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Factors affecting the distribution and abundance of two species of beach crab : Hemigrapsus oregonensis… Low, Charles James 1970

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FACTORS AFFECTING THE DISTRIBUTION AND ABUNDANCE OF TWO SPECIES OF BEACH CRAB, HEMIGRAPSUS OREGONENSIS AND HEMIGRAPSUS NUDUS by CHARLES JAMES LOW B.S.A., U n i v e r s i t y o f Guelph, 1967 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n the Department of ZOOLOGY V/e a c c e p t t h i s t h e s i s as con f o r m i n g to the r e q u i r e d s t a n d a r d THE UNIVERSITY OF BRITISH COLUMBIA November, 1970 In presenting this thesis in partial fu1filment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the Head of my Department or by his representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of The University of British Columbia Vancouver 8, Canada Date /- 7c ABSTRACT i i Hemigrapsus oregonensis and Hemigrapsus nudus, two beach crabs common on the P a c i f i c Coast of North America, show con-s i d e r a b l e v a r i a t i o n i n numbers, and species dominance from place to place. To determine why t h i s should be so, a number of experiments were performed., and observations made to de-termine what are the morphological, and p h y s i o l o g i c a l d i f f e r -ences between the two species, and how the crabs would be aff e c t e d by the d i f f e r e n t p h y s i c a l c o n d i t i o n s p r e v a i l i n g i n d i f f e r e n t places. In general, i t appeared that H. nudus could not t o l e r a t e muddiness of the substrate, while H. oregonensis tended to be eliminated from clean areas by predators. The abundance of the crabs appears to be c o r r e l a t e d with the amount of cover a v a i l a b l e . i i i TABLE OF CONTENTS PAGE ABSTRACT i i LIST OF TABLES i v LIST OF FIGURES v i i ACKNOWLEDGEMENTS v i i i INTRODUCTION 1 NATURAL HISTORY 4 BEHAVIOR 7 EXPERIMENTAL RESULTS 10 OBSERVATIONS ON DISTRIBUTION 11 EXPERIMENTS 13 FACTORS AFFECTION DISTRIBUTION: OXYGEN 13 FACTORS AFFECTING DISTRIBUTION: MUD 17 FACTORS AFFECTING DISTRIBUTION: SETTLEMENT . . . .21 FACTORS AFFECTING DISTRIBUTION: COMPETITION FOR COVER 22 FACTORS AFFECTING DISTRIBUTION: PREDATION. . . . .24 FACTORS AFFECTING DISTRIBUTION: SALINITY 26 FACTORS AFFECTING DISTRIBUTION: DESICCATION. . . .26 FACTORS CONTROLLING ABUNDANCE 28 DISCUSSION 30 CONCLUSION 34 BIBLIOGRAPHY 70 i v LIST OF TABLES TABLE PAGE I Reducing power of substrate samples, c l a s s i f i e d by the s p e c i e s of crab which dominated i n the census sample 35 I I Counts of the number of crabs reaching the surface of the water per hour. 37 I I I Counts of the number of crabs reaching the surface of the water per hour 38 IV Number of each species dead, presumably of s u f f o c a t i o n , r e l a t i v e to t h e i r choice to remain under cover, or to respond to a h o r i z o n t a l oxygen gradient 39 V Time to death of each member of p a i r s of crabs, matched f o r s i z e , i n water where most of the oxygen was removed by bubbling n i t r o g e n , i n f l a s k s which were corked to exclude a i r a f t e r the a d d i t i o n of the crabs. A l VI (A) Comparison of s u r v i v a l time w i t h i n matched p a i r s of H. oregonensis and H. nudus i n corked one l i t e r f l a s k s , without mud 42 (B) Comparison of s u r v i v a l time of matched p a i r s of H. oregonensis and H. nudus i n corked one l i t e r f l a s k s , with mud A3 (C) Comparison of s u r v i v a l times of H. nudus, with and without mud, i n corked one l i t e r f l a s k s kh V TABLE PAGE VI (D) Comparison of survival times of H. oregonensis i n corked one l i t e r f l a s k s with and without mud 45 VII Survival of crabs caged i n the f i e l d , i n areas with d i f f e r e n t substrate p a r t i c l e size d i s t r i b u t i o n s . . . . . . . . . 4 6 VIII Total number of crabs on di f f e r e n t substrates, counted 13 times for 7 days 48 IX Proportion of a l l crabs on beaches made up of those with a carapace width l e s s than 7 mm., r e l a t i v e to the dominance of the beach by the adults of the two species .49 X (A) Number of crabs out from under l i m i t e d cover. 200 H. oregonensis present 50 (B) Number of crabs out from under cover. 200 H. nudus present 50 (C) Number of crabs out from under cover. 100 of each species present 51 (D) Comparison of number of crabs out from under cover by species 51 XI Crab taken f i r s t when pairs of crabs, not matched for size, were thrown into a tank with an assortment of crab-eating f i s h 53 XII Crab predators. Species of birds and f i s h examined for crabs eaten 54 v i TABLE PAGE XIII Tolerance of the two species for low s a l i n i t y 57 XIV Time to death of the two species i n fresh (tap) water. 58 XV Time to death, i n hours, of the two species when kept i n dry dishpans 59 XVI Regression on crab census data, with the number of crabs as the dependent variable, and as independent variables; the reducing power of the substrate (0), the percent cover (R), the surf (S), the available food (F), the height of the sample on the beach (H), and the s a l i n i t y (N) 60 XVII Comparison of the number of crabs found i n plots 1, 2 and 3, before and af t e r rocks were removed from plot 1 and added to plot 2. . . . . .61 v i i LIST OF FIGURES FIGURE PAGE 1 Graph of % H. nudus against reducing power of the substrate 62 2 Change i n d i s t r i b u t i o n of H. nudus and H. oregonensis over time at d i f f e r e n t oxygen concentrations 63 3 Change i n d i s t r i b u t i o n t o t a l s of the two species with oxygen tension 65 A Size d i s t r i b u t i o n of H. nudus from San Juan Island, June, 1969 66 5 Size d i s t r i b u t i o n of H. oregonensis from Spanish Banks, February, 1968 . . . .67 6 Number of crabs l e f t a l i v e after addition of 30 of each species to a tank with predatory f i s h and cover 68 MAP 69 v i i i ACKNOWLEDGEMENTS I wish to thank Dr. J . R. E. Harger f o r h i s advice, c r i t i c i s m and patience, without which t h i s work could not have been c a r r i e d out. I a l s o would l i k e to express my a p p r e c i a t i o n f o r the i n t e r e s t shown i n t h i s work by Miss 0. Johannson, Miss S. Behrens, Mr. P. Breen and Mr. J . Stimpson. This work was supported i n part by a N a t i o n a l Research Council bursary, and by a U n i v e r s i t y of B r i t i s h Columbia f e l l o w s h i p . 1 INTRODUCTION Hemigrapsus nudus and Hemigrapsus oregonensis are small i n t e r t i d a l crabs which are widely distributed on the P a c i f i c North American Coast and occur, usually together, on beaches in the Puget Sound-Strait of Georgia area. The range of H. nudus. according to Schmidt ( 1921 ) , i s from Sitka, Alaska, to the Gulf of C a l i f o r n i a . H. oregonensis ranges from Prince Wil-liam Sound, Alaska, to the Gulf of C a l i f o r n i a . These crabs are found i n abundance on beaches covered with loose rock ranging i n size from cobbles to boulders. H. nudus i s found alone, i n small numbers, on s o l i d rock faces, where there are crevices for the crabs to shelter i n . H. oregonensis i s found alone on sheltered mud and sand f l a t s , where the sub-strate contains enough fine material to allow the crabs to dig holes. In both habitats, the crabs are found i n low numbers, usually on the order of 0.1 crabs per square meter. On beaches with loose rock, there can be as many as 500 crabs per square meter. Beaches where H. nudus i s dominant tend to have l i t t l e or-ganic and fine material i n the substrate, while where H. ore-gonensis i s the more numerous the beach i s usually muddy, with much organic material. The water above these muddy beaches i s usually quite turbid, even though there i s usually l i t t l e surf. Above the clean, sand and s h e l l beaches where H., nudus i s domi-nant, the water i s usually clear, even though there may be considerable surf. The muddy beaches dominated by H. oregonensis 2 tend to have a l o t of reduced m a t e r i a l i n the substrate. Under the surface, the muck i s black, and u s u a l l y smells s t r o n g l y of hydrogen sulphide. Where H. nudus i s the more common crab, as a r u l e there i s l e s s black m a t e r i a l , and that i s u s u a l l y a l o t deeper than i s the case on H. oregonensis dominated beaches. Dominance of a beach by one or the other of the two species i s unmistakeable. I t i s usual f o r one of the two species to make up 80% or more of a l l the crabs present. However, i n some instances i t i s p o s s i b l e that one species w i l l be replaced by the other i n a very short d i s t a n c e . Both species are found almost e x c l u s i v e l y between high and low t i d e l e v e l s . E i t h e r species can occupy the whole i n t e r -t i d a l , but where they are found together, the center of abun-dance of H. nudus w i l l be above that of H. oregonensis. Where H. nudus i s dominant, H. oregonensis w i l l be confined to a narrow s t r i p along the bottom of the beach, while where H. ore-gonensis i s dominant, H. nudus w i l l be confined to a narrow s t r i p along the top of the beach. In the narrow s t r i p s to which the subordinate species i s confined there are almost a l -ways many of the dominant species as w e l l . In these areas where a few of the r a r e r species e x i s t , at the top, or bottom of the beach, as the case may be, the number of crabs per u n i t area i s much l e s s than i n the center of the beach, where the dominant crab w i l l maintain a high population dens i t y . Thus, while both species are capable of l i v i n g from the top to the bottom of the i n t e r t i d a l , they both p r e f e r , i n the sense that the greatest populations are maintained, to l i v e 3 near the center of the i n t e r t i d a l , on beaches with a good cover i n g of loose rock. This study was undertaken to attempt to understand the reasons f o r the observed d i s t r i b u t i o n s and varying abundances shown by the crabs. NATURAL HISTORY The major d i f f e r e n c e s between the two species of Hemigrap-sus i s i n s i z e and h a i r i n e s s (Schmidt, 1921). H. nudus i s the l a r g e r of the two, ranging i n s i z e up to about mm., and has no h a i r oh the l e g s , while the h a i r i n the b r a n c h i a l openings i s sparse and coarse. H. oregonensis i s smaller, seldom excee-ding 35 mm., and has h a i r along the outer edges of the l e g s . I t a l s o has a dense mat of f i n e setae i n the openings to the b r a n c h i a l chambers. According to Knudsen (196A)> H. nudus females produce more eggs than those of H. oregonensis. The mean production of eggs by H. nudus i s 13,000 per female, while H. oregonensis females produce 7,650 each. The t i m i n g of reproduction d i f f e r s , w ith H. nudus females c a r r y i n g eggs, i n t h i s r e g i o n , i n A p r i l and May. H. oregonensis females carry eggs from mid-May to August. According to Hart (1935)> H. nudus eggs are s l i g h t l y l a r -ger than are those of H. oregonensis, and the l a r v a e s i m i l a r l y are l a r g e r . Apart from t h i s , the l a r v a e are morphologically very s i m i l a r . Both species go through one pre-zoeal stage, f i v e zoeal stages, and one megalopal stage before metamorpho- ; sin g to the f i r s t true crab stage. The time from hatching to the f i r s t true crab stage i s about f i v e weeks f o r both species. Knudsen (op. c i t . ) i n d i c a t e s that the food of the two species i s nearly i d e n t i c a l . He re p o r t s that a sample of 35 H. nudus, taken from Puget Sound, were found to have eaten mainly plant m a t e r i a l , i n c l u d i n g s e s s i l e diatoms and desmids, along with some green algae. They were a l s o found to have eaten a small amount of animal m a t e r i a l , and a few sand g r a i n s . A sample of 12 H. oregonensis had been-eating the same thi n g s , though he did not f i n d any animal t i s s u e . Both e x h i b i t much the same tolerance f o r p h y s i c a l f a c t o r s . Knudsen (op. c i t . ) r e p o r t s that both species stop feeding at temperatures below 6.9°C., and Dehnel ( i 9 6 0 ) shows a depres-sion, of r e s p i r a t i o n r a t e at low temperatures which i s . s i m i l a r f o r both species. R i c k e t s and C a l v i n and Hedgepeth ( 1 9 6 2 ) s t a t e that H. nudus i s somewhat more r e s i s t a n t to d e s i c c a t i o n , while H. ore-gonensis i s s l i g h t l y more t o l e r a n t of low s a l i n i t i e s . The crabs are preyed upon by a number of b i r d s , animals, and f i s h . P. A. Dehnel (pers. comm.) s t a t e s that r o b i n s (Turdus  mig r a t o r i u s ) eat crabs while foraging i n the i n t e r t i d a l . D.. Hatle r (pers. comm.) says that mink (Mustela vison) dive f o r them at high t i d e . M. West (pers. comm.) says that racoons (Procyon l o t o r ) w i l l t u rn over small stones f o r crabs, while C. Se.cor (pers. comm.) reported that a sample of racoon feces was made' up almost e n t i r e l y of crab remains. J . Ward (pers.; comm.) says that they are part of the d i e t of the glaucus wing g u l l (Larus glaucescens). Clemens and Wilby ( I 9 6 D report that crabs make up part of.the food of a number of f i s h , i n c l u d i n g the b i g skate (Raja b i n o c u l a t a ) , the diamond s t i n g r a y (Dasyatis  dipteruru's) T the p a c i f i c cod (Gadus macrocephalus), the p a c i f i c h a l i b u t (Hippoglosus s t e n o l e p i s ) , the lemon sole (Parophrys'"- . v e t u l i s ) , the rock sole (Lepidpsetta b i l i n e a t a ) , the s t a r r y 6 flounder ( P l a t i c h y s s t e l l a t u s ) , the cabezon (Sorpaenichthys  marmoratus), the red i r i s h l o r d (Hemilepidotus hemilepidotus), the b u f f a l o s c u l p i n (Enophrys b i s o n ) , and the staghorn s c u l p i n (Leptocottus armatus). I have not l i s t e d those f i s h which Clemens and Wilby say eat "crustaceans" as opposed to "crabs," which may or may not inc l u d e the crabs i n question, though I know from personal experience that other f i s h do i n f a c t eat them r e g u l a r l y , e s p e c i a l l y the kelp g r e e n l i n g (Hexagrammos  decagrammos). According to Cottam (1939)» & number of d i v i n g ducks r e g u l a r l y make use of the two species f o r food. These i n c l u d e the greater scaup (Aythya m a r i l a ) , the l e s s e r scaup (Aythya  a f f i n i s ) , the common goldeneye (Bucephala c l a n g u l a ) , the bar-rows goldeneye (Glaucionetta i s l a n d i c a ) , the bufflehead (Bucephala a l b e o l a ) , the o l d squaw (Clangula hymenalis), the harlequin duck ( H i s t r i o n i c u s h i s t r i o n i c u s ) . the white winged scoter ( M e l a n i t t a d e g l a n d i ) t and the su r f scoter (Oidemia  n i g r a ) . 7 BEHAVIOR In the course of the study, many behavioral sequences were observed. Because an a p p r e c i a t i o n of the behavior provides a b e t t e r understanding of the experimental r e s u l t s , these obser-v a t i o n s are summarized here. The response of the crabs to other i n d i v i d u a l s of t h e i r own or the other species i s quite v a r i a b l e . In the l a b , where cover was not provided, moulting crabs were often k i l l e d and eaten by other crabs i n the tanks. However, apart from t h i s , the crabs were never observed to k i l l each other, though a considerable amount of a g o n i s t i c behavior was observed. In these a g o n i s t i c encounters, there was no sign that the crabs responded d i f f e r e n t l y to other crabs of d i f f e r e n t species. They would f i g h t members of t h e i r own species as r e a d i l y , and i n the same manner, as crabs of the other species. The normal sequence of events i n one of these encounters i s as f o l l o w s . A crab would be i n the normal r e s t i n g p o s i t i o n , with i t s hindmost p a i r of l e g s touching a v e r t i c a l or overhang-i n g surface, which i t seemed to regard as "cover." Another crab would approach, normally walking along the edge of the cover, maintaining contact with the cover w i t h i t s hind l e g s , which were r a i s e d o f f the substrate f o r the purpose. When i t came w i t h i n about 0 .5 cm. of the s t a t i o n a r y crab,, that crab would launch an attack, making stabbing motions with the chelae, and o c c a s i o n a l l y b r i e f l y grasping-one l e g of the encroacher. The l a t t e r would respond s i m i l a r l y , and a f t e r a few seconds, . 8 one of the crabs would r e t r e a t . . U s u a l l y , a l a r g e r crab could d r i v e o f f a smaller one, but i f the two v/ere c l o s e l y matched i n s i z e , a male would d r i v e o f f a female. When cover was i n short supply, and when the crabs were crowded, they would not often show t h i s behavior, but u s u a l l y would seem to look on each other as something, to s h e l t e r under. This behavior pro-duced p i l e s of crabs i n the corners of the tanks. As a r u l e , the approaching crab would crawl under the one which was not moving. The crabs responded to food as though they could detect i t at some distance, provided that both the food and the crab were under water. This response appeared to be to a chemical stimulus, r a t h e r than to a v i s u a l one. The response u s u a l l y followed t h i s p a t t e r n : the food would be put i n the water; a l a t e n t period would occur, apparently while the scent of the food d i f f u s e d to the crab, then the crab would come running towards the food with i t s mouthparts gaping. I t would then sieze the food and tear i t i n t o pieces small enough to eat. Where there was no d i f f u s i o n of scent, as when e i t h e r the crab or the food v/ere out of water, there was no r e a c t i o n . The food had to be capable of e m i t t i n g a chemical stimulus. While broken barnacles and mussels v/ere' among the crabs' f a v o r i t e foods, i n that they produced the most a c t i v e response, i n t a c t mussels and barnacles, as a r u l e , produced no response at a l l , unless they were very small. I f they were small, and i f the crab happened to touch them, i t would t r y to break the s h e l l . Small amphipods and isopods appeared to be detected because of 9 t h e i r motion. I f the small animal were r e s t i n g on the bottom, crabs would walk r i g h t over them without appearing to n o t i c e them. However, i f they happened to swim by a crab, the crab would grab at them, and i f i t succeeded i n catching the smaller animal, i t would tear i t up and eat i t . The crabs are st r o n g l y t h i g m o t a c t i c . They p r e f e r r e d to maintain contact with cover with t h e i r r a i s e d hind l e g s at a l l times, whether they were moving or not. This became very apparent when there was a l a y e r of sediment on the bottom of the tank. The crabs would keep a path along the sides of the tank clean, by c o n t i n u a l l y walking along the w a l l s , while i n the middle of the tank, there would be an undisturbed l a y e r , with an occasional set of crab t r a c k s c r o s s i n g i t . They would accept as cover anything they could touch with t h e i r r a i s e d hind l e g s , though they seemed to p r e f e r cover which had an overhang under which they could hide. As a r u l e , H. nudus appeared more s t r o n g l y thigmotactic than H. oregonensis. This behavior has obvious adaptive s i g n i f i c a n c e , i n that i t keeps the crabs i n close contact with cover under which to s h e l t e r i n case of an attack by a predator. 10 EXPERIMENTAL RESULTS Since what f o l l o w s i s long and s e q u e n t i a l , a paragraph w i l l be devoted to o u t l i n i n g the experiments which were per-formed. A number of experiments were done to i n v e s t i g a t e d i f f e r -ences between the two species w i t h respect to t h e i r a b i l i t y to detect and r e a c t to gradients i n oxygen t e n s i o n . Then, e x p e r i -ments were performed to see whether there was a d i f f e r e n c e be-tween the two species i n t h e i r a b i l i t y to su r v i v e low oxygen tensions. The i d e a that mud a f f e c t e d the v i a b i l i t y of the crabs i n the presence of low oxygen tensions to a d i f f e r e n t extent i n the two species was t e s t e d , as was the i d e a that s u f f i c i e n t l y low oxygen tensions could develop normally i n the usual h a b i t a t of the crabs to a f f e c t v i a b i l i t y . Experiments were set up to i d e n t i f y i n t e r s p e c i f i c competition f o r cover, and to determine which of the two species would be d i s p l a c e d i n such competition. An experiment was performed to see whether the crabs showed d i f f e r e n t preferences f o r types of substrate. A number of areas were examined to i d e n t i f y the d i f f e r e n c e s between beaches which were occupied mostly by H. nudus, and those which were dominated by H. oregonensis. An attempt was made to determine whether dominance of beaches by one or the other of the two species could be due to d i f f e r e n t i a l settlement. To determine the e f f e c t of predation on the crabs, a number of experiments were enacted. The e f f e c t s of s a l i n i t y and d e s i c c a t i o n on the crabs were examined i n a s e r i e s of , , m o r t a l i t y n t e s t s . The 11 e f f e c t s of a number of environmental f a c t o r s on the abundance of the crabs v/as tested by using a m u l t i p l e r e g r e s s i o n analy-s i s on f a c t o r s measured or estimated during censuses of the crabs. F i n a l l y , cover v/as manipulated d i r e c t l y to t e s t the e f f e c t of changes i n cover a v a i l a b i l i t y on the number of crabs present. OBSERVATIONS ON DISTRIBUTION ' •; Seventeen beaches were i d e n t i f i e d as being "H. nudus domi-nated" or "H..oregonensis dominated." On these beaches, a l l crabs occupying a s t r i p , one h a l f meter wide, the f u l l height of the beach were c o l l e c t e d . This s t r i p was defined by t y i n g a s t r i n g at the top of the beach £where the barnacles stopped)-and s t r e t c h i n g i t to the bottom of the beach (where the l a m i -n a r i a , which are u s u a l l y permanently submerged, started) and again f a s t e n i n g i t . A s t i c k , one h a l f meter long, and notched i n the middle was then run down the s t r i n g . Any rock with more than h a l f i t s projected area w i t h i n the s t r i p so defined was turned over and the crabs under i t c o l l e c t e d , counted, sexed, and measured. The s t r i p s were d i v i d e d i n t o "samples," each f i v e meters long, and separate records were kept 'for each sam-p l e . From each sample, enough substrate was c o l l e c t e d , by scraping to a depth of approximately 0 . 5 cm, to f i l l a 6 f l u i d ounce ( 1 5 6 cc.) j a r . At the same time, v i s u a l estimates v/ere made;of the amount of cover on the beach, using as 100% a u n i -form l a y e r of rocks completely covering the beach, one stone deep; the food p o t e n t i a l l y a v a i l a b l e to the crabs i n the form 12 of other organisms; the amount of s u r f t h at the beach normally experienced, and the height of the middle of the sample on the beach. The c l a r i t y of the water was measured a t the time of sampling by s e c c h i d i s k readings,. and the s a l i n i t y , as d e n s i t y , measured with a hydrometer. In the l a b , the r e d u c i n g power of the sample of s u b s t r a t e was measured, u s u a l l y the day a f t e r c o l l e c t i n g i t . T h i s was done by p u t t i n g the samples of sub-s t r a t e i n t o one l i t e r f l a s k s , f i l l i n g those f l a s k s , and one e x t r a , the c o n t r o l , with c l e a n , aerated seawater, then c o r k i n g the f l a s k s so as to exclude a l l bubbles. A winkler a n a l y s i s was made on the oxygen present i n the water when i t was added, and another on each f l a s k a f t e r one hour. The d i f f e r e n c e be-tween the i n i t i a l value and the f i n a l value i s recorded as the r e d u c i n g power. At the same time, the t u r b i d i t y of the water with the samples of s u b s t r a t e was recorded on a s c a l e of from one, water which was completely c l e a r a f t e r the hour, to s i x , v/ater which was s t i l l extremely t u r b i d . I t was noted that water which was very t u r b i d tended to have a h i g h r e d u c i n g power, while water which c l e a r e d q u i c k l y tended to have l i t t l e oxygen removed by the sample (see Table I ) . At the same time, the r e d u c i n g power was c l o s e l y r e l a t e d to the s p e c i e s of crab which was most abundant on the beach. Where there were more H. nudus than H. oregonensis, the average r e d u c i n g power was 0 . 4 3 6 p a r t s per m i l l i o n of oxygen removed from one l i t e r of water per hour, while where H. oregonensis was more common, the average r e d u c i n g power was 1 . 9 2 3 p a r t s per m i l l i o n and the d i f f e r e n c e i s s i g n i f i c a n t (see Table I ) . 13 However, the r e l a t i o n s h i p between the r e d u c i n g power of the s u b s t r a t e and the percent of the crabs present made up by one or the other of the two s p e c i e s was not l i n e a r , but i n d i c a t e s a t h r e s h o l d e f f e c t (see F i g u r e 1 ) . T h i s shows that with a reducing power of below 1.3 ppm., the beach i s l i k e l y to be dominated by H. nudus, while with higher r e d u c i n g powers, H. oregonensis i s l i k e l y to be more common. With values between 0.7 and 1.5 p a r t s per m i l l i o n per hour, there i s u n c e r t a i n t y as to which crab w i l l dominate a s e c t i o n . However, these s e c t i o n s with i n t e r m e d i a t e v a l u e s of reducing power were a l l on beaches where the other s e c t i o n s f e l l i n t o e i t h e r the higher or lower c a t e g o r i e s of r e d u c i n g power. These other s e c t i o n s were dominated by the crab nor-mally found i n that range of r e d u c i n g power. Beaches tended to be muddy at the bottom, with a high r e d u c i n g power, while at the top, they tended to be sandy, and have a low r e d u c i n g power. In these ( h i g h or low) s e c t i o n s the subordinate crab kept a f o o t h o l d , but there were u s u a l l y some of the dominant species i n these areas as w e l l . EXPERIMENTS FACTORS AFFECTING DISTRIBUTION: OXYGEN To i n v e s t i g a t e the r e a c t i o n of the two s p e c i e s of oxygen gr a d i e n t s , and to determine i f there were a d i f f e r e n c e between the two s p e c i e s with r e s p e c t to t h e i r a b i l i t y to d e t e c t and r e a c t to these g r a d i e n t s the f o l l o w i n g experiments were set up. Ik Ten crabs of each species were put i n t o a bucket of clean seawater approximately 18 cm. deep. Cover, i n the form of a piece of p l a s t i c screen, was provided i n the bottom of the bucket, while another piece of screen was suspended so that i t extended from the bottom of the bucket to above the surface of 'the' water. . The crabs were able to climb t h i s v e r t i c a l screen. The crabs were then l e f t undisturbed f o r four hours, during which time they apparently l a r g e l y depleted the d i s s o l v e d oxy-gen i n the water. During t h i s time, they a l l stayed at the bottom of the bucket, under the cover. A f t e r t h i s time, Climb-i n g began. Counts were made of the number of crabs reaching the surface at short i n t e r v a l s (see Table I I ) . The r e s u l t s i n d i c a t e d that H. oregonensis climbs more than does H. nudus, showing a greater avoidance of the low oxygen area at the bottom of the bucket. To el i m i n a t e a p o s s i b l e e f f e c t of i n t e r s p e c i f i c d i s p l a c e -ment from cover, t h i s experiment was repeated, using two buckets, each w i t h 2 0 crabs of one species. To make sure that i t was, i n f a c t , oxygen d e p r i v a t i o n which was causing the crabs to ; climb, n i t r o g e n was bubbled through the water i n both buckets. This has the e f f e c t of removing a l l d i s s o l v e d gasses from s o l u -t i o n except n i t r o g e n . The number of crabs reaching the surface was again recorded (see Table I I I ) . Again, more H. oregonensis climbed out per u n i t time than H. nudus.* This experiment was terminated by the onset of m o r t a l i t y among the crabs. In the l a s t hour of the experiment, 8 H. nudus and 2 H. oregonensis di e d , presumably of s u f f o c a t i o n . This experiment i n d i c a t e s ' 15 that H. oregonensis responds more actively than H. nudus to an oxygen gradient. Since some H. nudus were seen to climb, i t would seem that they were not kept at the bottom by an i n a b i l -i t y to reach the surface. Other experiments detailed below show that H. nudus i s able to detect and react to oxygen gra-dients. Therefore, since there was a significantly greater mortality among the H. nudus, i t was concluded that the H. nudus was forced to suffocate rather than enact the appropriate behavior to leave cover in response to the oxygen gradient. In these two experiments, the crabs were responding to two gradients. The oxygen defi c i t at the bottom of the bucket tended to force them to climb to the surface, while the cover provided, in association with their thigmotatic response apparently induced them to remain at the bottom.^ To try to isolate the effect of low oxygen alone on the crabs, a hori-zontal gradient was set up, with the only cover being the wall of the tank. To produce this gradient, a holding tank, 122 by 70 by 30 cm. was divided lengthwise into two troughs. Each of these was divided into /f sections by soft polyethylene sheets, which ex-tended from the top of the trough to the bottom, but were attached only to the walls, thus acting as effective barriers to water circulation, but flexible enough that the crabs were able to squeeze under them. Water was added to a depth of 15 cm., and an oxygen gradient was established by bubbling a i r at one end of the trough, and nitrogen at the other. When the gradient stabilized, at 1.5> 2 . 0 , 5*6, and 6.8 parts per 16 m i l l i o n of oxygen i n the successive s e c t i o n s , 2 crabs of each species were added to each s e c t i o n . Then counts were made at 15 minute i n t e r v a l s f o r f i v e hours of the crabs i n each sec-t i o n . The number of crabs, averaged f o r the two troughs, i s p l o t t e d by hours f o r each of the oxygen tensions i n Figure 2 . ^ I n the lower oxygen tensions, H. oregonensis q u i c k l y reaches a greater density than H. nudus, while i n the opposite ends of •fe-ttle troughs, H. nudus seems to dominate. The t o t a l s over time of a l l counts are p l o t t e d i n Figure . 3 against oxygen tension. Again, i t i s apparent that H. ore-gonensis i s i n greater abundance i n the low oxygen tension areas, while.H. nudus dominates the high oxygen s e c t i o n s . ^ ^ That H. nudus w i l l remain under cover even i n the face of l e t h a l l y low oxygen concentrations was shown by an accident' which occurred i n the course, of an experiment not otherwise :> t r e a t e d i n t h i s paper/^(The troughs described i n experiment three was modified by the a d d i t i o n of stones to one end s e c t i o n i n each trough so the a i r was bubbling among the stones i n one trough, and at the opposite end to the stones i n the other_^) The nitrogen was turned o f f , and 114 H. nudus and 84 H. ore-gonensis were added to each trough. This was l e f t over a week-end, and on Monday both 'troughs had become f o u l . In the trough where the a i r was bubbling at the bare end of the trough, there were 36 H. nudus and 4 H. oregonensis dead. In the other trough, where the a i r was bubbling among the stones, there were 6 H. nudus and 1 H. oregonesis dead (see Table I V ) . ^There i s a s i g n i f i c a n t d i f f e r e n c e between the troughs f o r H._nudus, but 17 not f o r H. oregonensis i n m o r t a l i t y ) Since the crabs were • able to choose among the sec t i o n s by crawl i n g under the d i v i -ders, and had shown no h e s i t a t i o n i n doing so i n the absence of cover, i t was concluded that H. nudus would chose to remain / under cover even i n the face of l e t h a l l y low oxygen concentra-t i o n s , while H. oregonensis would chose to respond to the S oxygen gradient r a t h e r than.to the gradient i n c o v e r . ^ FACTORS AFFECTING DISTRIBUTION: MUD The d i f f e r e n c e between the two species with respect to t h e i r tendency to leave cover f o r oxygen, together with the f a c t that they d i f f e r i n the arrangement of the h a i r s i n t h e i r b r a n c h i a l openings, which would produce d i f f e r e n c e s i n t h e i r a b i l i t y to prevent ejlogging of the g i l l s , l e d me to form the hypothesis that there might be a d i f f e r e n c e between the species with r e -spect to t h e i r a b i l i t y to withstand low oxygen tensions, and that t h i s a b i l i t y might be modified by the clogging e f f e c t of mud i n the g i l l s . To t e s t t h i s i d e a , the f o l l o w i n g experiments v/ere performed. P a i r s of crabs, matched f o r s i z e , were put i n t o one l i t e r f l a s k s f i l l e d to the brim with seawater. Nitrogen was bubbled • through the water f o r 15 minutes to remove d i s s o l v e d oxygen. The f l a s k s were then corked so that no bubbles were i n c l u d e d . At one hour i n t e r v a l s , the f l a s k s were b r i e f l y i n v e r t e d to check whether the crabs were a l i v e . L i v i n g crabs would spread t h e i r l e g s as they f e l l through the water, while crabs c l a s s i f i e d as dead shows no a c t i v i t y . At each check, the number of crabs 18 that had died i n the i n t e r v a l was recorded (see Table V). In one case, the H. nudus and the H. oregonensis_died i n the same i n t e r v a l , while i n the other four f l a s k s the H..nudus died > f i r s t . On the average, H. oregonensis o u t l i v e d H. nudus by 3 . 9 hours. Twenty -six f l a s k s were prepared i n the same manner as i n the previous experiment. To h a l f of these, mud, t y p i c a l of the substrate on a beach dominated by H. oregonensis was added. The water was then aerated to s a t u r a t i o n . Matched p a i r s of, crabs were added, and hourly checks were performed. In the clean f l a s k s , i n ten cases, H. nudus died f i r s t , i n one, a t i e was recorded, and i n two, H. oregonensis died f i r s t . In the f l a s k s with the mud, H. nudus died f i r s t i n nine cases, at the same time i n two, and i n two of the f l a s k s , H. oregonen-s i s died f i r s t (see Tables VIA,,B, C, and D). The s u r v i v a l time f o r H. nudus was 1 8 . 1 8 hours i n the clean f l a s k s , and 1 2 . 3 0 hours i n the f l a s k s with the mud. For H. oregonensis, the average s u r v i v a l time i n the clean f l a s k s was ' 2 0 . 9 1 hours, and i n the f l a s k s with the mud, 1 9 . 1 0 hours. On d i s s e c t i o n , the g i l l s of H. nudus were found, to be clogged with mud, i n the sample from the muddy-flasks, while those of H. oregonensis ————— was found to be r e l a t i v e l y clean. Thus, i t seems that H. oregonensis i s able to withstand low oxygen tensions longer than H. nudus, and that the v i a -b i l i t y of H. nudus i n an environment with low oxygen i s con-s i d e r a b l y reduced i f mud i s a l s o present, while t h i s i s not true of H. oregonensis. 19 I f the e f f e c t of mud and low oxygen on a beach i s to t e l i m i n a t e H. nudus, i t i s necessary to demonstrate that low oxygen c o n d i t i o n s are l i k e l y to develop i n the microhabitat frequented by the crabs. A syringe was used to draw water sam-ples from under rocks where the crabs l i v e d , without d i s t u r b i n g the environment. Although considerable t e c h n i c a l d i f f i c u l t y was experienced i n g e t t i n g samples which were free of bubbles, three samples were obtained from a beach dominated by H. nudus and three from one where H. oregonensis was dominant. A micro-winkler t e s t was run on these samples. On the H. nudus beach, the samples contained i+.O, 2 . 5 and 1 . 3 p a r t s per m i l l i o n of oxygen, with the f i r s t sample being taken highest on the beach, and the l a s t , lowest. The samples from the H. oregonensis beach contained 2 . 0 , 1.3> and 0 . 0 ppm. This i s not a s i g n i g i c a n t d i f f e r e n c e (P = 0 . 1 0 ) , but i t must be emphasized that these samples were taken i n winter, i n a heavy rainstorm, when pre-sumably there would be a good flow of aerated water through the substrate. Thus, i t i s l i k e l y that these are nearly as high oxygen concentrations as ever occur, and i t does .'demon-,; ^ ~"> s t r a t e that 3?ow oxygen tensions can develop i n the water under the rocks where the crabs live.*' " .t- _s To i n v e s t i g a t e the e f f e c t of beach muddiness on s u r v i v a l of the two species, crabs were caged i n c l o s e l y adjacent areas which d i f f e r e d considerably i n muddiness. Sets, each of three cages, were placed i n a clean, rocky area, i n a clean sandy \ area, i n a muddy rocky area, and i n a muddy sandy area, at Deception Pass State Park, Washington. The cages were a l l 2 0 w i t h i n a hundred meters of each other, and w i t h i n 3 0 cm of each other v e r t i c a l l y . In each set, one cage contained 2 0 H. nudus, one contained 2 0 H. oregonensis, and one contained 10 of each species. Four rocks were placed on p r o j e c t i n g side f l a p s . The cages were of 1/4 i n c h hardware c l o t h , coated with epoxy r e s i n to prevent c o r r o s i o n , and were 60 by 60 by 15 cm. A f t e r three months, the cages were l i f t e d , and the s u r v i v i n g crabs were; counted (see Table V I I ) . In general, s u r v i v a l of H. nudus was be t t e r than that of H. oregonensis. This may be because the bottoms of the cages prevented the crabs from burrowing i n t o the substrate, so that there was considerable m o r t a l i t y from f r e e z i n g , as t h i s experiment was run during what turned out to be one of the c o l d e s t winters on record f o r t h i s area. How-ever, i n both muddy areas, H. oregonensis survived b e t t e r than on the clean areas, while H. nudus d i d b e t t e r i n the clean : areas. To t e s t whether the crabs displayed a preference f o r the type of substrate to which they seemed, on morphological grounds to be adapted, the f o l l o w i n g experiment was set up. A box was d i v i d e d i n t o three troughs, each .150 by 20 by 15 cm. These were i n turn d i v i d e d i n t o 4 s e c t i o n s by p l e x i g l a s s d i v i d e r s , 3 cm high which the crabs were e a s i l y able to climb over. "Normal" substrate was d i v i d e d i n t o three f r a c t i o n s , that which s e t t l e d out of a water column i n 3 0 seconds (the sand), the f r a c t i o n which s e t t l e d out i n more than 3 0 and l e s s than 1 8 0 seconds ( s i l t ) , and the remainder, which took longer than 1 8 0 seconds to 'settle (the mud). These were put i n three of the 21 four sections with clean water i n the fourth. The arrangement of the substrates was randomized among the four sections of each trough, and water was added to within 0.5 cm of the top of the dividers. Five crabs of each species were added to each section. Counts of the number of crabs i n each section were made thirteen times i n 7 days (see Table VIII). H. oregonensis was found to prefer the section containing the s i l t , where they regularly buried themselves, while H. nudus avoided that sec-tion. FACTORS AFFECTING DISTRIBUTION: SETTLEMENT Some beaches are dominated by one species, and some by another. This s i t u a t i o n could arise i f the larvae of the crab could, or would, only s e t t l e on beaches to which they were adapted. To test i f t h i s were the case, samples of crabs were exam-ined from a l l the beaches where censuses were taken, separating the crabs into two size categories, those l e s s than 7 mm wide (juveniles), and those larger than t h i s . In general, the c o l -l e c t i o n of the smallest sizes was incomplete (see Figures k and 5) due to the differences between the adults and juveniles. When the young crabs are suddenly exposed, they remain immobile, while the larger ones usually move. Since these small crabs are similar i n size, shape, and colour to the larger p a r t i c l e s of the substrate, t h i s behavior makes them very d i f f i c u l t to locate. However, since the young of both species behave i n much the same way, and since the same sort of res u l t was found 22 on both H. nudus and on H. oregonensis dominated beaches, i t seems safe to assert that t h i s underrepresentation of the small crabs i s not biased towards one or the other of the species. In general, i t was found that the rarer species had a much higher r a t i o of young to old crabs than the dominant species (see Table IX). Apparently, the larvae metamorphosed where they happened to be when the time came for them to reach the f i r s t true crab stage and s e t t l e to the bottom. Thus, s e t t l e -ment of the l a r v a l stages of the two species produces a more equal r a t i o of the two species than would be expected from an examination of the adults i n an area. This, i n turn, indicates that there must be either emmigration or mortality reducing the numbers of the subordinate species. Since a beach may be dominated by one species of crab for miles, and since the mo-t i l i t y of small crabs i s very low, emigration i s not l i k e l y . Therefore, the subordinate species presumably suffers heavier mortality than the dominant one. FACTORS AFFECTlMG- DISTIRUBTION: COMPETITION FOR COVER As mentioned i n the introduction, the crabs are strongly thigmotactic. However, from the experiments on oxygen, where cover was provided, i t seemed that H. nudus was more strongly attracted to cover than H. oregonensis. To test i f t h i s i s true, the following experiments were set up. A small screen of p l a s t i c approximately 15 by 15 cm. was placed i n a c h i l d ' s c i r c u l a r wading pool 1 6 0 cm i n diameter. The screen was raised on stainless steel legs about 3 cm, and 23 was the only cover a v a i l a b l e to the crabs, as the side of the pool met the bottom i n a curve about 3 cm i n r a d i u s , so that there was no edge f o r the crabs to back up against. Seawater was added to a depth of 5 cm, and 2 0 0 H. oregonensis were added. Counts of the number of crabs not under cover were made ea r l y each morning f o r 9 days (see Table X). These crabs were then removed, the water was changed, and then 2 0 0 H. nudus were added, and s i m i l a r l y observed f o r 10 days. F i n a l l y , these were replaced with 1 0 0 crabs of each species, and the observations continued. When H. oregonensis was alone, there was an aver-age of 2 5 . 6 6 crabs exposed. When H. nudus was alone, there was an average of 9 - 5 5 crabs exposed. When the two species were together, there were 4 . 8 H. nudus and 4 9 . 8 H. oregonensis ex-posed. Since H. nudus i s , on the average, about one quarter l a r g e r than H. oregonensis, one would expect that H. nudus : alone would be out from under cover more•than H. oregonensis alone. This was not the case, so one may conclude that H. nudus does,tend to stay under cover more than H. oregonensis. At the same time, when the two crabs were together, there were about four times as many H. oregonensis exposed as would have been expected from the occasion when they were alone i n the pool. Thus, i t seems that H. oregonensis i s more prone to 1 wander exposed than i s H. nudus when e i t h e r i s alone, but when they are together, H. nudus d i s p l a c e s H. oregonensis from l i m i t e d cover. • 2k FACTORS AFFECTING DISTRIBUTION: PREDATION As was demonstrated above, mud with low oxygen tensions may cause H. nudus to be eliminated by s u f f o c a t i o n from beaches which are dominated by H. oregonensis. However, a clean sub-s t r a t e i s i n no way detrimental to the welfare of H. oregonen-s i s . Therefore, another mechanism must operate to reduce the s u r v i v a l of H. oregonensis on beaches which are dominated by H. nudus, si n c e , as mentioned above, there i s presumably d i f -f e r e n t i a l m o r t a l i t y a c t i n g on the crabs producing the high r a t i o of young to a d u l t crabs among the subordinant species. As ; demonstrated above, H. nudus w i l l d i s p l a c e H. oregonensis from cover. This would tend to expose H. oregonensis to a greater r i s k from predators than would be experienced by H. nudus. Predators could a l s o be t a k i n g H. oregonensis p r e f e r e n t i a l l y when o f f e r e d a free choice between the species. To these these ideas, the f o l l o w i n g experiments were performed. P a i r s of crabs were thrown i n t o a tank with a group of f i s h which are r e g u l a r l y found near shore, and are able to forage over the i n t e r t i d a l at high t i d e . These included k e l p g r e e n l i n g (Hexagrammos decagrammos), l i n g c o d (Ophiodon elonga-t u s ) , s t a r r y flounder ( P l a t i c h t h y s s t e l l a t u s ) , cabezon (Scor-paenichthys marmoratus), and staghorn s c u l p i n (Leptocottus  armatus). The crab of each p a i r which v/as taken f i r s t was recorded (see Table X I ) . I t was found ttfcat the f i s h d i d not s e l e c t crabs by species. To t e s t whether the crabs are equally a v a i l a b l e when cover i s provided, 30 crabs of each species were placed i n a 25 tank .at the Vancouver P u b l i c Aquarium. Ten stones were added f o r cover, then 8 s t a r r y flounders ( P l a t i c h t h y s s t e l l a t u s ) , 6 staghorn s c u l p i n (Leptocottus armatus), 5 s t r i p e d seaperch (Embiotoca l a t e r a l i s ) , 1 b u f f a l o s c u l p i n (Enophrys b i s o n ) , 3 lemon so l e s (Parophrys v e t u l u s l , 3 C~0 soles (Pleuronichthys  coenosus), 1 cabezon (Scorpaenichthys marmoratus), and a l i n g -cod (Ophiodon elongatus) v/ere added. A l l f i s h were l a r g e enough to take crabs. The number and species of crabs remaining were recorded each day f o r the f o l l o w i n g 13 days (see Figure 6 ) . There was high m o r t a l i t y the f i r s t n i g h t , and a f t e r t h i s , a slow a t t r i t i o n . M o r t a l i t y f e l l most h e a v i l y on H. oregonensis. To t e s t i f the greater l o s s of H. oregonensis obtained ,in the f i e l d , a number of b i r d s and f i s h were c o l l e c t e d (see Table XII) at Bowen I s l a n d , Bowyer I s l a n d , San Juan I s l a n d , and Decep-t i o n Pass State Park, Washington (see map). The /proportions of H. nudus to H. oregonensis i n these areas are r e s p e c t i v e l y 6 . 2 to 1, 5 . 0 to 1, 7 . 3 to 1, and 9 . 8 to 1 . I f a l l the animals c o l l e c t e d are lumped together, on the grounds that these were a l l H. nudus dominated beaches, one would expect to f i n d , on the average 7 . 5 7 H^nudus to 1 H. oregonensiskin the stomachs, i f the predators v/ere t a k i n g the crabs i n proportion to repre-s e n t a t i o n . There v/as a h i g h l y s i g n i f i c a n t r e v e r s a l of t h i s expectation, w i t h a t o t a l of 13 H. nudus and 38 H. oregonensis being found. Thus, i t seems that where the two species are found together, the displacement of H. oregonensis from cover by H. nudus might v/ell act to expose H. oregonensis to a greater r i s k from predators. ;' The f a c t that H. nudus i s more s t r o n g l y . 26 thigmotatic than H. oregonensis tends to keep t h i s species c l o s e r to cover, and so act s to spare i t from predation. ; FACTORS AFFECTING DISTRIBUTION: SALINITY R i c k e t s et a l . ( 1 9 6 2 ) advanced the hypothesis that gra-d i e n t s of s a l i n i t y might c o n t r o l the d i s t r i b u t i o n of the two species. This does not seem l i k e l y . I t i s known that f r e s h water f l o a t s " on s a l t , yet H. nudus, the species which they f e e l i s l e s s t o l e r a n t of low s a l i n i t y , i s i n v a r i a b l y found to have i t s center, of d i s t r i b u t i o n higher than H. oregonensis where jthe two occur on the same beach. To f u r t h e r t e s t t h i s hypothesis, the f o l l o w i n g experiments were run. Into a hol d i n g tank were.placed 3 0 6 H. nudus and 161 H. oregonensis. Seawater, of 27 p a r t s per thousand of s a l t s was added. Over s i x days, the water was p r o g r e s s i v e l y d i l u t e d to 4 p a r t s per thousand, at which point approximately 50% m o r t a l i t y had occurred (see Table X I I I ) . The d i f f e r e n c e i n m o r t a l i t y be-tween the two species was not found to be s i g n i f i c a n t . Nine crabs of each species were placed i n tap water, and the time to death was recorded f o r each i n d i v i d u a l (see Table XIV). Again, there was no s i g n i f i c a n t d i f f e r e n c e between the species i n time to death. FACTORS AFFECTING DISTRIBUTION: DESICCATION R i c k e t s et a l . ( 1 9 6 2 ) s t a t e that H. nudus i s more r e s i s -tant to d e s i c c a t i o n than i s H. oregonensis. 27 .This'was'tested, by p u t t i n g 23 i n d i v i d u a l s of each species, matched.for s i z e , i n dry dishpans, and r e c o r d i n g the time to death. The crabs were presumed to be dead when they no longer responded to tough. The average s u r v i v a l time, f o r H. nudus was 8 . 7 4 hours, and f o r H. oregonensis, 7 . 8 4 hours (see.Table XV).. The d i f f e r e n c e between the two species i s s i g n i f i c a n t . -This a b i l i t y of H. nudus to survive d e s i c c a t i o n , with i t s pre-ference f o r a sandy, r a t h e r than a muddy substrate, probably may account f o r the f a c t that H. nudus l i v e s higher on the beach than H. oregonensis. FACTORS CONTROLLING ABUNDANCE In order to determine what f a c t o r s set a l i m i t to the num-ber of crabs that can l i v e i n an area, f o r the sake of s i m p l i -c i t y , the two species were considered to be e c o l o g i c a l l y equi-valent, so that 3} simple sum of the crabs present i s regarded as the abundance. As i t was not p o s s i b l e to get a good e s t i -mate of the numbers of crab larvae which might be s e t t l i n g , i t was assumed that the number of crabs i n an area i s a goeS.d measure of the c a r r y i n g c a pacity of the area, and that enough young crabs s e t t l e each year to saturate the area. A m u l t i p l e r e g r e s s i o n was performed on the number of crabs present i n each s e c t i o n of beach against a l l the other v a r i a b l e s which were measured- or estimated f o r that s e c t i o n during the census (see Table XVI). These v a r i a b l e s were: the reducing power of the substrate, the percent cover, the su r f , the a v a i l -able food, the- height of the s e c t i o n on the beach, and the 28 s a l i n i t y . I t was found that the amount of cover was most c l o s e l y associated with the number of crabs present, with l e s s close a s s o c i a t i o n with the height on the beach and the redu-c i n g power of the substrate. A more accurate way of measuring the amount of cover than a v i s u a l estimate should give an even bet t e r c o r r e l a t i o n , since i t was always quite obvious that the number of.crabs was a f u n c t i o n almost s o l e l y of the a v a i l a b i l i t y of cover. I f the number of crabs were a f u n c t i o n of cover, then manipulation of the amount of cover should be r e f l e c t e d i n changes i n the number of crabs. This was t e s t e d by .an e x p e r i -ment. Two adjacent squares, 2.5 meters on a s i d e , were marked out. Both had about 100% cover of cobble s i z e d rocks. A cen-sus of these was done, with another count of the crabs i n a t h i r d p l o t f o r a c o n t r o l . Then h a l f the rocks i n p l o t 1 were moved to p l o t 2. A f t e r 2 months, another census was done on the three p l o t s (see Table XVII). At the s t a r t of the e x p e r i -ment, there were s i g n i f i c a n t d i f f e r e n c e s among the three p l o t s . This could be a t t r i b u t e d to the inherent v a r i a b i l i t y i n the amount of cover i n the three areas. Although the cover was estimated as. 100% i n a l l three areas, t h i s was based on a v i s -u a l survey of the rocks.' The cover to which the crabs a c t u a l l y respond i s the amount of c r e v i c e space, and they use only c r e -v i c e s w i t h i n a c e r t a i n range of s i z e s . This c r e v i c e space i s not amenable to accurate measurement without considerable d i s -r u p t i o n of the beach. At any r a t e , the change i n numbers of crabs i n response' 29 to the changes i n cover was h i g h l y s i g n i f i c a n t . There was a l o s s of crabs from the area where rocks were removed, a gain of crabs where the rocks were placed, and no s i g n i f i c a n t change i n the unaltered t h i r d p l o t . This change i n crab numbers i n response to changes i n cover supports the id e a that the number of crabs on a beach i s l a r g e l y a f u n c t i o n of cover. 30 DISCUSSION Previous s t u d i e s of competing s p e c i e s i n the i n t e r t i d a l (Connel, 1961; Harger, Ph.D. t h e s i s , U.C.S.B., 1967) showed that adaptions to p h y s i c a l c o n d i t i o n s play an important p a r t i n determining the outcome of competition. The mussels which Harger worked on were found to have b e h a v i o r a l and p h y s i c a l d i f f e r e n c e s which caused one s p e c i e s to be b e t t e r adapted to v/ithstand heavy wave shock, while the other s p e c i e s could l i v e i n the s i l t y c o n d i t i o n s found i n q u i e t e r waters. At i n t e r -mediate values of these two £#c$J*£*rs, the s p e c i e s c o e x i s t e d . The b a r n a c l e s which Connel worked on had s i m i l a r s o r t s of d i f f e r e n c e s . These produced, i n one s p e c i e s , g r e a t e r t o l e r a n c e of d e s i c c a t i o n , and i n the other, the a b i l i t y to s u r v i v e severe crowding. In t h i s study, the crabs showed d i f f e r e n c e s i n behavior, morphology, and p h y s i o l o g y which again could be presumed to determine the outcome of p o s s i b l e competition. H. oregonensis i s able to t o l e r a t e muddiness and low oxygen i n i t s microhabi-t a t , while H. nudus, although unable to s u r v i v e under these c o n d i t i o n s , i s b e t t e r able to avoid p r e d a t o r s . In general, i t seems that at one end of the d i f f e r e n t g r a d i e n t s , one s p e c i e s of animal i s unquestionably s u p e r i o r to the other, which simply cannot s u r v i v e , and at the other end, the opposite i s t r u e . Only i n the middle of these g r a d i e n t s can there be competition, i n that both s p e c i e s are able to l i v e there. T h i s s e p a r a t i o n at the ends of the g r a d i e n t s seems to 31 be an adaptive strategy of b e n e f i t to the animals i n v o l v e d . I t i s usual f o r the p h y s i c a l c o n d i t i o n s at the ends of the gradients to be extreme, with respect to the animal's t o l e r -ance f o r the range of f a c t o r s i n v o l v e d . Thus, where c o n d i t i o n s approach the l i m i t of tolerance of one of a p a i r of p o t e n t i a l competitors and exceed that of the other, the one with the greater tolerance i s able to s u r v i v e , probably p a r t l y because i t i s not handicapped by being forced to compete. This pre-sumably would allow i t to devote more of i t s energy to opera-t i o n s which are fundamental to i t s economy. With these crabs, such operations could be the gathering and consuming of food, r e s p i r a t i o n , and avoidance of predators. Where the two species l i v e together, at l e a s t some energy and time must be devoted to competitive i n t e r a c t i o n s . I t i s not s u r p r i s i n g , then, that competition may be deduced to be occuring only where the en-vironmental c o n d i t i o n s are c l o s e r to optimal f o r both species, and hence, l e s s energy must be devoted to bare s u r v i v a l . The mussels and barnacles a l s o seem to c o e x i s t only where the c o n d i t i o n s are apparently good enough that energy may be spared by both, f o r non-essential operations. The mussels l i v e together where wave shock seldom reaches i n t e n s i t i e s great enough to tear apart the clumps, but i s u s u a l l y enough to keep the clumps reasonably f r e e from s i l t (Harger, op. c i t . ) . The barnacles which Connell worked on c o e x i s t where submergence i s frequent enough to prevent severe d e s i c c a t i o n , but where the barnacles are not so crowded that space to grow i s c r i t i c a l . With these animals, t h i s area of coexistance i s a rather narrow 32 l i n e on the shore. The crabs mix i n l a r g e numbers where cover and food are abundant. Here again, c o n d i t i o n s seem to be c l o s e r to optimal fo r both species than the places where only one or the other species l i v e s . In each of these cases, there seems to be a species which i s , i n some sense, f u g i t i v e . This species can stand wider environmental f l u c t u a t i o n s than the other, but cannot suceed when i t i s confronted with the " l e s s hardy" species i n i t s s u i t e d environment, though, i f t h i s l e s s hardy species i s ab-sent, the f u g i t i v e species i s capable of occupying a consider-ably l a r g e r p o r t i o n of the h a b i t a t open to the two species. In the case of the mussels, M y t i l u s c a l i f o m i a n u s can l i v e i n f a i r l y quiet water, i f i t i s not overgrown by M. e d u l i s . The barnacle Chthamalus s t e l l a t u s can l i v e most of the way down the shore, i f Balanus balanoides i s excluded. H. oregonensis can l i v e on clean beaches, but only i f H. nudus i s not present. In a l l these cases, continuing competition i s made p o s s i b l e by the f a c t that these animals have planktonic l a r v a e . This allows c o n t i n u a l input of young animals to areas where they may compete. Competition can be i n v e s t i g a t e d i n these s i t u a t i o n s with ease, since they a l l conform to Parks' ( 1 9 6 2 ) c r i t e r i a . In each case, there are two d i s t i n c t and i d e n t i f i a b l e species, they can be studied as s i n g l e and mixed species populations, they are r e a d i l y countable, they can be moved without appre-c i a b l e apparent trauma to i n d i v i d u a l s , and i t i s p o s s i b l e to manipulate the environment to some extent. 33 I t seems then, that a l l these species have a range of tolerance f o r c e r t a i n p h y s i c a l f a c t o r s . Within a species p a i r , t h i s range extends f a r t h e r f o r one species than the other, i n the d i r e c t i o n of the refuge which the f u g i t i v e spe-c i e s takes advantage of. However, i t a l s o seems that t h i s range of tolerance i n c l u d e s optima f o r both species, and that these optima are often quite c l o s e together, i f not c o i n c i d e n t . Evidence f o r t h i s i s the f a c t t h a t , i f the set of c o n d i t i o n s which i s outside the range of tolerance of one competitor i s excluded from c o n s i d e r a t i o n , both animals can l i v e throughout the range, and the number of animals which can be maintained i n t h i s "optimum" area i s g e n e r a l l y much higher than the num-ber that can be maintained i n the refuge of the f u g i t i v e species. One could speculate that t h i s coincidence of optima may be evidence f o r recent evolutionary separation of the two species, and p o s s i b l y , that e v o l u t i o n w i l l continue to separate the two, f u r t h e r reducing the incidence of competition betv/een them, u n t i l , at the "millenium," there w i l l be a sharp l i n e of de-marcation between the ranges of the two species, w i t h no over-l a p at a l l , but t h i s seems u n l i k e l y i n the face of the plank-t o n i c breeding mechanism adopted by these animals. 3k CONCLUSION The d i s t r i b u t i o n of Hemigrapsus nudus and H. oregonensis i s controlled by the int e r a c t i o n of predators and the reducing power of the substrate. In the absence of mud, H. nudus w i l l displace H. oregonensis from cover, and so expose i t to d i s -proportionate great predation pressure, leaving H. nudus to dominate the beach. In the presence of fine mud with high reducing power, int e r v a l s of low oxygen a v a i l a b i l i t y i n the microhabitat occu-pied by the crabs are l i k e l y to occur. H. nudus tends to re-main i n the microhabitat and suffocate, while H. oregonensis w i l l leave. H. oregonensis i s also better adapted morpholo-g i c a l l y to prevent clogging of the g i l l s with mud, and i s able to withstand l e t h a l l y low oxygen l e v e l s longer than H. nudus. The number of crabs which can l i v e on a beach i s a function of the amount of cover, measured as the abundance of rocks under which the crabs can shelter. 35 TABLE I Reducing power of s u b s t r a t e samples, c l a s s i f i e d by the s p e c i e s of crab which dominated ( i . e . , made up more than 50% of the crabs) i n the census sample. Measurement i s of r e d u c t i o n of oxygen t e n s i o n ( i n p a r t s per m i l l i o n ) from water i n a one l i t e r f l a s k , over a p e r i o d of one hour. T u r b i d i t y i s recorded by a v i s u a l estimate, on a s u b j e c t i v e s c a l e of from 1 to 6 , with 1 being the c l e a n e s t . H. nudus Beaches H. oregonensis Beaches Oxygen T u r b i d i t y Oxygen T u r b i d i t y 0 . 5 0 0 . 1 1 0 . 2 6 0 . 2 7 0 . 0 5 0 . 1 0 0 . 8 5 0 . 1 0 0 . 1 0 0 . 9 0 0 . 7 0 0 . 7 0 0 . 3 9 0 . 3 5 0 . 3 7 0 . 9 0 0 . 2 1 0 . 5 7 o . 8 o 0 . 4 0 0 . 2 9 0 . 2 4 0 . 3 2 0 . 2 8 0 . 7 5 0 . 4 0 0 . 1 0 0 . 2 0 o . 3 9 0 . 1 8 0 . 1 0 0 . 0 5 2 1 1 1 1 1 3 1 1 3 3 3 2 2 2 3 2 3 3 2 1 2 3 2 3 2 1 1 2 1 1 1 1 . 8 4 1 . 4 3 4 3 5 6 k 4 4 k 4 4 3 4 k 4 5 5 3 6 3 5 2 6 5 5 5 5 5 6 3 1 . 8 8 1 . 3 0 1.26 1 . 4 2 0 . 9 4 1 .55 1 . 9 2 1 . 10 1.62 1 .72 1 . 0 0 2 . 0 5 1 . 2 0 2 . 0 1 0 . 5 0 3 . 0 5 2 . 5 0 2 . 7 0 2 . 8 0 2 . 9 0 2 . 1 5 3 . 1 2 1 . 6 0 TOTAL 5 5 . 7 7 MEAN 1 . 9 2 3 36 TABLE I (Continued) H. nudus Beaches Oxygen T u r b i d i t y 0 . 6 5 0 . 1 2 0 . 6 5 0 . 0 5 0 . 1 0 0 . 8 5 1 . 0 0 0 . 3 0 0 . 1 0 2 . 1 3 1 . 0 5 1 . 10 0 . 7 4 0 . 2 9 2 1 2 1 1 5 4 2 1 5 4 1 2 2 TOTAL 20.06 MEAN 0 . 4 3 6 Comparing the reducing power of the two sets of samples: Students t t e s t t = 4 5 . 8 0 3 D.F. = 73 P. i s much l e s s than . 0 0 1 Conclusion: Reducing p o t e n t i a l i s greater on H. oregonensis dominated beaches than on those where H. nudus""is more abundant. 37 TABLE I I Counts of the number of crabs reaching the surface of the water per hour. Oxygen d e f i c i t set up by the r e s p i r a t i o n of the crabs. 10 crabs of each species i n one bucket, with 7 inches of water, and cover, i n the form of a piece of screen, at the bottom of the bucket. T o t a l s Averages # Counts Hour Made H. oregonensis H. nudus H. oregonensis H. nudus 0 to A 0 0 0 0 5 8 24 4 3 . 0 0 . 5 6 11 62 27 5 . 6 2 . 5 7 8 72 32 9 . 0 4 . 0 8 1 0 78 27 7 . 8 2 . 7 TOTALS 2 3 6 9 0 I t i s obvious that H. oregonensis e x h i b i t s a greater tendency to climb and expose i t s e l f than H. nudus when both are faced with a choice of c l i m b i n g to avoid low oxygen, or remaining under cover i n the region of low oxygen. 38 TABLE I I I Counts of the number of crabs reaching the surface of the water per hour. Oxygen removed by bubbling n i t r o g e n . Species i n separate buckets, both with cover at the bottom, and 7 inches of water. 2 0 crabs i n each bucket. Hour # Counts T o t a l H. oregonensis T o t a l H. nudus Average H. oregonensis Average H. nudus 1 6 3 0 0 . 5 0 . 0 2 6 2 5 7 4 . 2 1 . 2 3 6 4 4 21 7 . 3 3 . 5 4 6 52 37 8 . 7 6 . 2 5 6 70 41 1 1 . 7 6 . 8 6 6 87 4 6 1 4 . 5 7 . 7 7 6 89 45 1 4 . 8 7 . 5 8 6 81 31 1 3 . 5 5 . 2 TOTALS 451 2 2 8 Number dying i n Hour 8 = 2 H. oregonensis and 8 H. nudus. Conclusion: H. oregonensis e x h i b i t s a greater tendency to expose i t s e l f than H. nudus, when faced with a choice of r e -sponding to an oxygen gradient by cli m b i n g or remaining under cover i n the face of low oxygen. I t i s also apparent that H. nudus w i l l , i n some cases, at l e a s t , remain under cover even when oxygen reaches l e t h a l l y low l e v e l s . 39 TABLE IV Number of each species dead, presumably of s u f f o c a t i o n , r e l a -t i v e to t h e i r choice to remain under cover, or to respond to a h o r i z o n t a l oxygen gradient. There were i n i t i a l l y 114 H. nudus and 8A H. oregonensis i n each lane. Lane 1 Section Condition # Dead 1 A i r No cover 0 No cover No cover Cover 0 0 36 H. nudus A S. oregonensis Lane 2 Condition # Dead No cover 6 H. nudus 1 H. oregonensis A i r No cover No cover Cover Testing the d i f f e r e n c e s between the two species with respect to t h e i r choice to leave cover to respond to l e t h a l l e v e l s of low oxygen and/or high hydrogen sulphide. I f there were no d i f f e r e n c e between the species, one would expect that an equal number of each would have died i n each lane. In Lane 1, there were AO dead crabs. I f they responded the same, the expectation would be that there should be 2 0 of each species. Chi square = ( 2 0 - 3 6 ) 2 / 2 0 + ( 2 0 - A ) 2 / 2 0 = 2 5 . 6 P r o b a b i l i t y of t h i s c h i square = l e s s than . 0 0 1 with 1 d.f. In Lane 2 , there were 7 dead crabs, so the expected d i s t r i b u -t i o n i s 3 . 5 of each species. Chi square = ( 3 . 5 - 6 ) 2 / 3 . 5 + ( 3 . 5 - D 2 / 3 . 5 = 3 - 5 7 P r o b a b i l i t y of t h i s c h i square = between 0 . 1 and 0 . 0 5 AC-TABLE IV (Continued) Conclusion; In Lane 1, H. nudus died i n s i g n i f i c a n t l y greater numbers than d i d H. oregonensis. In Lane 2, while the d i f -ference between th"e species i s not s t a t i s t i c a l l y s i g n i f i c a n t , the trend i s s i m i l a r to that of Lane 1. This i n d i c a t e s that H. nudus w i l l respond to cover to a greater extent than w i l l !• oregonensis when faced w i t h a choice of l e a v i n g cover to respond to low oxygen l e v e l s . 41 TABLE V Time to death of each member of p a i r s of crabs, matched f o r s i z e , i n water where most of the oxygen was removed by bubbling nitrogen, i n f l a s k s which were corked to exclude a i r a f t e r the a d d i t i o n of the crabs. Time to death i n hours  H. nudus 6 . 7 5 5 . 0 0 5 . 0 0 5 . 0 0 5 . 0 0 TOTAL 2 6 . 7 5 Matched p a i r s t t e s t t = 4 . 0 9 6 d.f. = k P between . 0 1 and . 0 0 5 Conclusion: H. nudus d i e s s i g n i f i c a n t l y sooner than H. oregonensis v/Ken they are placed i n p a i r s i n deoxygenated water. H. oregonensis 1 2 . 0 0 5 . 0 0 6 . 7 5 1 0 . 5 0 1 2 . 0 0 4 6 . 2 5 . Di f f e r e n c e ( y ) 5 . 2 5 0 . 0 0 1 . 7 5 5 . 5 0 7 . 0 0 1 9 . 5 0 42 TABLE VI (A) Comparison of s u r v i v a l time w i t h i n matched p a i r s of H. ore-gonensis and H. nudus i n corked one l i t e r f l a s k s , without mud. Matched p a i r s t t e s t . N = 1 3 . Time i n hours to death Y (H.o H. n H. o. 1 0 . 0 2 0 . 0 1 0 . 0 1 0 . 0 1 3 . 3 3 . 3 1 3 . 3 1 3 . 3 0 . 0 . 2 0 . 0 2 0 . 0 0 . 0 1 3 . 6 1 3 . 3 - 0 . 3 2 5 . 0 2 0 . 0 - 5 . 0 1 1 . 0 2 0 . 0 9 . 0 1 1 . 0 2 0 . 0 9 . 0 2 0 . 0 2 2 . 0 2 . 0 2 5 . 5 2 7 . 0 1 .5 2 5 . 0 2 7 . 0 2 . 0 2 5 . 0 2 8 . 0 3 . 0 2 7 . 0 2 8 . 0 1 . 0 AVERAGE 1 8 . 1 8 2 9 . 9 1 2 . 7 3 Paired t t e s t Sum of Y = 3 5 . 5 Mean Y = 2 . 7 3 (Sum of Y ) 2 = 1 2 6 0 . 2 5 (Sum of Y) 2/N = 9 6 . 9 4 2 Sum of (Y 2) = 3 1 8 . 2 3 Sum of squares = 1 1 6 3 . 3 0 8 s 2 = 13 .617 S2/N = 1 .047 S 2/# = 1 . 8 2 3 t = 2 . 7 3 / 1 . 0 2 3 = 2 . 6 6 8 D.F. = 12 P = . 0 0 1 approx Conclusion: The two species d i f f e r s i g n i f i c a n t l y i n time to death, i n a clean environment, w i t h H. nudus dying f i r s t . 43 TABLE VI (B) Comparison of s u r v i v a l time of matched p a i r s of H. oregonen-s i s and H. nudus i n corked one l i t e r f l a s k s , w i t h mud. Matched p a i r s t t e s t . Time i n hours to death H. n. H. o. Y (H.o 1 2 . 7 1 1 . 5 - 1 . 2 1 1 . 6 1 2 . 7 1 . 1 1 1 . 6 2 0 . 0 8 . 4 1 0 . 0 2 2 . 0 1 2 . 0 1 1 . 0 2 0 . 0 9 . 0 2 0 . 0 2 0 . 0 0 . 0 1 2 . 0 2 0 . 0 8 . 0 1 3 . 3 2 0 . 0 6 . 7 1 2 . 7 1 1 . 7 - 1 . 0 1 2 . 0 2 0 . 0 8 . 0 1 1 . 0 2 5 . 2 1 4 . 2 1 1 . 0 2 5 . 2 1 4 . 2 1 1 . 0 2 0 . 0 9 . 0 AVERAGE 1 2 . 3 1 9 . 1 6 . 7 8 Paired t t e s t Sum of Y = 8 8 . 1 Mean Y = 6 . 7 8 (Sum of Y ) 2 = 7 7 6 1 . 6 1 (Sum of Y) 2/N = 5 9 7 . 0 4 7 Sum of (Y 2) = 9 5 2 . 4 5 SS = 3 5 5 . 4 0 3 S 2 = 2 9 . 6 1 6 S 2/N 1 = 2.277 S 2/N ? = 1 . 5 0 9 t = 6 . 7 8 / 1 . 5 0 9 = 4 . 4 9 3 D.F. = 12 P = l e s s than . 0 0 1 Conclusion: H. oregonensis l i v e s s i g n i f i c a n t l y longer i n a muddy, oxygen poor, environment than H. nudus. 44 TABLE VI (C) Comparison of survival times of H. nudus. with and without mud, i n corked one l i t e r f l asks. Time i n hours to death. Simple t test. Without Mud With Mud 10.0 12.7 10.0 11.6 13.3 11.6 20.0 10.0 13.6 l l . o 25.0 20.0 11.0 12 .0 11.0 13.3 20.0 12.7 25.5 12.0 25.0 11.0 25.0 11 .0 27.0 11.0 AVERAGES 18.18 12.30 Simple t test N = 26 Sum of y = 396.3 mean y^ = 18.18 mean y 2 = 12.30 (Sum of y ) 2 = 157053.69 (Sum of y) 2/N = 6040.53 sum of (y 2) = 6898.69 SS = 858.16 S 2 = 24.523 S2/N = 0.943 S2/N^ = 0 . 9 7 H t = 18.18 - 12.30/0 .9711 = 6.0549 D.F. = 24 P = l e s s than .005 Conclusion: H. nudus dies s i g n i f i c a n t l y sooner on a muddy environment than i n a clean one. 45 TABLE VI (D) Comparison of survival times of H. oregonensis i n corked one l i t e r flasks with and without mud. Time to death i n hours. Simple t test. Without Mud With Mud 20.0 11 .5 13.3 12.7 20.0 20.0 13.3 22 .0 13.3 20.0 20.0 20 .0 20.0 20 .0 20.0 20.0 22.0 H . 7 27.0 20.0 27.0 25.2 28.0 25.2 28.0 20.0 AVERAGES 20.91 19.10 Simple t test N = 26 Sum of y = 520.2 mean y], = 20.91 mean y 2 = 19.10 (sum of y ) 2 = 270608.OA (sum of y) 2/N = IO408.OO sum of (y 2) = 11025.18 SS = 617.18 S 2 = 595.756 S2/N = 22.913 S2/N* = A.7868 t = 20.91 - 19.10/A .7868 = 0.378A D.F. = 24 P = greater than .05 Conclusion; Mud does not affe c t the survival time of ri. oregonensis s i g n i f i c a n t l y . 46 TABLE VII Survival of crabs caged i n the f i e l d , i n areas with di f f e r e n t substrate p a r t i c l e size d i s t r i b u t i o n s . Duration - 3 months. 20 crabs per cage. Number Left Alive Out of Area Cage H. nudus H. oregonensis Muddy- 1 5 20 Rock 2 6 0 10 & 10 3 1 20 Clean 1 12 20 Rock 2 8 2 10 & 10 3 0 20 Muddy 1 14 20 Sand 2 9 10 & 10 3 11 20 Clean 1 17 20 Sand 2 4 0 10 & 10 3 0 20 X expected i s the average of the column or row of the contin gency table. Contingency Table Mud Clean Mud Clean Rock Rock Sand Sand Total E X 2 H.n alone 5 12 14 17 48 12.0 6.49 H.n with H.o 6 8 9 4 27 6.75 2.18 H.o alone 1 1 11 0 12 3.00 28.66 H.o with H.n 0 2 9 0 11 2.75 19.90 Total 12 22 43 21 98 E 3.0 5.5 10.75 5.25 X 2 8.66 16.55 1.55 37.10 D.F. for each X 2 = 3 . X 2 greater than 7.83 are s i g n i f i c a n t . hi TABLE VII (Continued) In the column of Chi square values, significance indicates that there i s a difference within a species between di f f e r e n t areas. From these, i t may be concluded that H. oregonensis, both alone and with H. nudus. shows a better survival i n the muddy sand area, while H. nudus i s not affected by the d i f -ferent areas s i g n i f i c a n t l y . In the row Chi square values, significance indicates that there i s a difference between the species within any one area. From these, one may conclude that only i n the muddy sand area i s there not a s i g n i f i c a n t difference i n the sur v i v a l of the two species. 48 TABLE VIII Total number of crabs on d i f f e r e n t substrates, counted 13 p times for 7 days. Significance calculated by X on o r i g i n a l data, using as the expected frequency the number of crabs per trough divided by the numbers of sections per trough. Trough Species Sand S i l t Mud Clean 1 H. nudus 77 51 48 37 H. oregonensis 2 0 58 51 44 2 H. nudus 65 32 3 4 60 H. oregonensis 60 49 43 49 3 H. nudus 49 23 4 6 69 H. oregonensis 65 95 30 13 Totals H. nudus 191 1 0 6 128 166 H. oregonensis 145 2 0 2 124 106 X 2 : for a l l 3 troughs. N = 39 X 2 Sand: 2- nudus 43.4 2« oregonensis 65.4 Mud : 2- nudus 41.4 2- oregonensis 59.4 S i l t : 2- nudus 76.2 2- oregonensis 67.6 Clean :H. nudus 35.2 2- oregonensis 66.8 X 2 for significance at 0 . 0 5 l e v e l i s 4 3 . 7 7 H. nudus avoids s i l t s i g n i f i c a n t l y . H. oregonensis s i g n i f i c a n t l y prefers s i l t to the other substrates. RO-TABLE IX Proportion of a l l crabs on beaches made up of those with a carapace width l e s s than 7 mm., r e l a t i v e to the dominance of the beach by the adults of the two species. Dominant Species Subordinate Species # Small # Large Ratio # Small # Large Ratio 1 96 0.0104 2 37 0.0541 + 6 42 0.1429 0 27 0.0 -80 1394 0.0547 39 82 0.4756 + 22 973 0.0226 14 81 0.1728 + 175 1580 0.1168 99 29 3.4138 + 43 388 0.1108 0 29 0 .0 -78 656 0.1189 8 55 0.1455 + 1 630 0.0016 9 116 0.0776 + 2 149 0.0134 5 15 0.3333 + 22 882 0.0249 32 237 0.1350 + 13 398 0.0326 6 5 1.2000 + 41 1365 0.0300 27 69 0.3913 + 8 268 O.0305 10 82 0.1220 + Sign test. N = 13 , x = 2 Probability = .002 (Table A, Siegel) Conclusion; The r a t i o of young to old i s greater for the sub-ordinate species than for the dominant species on most beaches, which indicates that the subordinate species i s suffering d i f -f e r e n t i a l mortality among the young crabs, i f one may assume that settlement i s approximately equal between the two species. 50 TABLE X (A) Number of crabs out from under l i m i t e d cover. 200 H. oregon- ensis present. Day # Crabs Exposed 1 0 2 0 3 9 4 27 5 32 6 17 7 31 8 62 9 _52_„ Mean 2 5 . 7 TABLE X (B) Number of crabs out from under cover. 200 H. nudus present. Day # Crabs Exposed 1 8 2 31 3 13 k 3 5 6 7 1 8 8 9 1 10 _17 Mean 9»5 51 TABLE X (C) Number of crabs out from under cover. 100 of each species present. # H. nudus # H. oregonensis Day Exposed Exposed  1 5 45 2 7 60 3 3 50 4 45 5 8 57 6 2 49 7 4 38 8 - L -Mean A .8 4 9 . 8 TABLE X (D) Comparison of number of crabs out from under cover by species. (A) X 2 between H. nudus and H. oregonensis when both are alone. E = average of t o t a l s = (231 + 9 5 ) / l 9 = 1 7 . 1 6 X 2 = 3 3 7 . 5 5 D.F. = 18 P . 0 0 1 Conclusion; When the species are tested separately, H» oregonensis i s exposed s i g n i f i c a n t l y more than TL nudus. (B) X 2 between H. nudus and H. oregonensis when they are confined together. "~ E = average of t o t a l s = (39 + 3 9 7 ) A 6 = 2 7 . 2 5 X 2 = 3 0 7 . 9 2 52 TABLE X (D) (Continued) D.F. = 1 5 P . 0 0 1 Conclusion: When both species are confined with limited cover, H. oregonensis i s exposed s i g n i f i c a n t l y more than H.""nudus. """ (C) X 2 between H. oregonensis i n presence of H. nudus and 2* oregonensis i n absence of H. nudus. "~ E = average of t o t a l s = ( 3 9 7 + 2 3 D / 1 7 = 2 3 . 3 7 X 2 = 5 2 1 . 6 2 D.F. = 16 P . 0 0 1 Conclusion: H. oregonensis i s s i g n i f i c a n t l y displaced from cover by~H.""nudus. 53 TABLE XI Crab taken f i r s t when p a i r s of crabs, not matched f o r s i z e , were thrown i n t o a tank with an assortment of c r a b - e a t i n g f i s h . Crab of P a i r Taken F i r s t H. nudus: 19 H. oregonensis: 1 5 Chi Square = 0 . 4 7 0 5 N.S. D.F. = 1 Conclusion: The f i s h appear to take the crabs on a random b a s i s , r a t h e r than d i s t i n g u i s h i n g between the two species. 54 TABLE XII Crab predators. Species of b i r d s and f i s h examined f o r crabs eaten. # Predators # H. # H. Species Taken nudus oregonensis Crow Corvus brachyrhyncus 5 1 0 Glaucous wing g u l l Larus glaucescens 7 0 0 Surf scoter M e l a n i t t a p e r s p e c e l l a t a 4 0 0 Buonapart g u l l Larus P h i l a d e l p h i a 1 0 0 Harlequin duck H i s t r i o n i c u s h i s t r i o n i c u s 4 5 13 Eared grebe Colymbus n i g r i c o l l i s 3 0 0 Barrows goldeneye Bucephalus i s l a n d i c a 5 2 6 Bairds cormorant Phalacorcorax pelagicus 4 1 0 Pigeon guillemot Cepphus columba 2 0 0 Red breasted merganser Mergus s e r r a t o r 3 0 0 Marbled murrelet Brachyramphus marmoratus 5 0 0 V/estern grebe Aechomophorus o c c i d e n t a l i s 3 0 0 Common loon Gavia immer 1 0 0 M ll rd duck Anas platy hynchus 2 Double cr sted cormorant Ph corco ax a u r i t u s 55 TABLE XII (Continued) # Predators # H. # H. Species Taken , nud"us oregonensis Starry flounder Platichthys s t e l l a t u s 1 1 0 Cabezon Scorpaenichthys marmoratus 1 0 0 Red i r i s h l o r d Hemilepidotus hemilepidotus 2 0 0 Ling cod Ophiodon elongatus 8 0 0 Kelp greenling Hexagrammos decagrammos 15 3 19 Quillback rockfish Sebastodes maliger 11 0 0 Black r o c k f i s h Sebastodes melanops 8 0 0 Striped seaperch Embiotoca l a t e r a l i s 1 0 0 Total H. nudus taken by a l l predators = 13 . Total H. oregonensis taken by a l l predators = 3 8 . Crab population of beaches where predators were obtained Place H. nudus H. oregonensis Ratio H.n to H.o Bowyer Island 642 12? 5.0 to 1 ' Bowen Island 322 52 6.2 to 1 San Juan Island 3276 448 7.3 to 1 Deception Pass 2250 230 9.8 to 1 Total 6490 857 7.57 to 1 Overall Ratio = 7.57 H. nudus to 1 H. oregonensis. Therefore, one would expect, on the basis of random choice of crabs by predators, that there should be about 7.57 H. nudus to 1 H. oregonensis i n predator stomachs. 56 TABLE XII (Continued) Total crabs taken by predators = 51• Therefore, the expected d i s t r i b u t i o n of species i s : 51 x 7 .57 /8 .57 = 45.05 H. nudus 51 x 1 .0 /8 .57 = 5.95 H. oregonensis Observed H, nudus = 13 Observed H. oregonensis • 38 X 2 = (45.05 - 1 3 ) 2 A 5 . 0 5 + (5.95 - 3 8 ) 2 / 5 . 9 5 = 195.44 D.F. = 1 Probability i s much l e s s than .001 Conclusion: H. oregonensis i s taken by predators to a much greater extent than i s H. nudus. 57 TABLE XIII Tolerance of the two species for low s a l i n i t y . A count of the number of crabs found dead after 6 days of r a i n diluted the seawater i n the container from 27 parts per thousand of s a l t s to 4 parts per thousand. H. nudus H. oregonensis Totals Live 141 83 224 Dead 165 78 243 Totals 306 161 467 Chi square. If the tolerance of the two species for fresh water i s i d e n t i c a l , then one would expect that the same pro-portion of each species would be dead. The proportion of a l l crabs dead i s 0 . 5 2 7 . Thus the number of H. nudus expected dead i s 159 .224, and the number of H. oreponensis expected dead i s 8 4 . 8 6 5 . X 2 = ( 1 5 9 . 2 2 4 - 1 6 5 ) 2 A 5 9 . 2 2 4 + ( 8 4 . 8 6 5 - 7 8 ) 2 / 8 4 . 8 6 5 = . 7 6 4 8 D.F. = 1 The difference between the species i s not s i g n i f i c a n t . 58 TABLE XIV Time to death of the two species i n fresh (tap) water. In hours. H. nudus H. oregonensis 20 20 22 21 20 16 24 28 18 22 20 30 24 22 12 18 16 26 Randomization test for 2 independent variables t = .748 D.F. = 16 P = 0.20 Conclusion: There i s no s i g n i f i c a n t difference between the two species i n time to death. 59 TABLE XV Time to death, i n hours, of the two species when kept i n dry dishpans. Death i n individ u a l s recorded when they no longer responded to touch. Totals Averages H. nudus 5.75 5.75 5.75 5.75 6.75 7.5 7.5 7.5 7.5 8.0 8.0 8.0 8.0 8.0 8.0 10.0 10.0 11.0 11.0 12.0 12.0 12.0 12.0 209.75 8.74 H. oregonensis 3.0 3.0 5.75 5.75 5.75 7.5 6.5 6.5 6.5 8.0 9.0 9.0 9.0 9.0 9.0 10.0 6.5 6.5 8.0 8.0 9.0 9.0 10.0 188.25 7.84 Mann-Whitney U test U = 219 Z = 3.13 P. = less than .0007 Conclusion: H. nudus survives desiccation longer than H. oregonensis. 60 TABLE XVI Regression on crab census data, with the number of crabs as the dependent variable, and as independent variables; the reducing power of the substrate (0 ) , the percent cover (R), the surf (S), the available food (F), the height of the sample on the beach (H), and the s a l i n i t y (N). Retabulated from computer printout of variance-covariance matrix. Variable Proportion of Sum of Squares Accounted For R 0 S 25.316 214.17 19.997 F N H . 0 0 3 5 4 7 2 . 0 1 6 . 0 0 3 4 5 Conclusion: The amount of cover was the most important v a r i -able associated with the number of crabs per sample. 61 TABLE XVII Comparison of the number of crabs found i n plots 1, 2 and 3, before and after rocks were removed from plot 1 and added to plot 2. I n i t i a l rock cover i n each area was approximately 100%. After moving the rocks, plot 1 had 50% cover, and plot 2 had 150%, while plot 3 remained unaltered. The species are treated as equivalent. Before Moving Rocks Plot 1 Plot 2 Plot 3 229 176 198 After Moving the Rocks Plot 1 Plot 2 Plot 3 123 331 208 Change Plot 1 Plot 2 Plot 3 -106 +155 +10 Conclusion: I t i s obvious that there was a s i g n i f i c a n t change i n the number of crabs i n the time between the f i r s t and second counts. 62 H. n u d u s fcfi 9 0 sO 7 0 u 50 3d xo •2 L* r \f •, . « * » • -»—t. 1 V o $.5" A r 3.0 Reducing power of substrate, i n p a r t s per m i l l i o n per hour. FIGURE 1: Graph of % H. nudus against reducing power of the substrate. FIGURE 2: Change i n d i s t r i b u t i o n of H. nudus and H. oregon-ensis over time at d i f f e r e n t oxygen concentrations. 64 FIGURE 2 (Continued) zo It 0 I *> 3 § c r a b s •V J / N y c \ I H—AC ^ 1 1 / —.• * / y ( J ' t 3 * fHruRs -Note that at low oxygen tensions, H. oregonensis outnumbers H. nudus, while the opposite i s true at high oxygen tensions, 65 P a r t s per m i l l i o n Note that as the oxygen tension r i s e s H. nudus replaces H. oregonensis. FIGURE 3: Change i n d i s t r i b u t i o n t o t a l s of the two species with oxygen tension. 66 N u m b e r s o f H. n u d u s c o 1 1 e c t e d 0 o v •••*--h;-%-'";£~j, lJf 32 S i z e s , i n mm., of the crabs FIGURE k: Size d i s t r i b u t i o n of H. nudus from San Juan I s l a n d , June, 1969. ~ 67 O F Sizes, i n mm., of the crabs FIGURE 5: Size d i s t r i b u t i o n of H. oregonensis from Spanish Banks, February, 1 9 6 8 . 68 c r a b s s t i 1 1 a 1 i v e H. nudus  H. oregonensis xxxxxxx 1 2f> \ *L \ 14 \ i If \ 31 \ lb ,4 \ \ /s /-V. a to t y—> N ,/ • " J T *—* <—i I i_ ••• % / I 1 ¥ * L r \ 5 Days FIGURE 6: Number of crabs l e f t a l i v e a f t e r a d d i t i o n of 3 0 of each species to a tank w i t h predatory f i s h and cover. 69 A M P OF p f ? & 3 > A T 0 R CGJLJL ECTiVC- AKEA* — BIBLIOGRAPHY 70 CLEMENS, W.A. AND WILBY, G.V. 1967. Fishes of the P a c i f i c Coast of Canada. Queens Printer, Vancouver. CONNEL, J . 1961a. E f f e c t of Competition, Predation by Thais l e p i l l u s . and Other Factors on Natural Popula-tions of the Barnacle, Balanus balanoides. Ecol. Monographs, 3 1 : 61-104. COTTAM, C. 1939. Food Habits of North American Diving Ducks. U.S.D.A. Tech. B u l l . # 643. CROMBIE, A.C. 1947. I n t e r s p e c i f i c Competition. J . An. Ecol. 16: 44 -73 . DEHNEL, P.A. i 9 6 0 . Effect of Temperature and S a l i n i t y on the Oxygen Consumption of two I n t e r t i d a l Crabs. B i o l . B u l l . 118. 2 : 215-249. ELTON, C.S. 1935. Animal Ecology. Second Edition, London. HARGER, J.R.E. 1967. Population Studies on Mytilus Communi-t i e s . Ph.D. Thesis, U.C.S.B., 1967. HART, J.F.L. 1935. The Larval Development of B r i t i s h Colum-bia Brachyura I. Can. J . Research, 12 : 411-432. HIATT, R.W. 1948. The Biology of the Lined Shore Crab, Pachygrapsus crassipes. Pac. S c i . 2 : 135-213. KNUDSEN, J.W. 1964. Observations on the Reproductive Cycles and Ecology of the Common Brachyura and Crablike Anomura of Puget Sound. Pac. S c i . 8: 3 -33 . RICKETTS, E.F., CALVIN, J . AND HEDGEPETH, J.W. 1962. Between P a c i f i c Tides, Third Edition, Revised by J . W. Hedge-peth. SCHMIDT, W.L. 1921. The Marine Decapod Crustacea of C a l i f o r -nia. Univ. of C a l i f . Publ. Zool. 23 : 1-476. SIEGEL, S. 1956. Nonparametric S t a t i s t i c s for the Behavorial Sciences. McGraw H i l l , Toronto. 

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