapted  i t s v i s u a l s y s t e m i s most  involved  owing to  how  t r a n s i t i o n between a t e r r e s t r i a l  what stages' are  and  required.  w o u l d t h u s seem t h a t t h e p i n n i p e d i n the  in a i r  the  This s t r u c t u r a l  c e t a c e a n eye eye  bottom  upper h a l f of  i t w o u l d seem f r o m t h e s e  t h a t the p i n n i p e d  point"  the  b e e n c o n f i r m e d , h o w e v e r , and  of cetaceans,  Nevertheless,  on  a t the  s p e c i a l method o f s e e i n g  "amphyophthalmia".  o r g a n i z a t i o n has visual habits  a i r images r e c e i v e d  tapetum  and  are  63  SUMMARY  This  s t u d y shows t h a t  '  t h e h a r b o u r s e a l eye i s m o r p h o l o g i c a l  d i f f e r e n t f r o m t h a t o f t e r r e s t r i a l mammals a n d a q u a t i c b e i n g a d a p t e d f o r v i s i o n i n b o t h a i r and w a t e r . a  s p h e r i c a l lens  of  excessive  In a i r , a u n i q u e d i o p t r i c mechanism  t h e r e t i n a and p u p i l p r e v e n t t h e need f o r  accommodation.  This  r e f r a c t i v e e r r o r s under h i g h effectiveness  mechanism a l s o  light  as t h e p u p i l e n l a r g e s w i t h  of  light  , was a l s o  completed  type B photoreceptor c e l l s being p o s i t i v e l y i d e n t i f i e d the f i r s t  laterally  time i n a pinniped.  A very extensive  network  c o n d u c t i n g e l e m e n t s was a l s o o b s e r v e d ,  impling  perhaps a high retina.  information  Owing t o t h e v e r y  c e l l s also present, light the  but loses  decreasing  A d e t a i l e d study o f r e t i n a l organization  for  corrects the  intensities,  intensity.  with  I t possesses  t o compensate f o r t h e l o s s i n e f f e c t i v e n e s s  t h e cornea under water.  incorporating  mammals,  processing  a b i l i t y within the  l a r g e number o f t y p e C p h o t o r e c e p t o r  t h e r e t i n a may b e assumed t o b e a l s o  very  s e n s i t i v e , a c o n d i t i o n w h i c h meets t h e r e q u i r e m e n t s o f  d i o p t r i c mechanism; a s e n s i t i v e r e t i n a i s r e q u i r e d f o r  p h o t o p i c v i s i o n owing t o t h e e f f e c t o f the stenopeic  aperture,  as w e l l a s f o r n o c t u r n a l  also  vision.  observed, but t h e i r existence considered  i n the normal l i v i n g  u n l i k e l y on t h e b a s i s  organization  R e t i n a l f o l d s were  animal i s  o f p r e s e n t knowledge.  The  o f the harbour s e a l r e t i n a i s thus b a s i c a l l y  64  similar  to that reported  f o r the harp s e a l  (Nagy and  Ronald,  1970) . R e f r a c t i v e o b s e r v a t i o n s h a v e shown t h a t t h e h a r b o u r eye  i s v e r y m y o p i c and  a s t i g m a t i c i n a i r , and  w i t h no d i s c e r n a b l e a s t i g m a t i s m corneal curvature observations function  and  is horizontal, h i s and  in a i r .  i n water.  This  w i t h t h a t r e p o r t e d by  Walls'  h i s t o l o g i c a l observations. d i s c r i m i n a t i n g a 1 mm and  water.  supports acuity  The  gap  (1942) t h e o r i z i n g as  i n the s t r i c t  least (1893)  (1970) f o r t h e h a r p  to l i g h t ,  to i t s  i s a l s o i n agreement  as  seal.  the r e l a t i v e l y suggested  by  a t a d i s t a n c e o f 1.7  that v i s u a l  s e n s e was  m  of  i n both a i r  i n both  s e n s i t i v i t y and  measured.  high  the  Both s e a l s t e s t e d were capable  s i m i l a r i t y of these r e s u l t s  the contention  axis of 1  Behavioural experiments a l s o confirmed s e n s i t i v i t y o f t h e h a r b o u r eye  hypermetropic  confirming Johnson s  latter observation Piggins  The  seal  media not  visual  65  G l o s s a r y o f Terms  cycloplegia  - p a r a l y s i s o f the c i l i a r y muscle;  paralysis of  accommodation. diopter  - t h e r e f r a c t i v e power o f a l e n s w i t h  o f one meter refractive  distance  (assumed a s a u n i t o f m e a s u r e m e n t f o r  power).  d i o p t r i c mechanism - p e r t a i n i n g involved  a focal  t o t h e eye  structures  in refraction  f u n d u s c o p i c examination - examination o f t h e fundus o f the eye keratometer the  - an i n s t r u m e n t f o r m e a s u r i n g  cornea  Placido's  disc - a disc with  used  i n examining  retinoscopy  - an o b j e c t i v e  diagnosing,  slit  c i r c l e s marked on i t  or opening.  method f o r i n v e s t i g a t i n g ,  and e v a l u a t i n g  by p r o j e c t i o n observation  concentric  the cornea.  s t e n o p e i c - having a narrow  on  the curves of  r e f r a c t i v e e r r o r s o f the eye,  o f a beam o f l i g h t  i n t o t h e eye and  o f t h e movement o f t h e i l l u m i n a t e d  the r e t i n a l  surface  eye o f t h e emergent  area  and o f t h e r e f r a c t i o n by t h e  rays.  66  REFERENCES  A b e r c r o m b i e , M.,  1946,  microtome  sections.  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