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Some aspects of the behavioural ecology of two amphipod species in Marion Lake, British Columbia Bryan, Anthea D. 1971

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SOME ASPECTS OF THE BEHAVIOURAL ECOLOGY OF TWO AMPHIPOD SPECIES IN MARION LAKE, BRITISH COLUMBIA by Anthea D. Bryan B.Sc, University of V i c t o r i a , 1965 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in the Department of ZOOLOGY Ye accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA September, 1971 In p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t of the r e q u i r e m e n t s f o r an advanced d e g r e e a t the U n i v e r s i t y o f B r i t i s h C o l u m b i a , I a g r e e t h a t t h e L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and S t u d y . I f u r t h e r a g r e e t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g o f t h i s t h e s i s f o r 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 the Head o f my Department or by h i s r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g or p u b l i c a t i o n o f t h i s thes.is f o r f i n a n c i a l g a i n s h a l l not be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . Department o f The U n i v e r s i t y o f B r i t i s h C olumbia Vancouver 8, Canada Date Ijtjp&snAvu : WI i i ABSTRACT Two benthic amphipod species (Crangonyx richmondensis var occidentalis Hubricht and Harrison and Hyalella azteca Saussure) l i v e together i n Marion Lake, but, as previous workers have shown, d i f f e r markedly in the d i s t r i b u t i o n of their numbers. The aim of the study was to compare the behaviour of the two species and to see i f any differences might account for the d i f -ferences i n d i s t r i b u t i o n . Particular attention was paid to movement and feeding behaviour. Crangonyx was equally abundant on the bottom at a l l depths in the lake; Hyalella was abundant i n the shallows ( 1m) and scarce i n the deeper waters ( 3 m). Numbers of each species d i f f e r e d among areas i n the lake. Crangonyx usually moved by crawling over the mud, and s e l -dom swam. I t seemed to find food by i t s smell or motion or both, and to recognize food by touch or taste .. It attacked l i v i n g prey by pouncing on i t and grasping i t with i t s gnatho-pods. In contrast, Hyalella moved mainly i n a series of swims and pauses. Hyalella fed by grazing on the mud and rooted aqua-t i c s , and ingesting detritus. Both species moved away from sources of l i g h t . ¥hen food was present i n various d i s t r i b u t i o n s , the search patterns of Crangonyx di f f e r e d : some animals turned back and thoroughly searched the area where they had just found food; others crawled along a r e l a t i v e l y straight path; yet others behaved i n an intermediate manner. In general, Crangonyx seemed to be i i i quicker at finding clumped food than uniformly distributed food. In the absence of food, some individual Crangonyx turned more than others. Hyalella searched by swimming, pausing on a surface, and remaining where food was present. Probably neither species moves frequently between depth zones, although both species are mobile enough to do so. Marked amphipods placed at 1 - and 3-m depths i n the lake were recaptured in samples taken 1 and 4 m from the release point after an hour. Fewer marked amphipods of either species were recaptured i n the deep area than i n the shallow area, indicating that they l e f t the deep area faster. Of the behavioural differences noted, only the difference i n feeding habits between the species seems l i k e l y to account, at least i n part, for the difference i n the d i s t r i b u t i o n of their numbers. Recent work by Dr. Hargrave has shown that epibenthic algal production decreases with depth, though not as sharply as Hyalella numbers. In laboratory substrate-choice experiments, Hyalella* 1chose areas of abundant food. Its assimilation e f f i -ciency and growth d i f f e r e d when i t fed on d i f f e r e n t sediment microflora. Dr. Grtfendling found that the abundance of the algal groups i n Marion Lake di f f e r e d with depth. The d i s t r i b u t i o n of Hyalella may be related to the d i s t r i b u t i o n of certain species of algae i n i t s diet. The d i s t r i b u t i o n of food for Crangonyx is unknown. iv TABLE OF CONTENTS Page TITLE PAGE ± ABSTRACT ± ± TABLE OF CONTENTS i v LIST OF TABLES v i LIST OF FIGURES v i i i ACKNOWLEDGEMENTS i x INTRODUCTION 1 MATERIALS AND METHODS 3 Observations on Amphipod Behaviour at Marion Lake 3 Laboratory Conditions and Procedure 3 1. Crangonyx searching - food absent 3 2. Crangonyx searching - food present 4 Bottom Samples 5 Movement Studies 9 RESULTS 11 Aspects of Behaviour 11 1. Locomotion 11 2. Feeding Habits 11 3. Approach to food and food recognition 12 4. Intra- and i n t e r - s p e c i f i c encounters 14 5. Reaction to l i g h t 16 Searching by Crangonyx 16 1. Food absent 16 2. Pood present 23 A. Food clumped 23 V B. Food uniformly distributed 27 C. Food i n straight line 27 Searching by Hyalella 31 Distr i b u t i o n of Amphipods 31 Movement Studies 44 DISCUSSION 48 Distr i b u t i o n of Amphipods 48 Food Supply for Crangonyx 49 Food Supply for Hyalella 49 Movement 51 Searching by Crangonyx 54 SUMMARY 57 BIBLIOGRAPHY 59 v i LIST OF TABLES Page I. Number of paths taken i n each dir e c t i o n by a 17 Crangonyx moving out from the centre of a con-tainer where the direction of the strongest l i g h t source was 353 to 7 . II. Frequency of di r e c t i o n of paths taken by eight 17 Crangonyx under uniform l i g h t conditions; animals were released i n the centre of a container and the angle of their paths from a fixed compass direction measured. III. Frequency of angles turned from previous path dir e c t i o n 18 by a Crangonyx at one-second intervals i n a lakeside tub where mud but no prey was present. IV. Angles turned from previous path d i r e c t i o n by eight 21 Crangonyx i n uniform l i g h t conditions. V. Time taken by eight Crangonyx i n uniform l i g h t 22 conditions to cross each of three concentric c i r c l e s ; individual animals were released in the centre of the container. VI. Analysis of search times for Crangonyx i n the Clumped 25 Food Experiment. VII. Analysis of search times for Crangonyx i n the Uniform 28 Food Experiment. Vm. Overall mean search times (sec) of a l l Crangonyx i n 30 Clumped, Uniform, and Straight Line Food Experiments. a. only animals used i n later experiments included. b. a l l animals used i n Clumped Food Experiment i n -cluded. IX. Eff e c t of month, station, and depth on number of 35 Hyalella. A. Analysis of variance by month and station. B. Analysis of variance by month and depth. C. Comparison of v a r i a b i l i t y i n Hyalella numbers among stations and depths. X. Average size of Hyalella and of Crangonyx males at 41 different depths i n A p r i l , 1969. XI. Effect of month, station, and depth on number of 42 Crangonyx. A. Analysis of variance by month and station. B. Analysis of variance by month and depth. C. Comparison of v a r i a b i l i t y i n Crangonyx numbers among stations and dejfchs. v i i XII. Total number of marked Hyalella and Crangonyx 46 recaptured within a 1-m radius of the release point at 1 - and 3-m depths during two movement studied in Marion Lake. 1000 marked Hyalella and 165 marked Crangonyx were released at each depth, ; Data for July are based on seven samples at each depth, 2 hr after the amphipods were released; data for August, on eight samples, 45 min after release. 2 XIHi Density of unmarked amphipods per 506.25 cm 46 i n a l l samples taken at 1- and 3-m depth for movement studies. Number of samples at each depth was 15 i n July, 16 i n August. v i i i LIST OF FIGURES Page 1. Food distr i b u t i o n s used i n laboratory experiments on 6 searching by Crangonyx. Each black dot represents a piece of frozen Tubifex about 2 mm long. A. Food clumped; B. Food uniformly distributed; C. Food i n straight l i n e . 2. Contour map of Marion Lake, B. C., showing sampling 8 areas (A to T). 3. Paths of Individual Crangonyx marked off i n 4-sec 20 intervals i n a container where food was absent. A. turner; B. non-turner. Numbers between demarcations refer to the time in t e r v a l between those demarcations when less than 4 sec. 4. Density of Crangonyx and Hyalella at 1 - and 3-m depths 32 i n 1967. V e r t i c a l lines represent standard errors. Each point is the average from 4-5 samples. 5. Average midday temperature at the mud surface at 1 - 33 and 3-m depths i n 1967. 6. Density of Hyalella and Crangonyx at different depths 37 i n 1968 and 1969. V e r t i c a l lines represent standard errors. A. Hyalella; B. Crangonyx. Numbers along the x-axis i n B represent sample sizes for both A and B. 7. Size d i s t r i b u t i o n of Hyalella at different depths from 40 May to September and i n December, 1969. 8. Size d i s t r i b u t i o n of Crangonyx at a l l depths i n 1968 45 and 1969. ix ACKNOWLEDGEMENTS I am very grateful to my supervisors, Dr. D. H. Chitty and Mr. N. E. Gilbert, for their help and encouragement throughout the study. Dr. I. E. Efford, Dr. C. S. Holling, and Mrs. J. Maynard c r i t i c a l l y read the thesis. I am indebted to Mr. K. Tsumura for help with both f i e l d and laboratory work, to Dr. D. M. Ware and Mr. W. R. Black for help with the construction of equipment, and to Mr. I. Tesaki for help with the measurement of amphipods. Mrs. D. Lauriente provided assistance by writing computer programs to summarize data. Financial assistance was provided by the National Research Council of Canada and the Department of Zoology at the University of B r i t i s h Columbia. 1 INTRODUCTION When two morphologically similar species l i v e i n the same habitat, but d i f f e r i n r e l a t i v e abundance in di f f e r e n t parts of i t , a comparison of the behavioural ecology of the two may c l a r -i f y the reasons for this difference i n abundance; i t may estab-l i s h whether these species can coexist without interference. This study i s concerned withthe behaviour a l ecology of two such amphipod species i n Marion Lake, B r i t i s h Columbia. Marion Lake, described by Efford (1967^ is a small lake (surface area, 33 acres) with a mean depth of 2.4 m and an aver-age maximum depth of about 5.5 m, though the lake l e v e l fluctuat considerably. The lake bottom is covered with a flocculent ooze weed-beds are sparse. Hyalella azteca Saussure, the only described species of Hyalella i n North America, is widely spread in permanent fresh water on this continent (Bousfield, 1958); i t has been studied f a i r l y extensively (see Cooper, 1965). Females produce several broods during the summer; adults l i v e for 12-16 months. Cran-gonyx richmondensis occidentalis Hubricht and Harrison occurs i n 'bog ponds, small lakes and their outflows' i n B r i t i s h Columbia and Washington; two other subspecies are found i n Eastern North America. Very l i t t l e work has been published about this subspecies. Crangonyx females l i v e from 13-14 months and produc a single brood in the spring; males l i v e from 16-23 months (Mathias, 1971). 2 Both Hamilton (1965) and Mathias (1971) studied the amphipods in Marion Lake. Mathias compared the population energetics of the amphipods in r e l a t i o n to depth and found marked differences i n the d i s t r i b u t i o n of numbers of the two species between May and October. 2 The mean summer standing crop of Hyalella was high (1952/m ) at 1-m depth, but was low (75/m ) at depths greater than 2.5 m. On 2 the other hand, Crangonyx occurred i n uniform density (283/m ) at a l l depths, except that numbers were s l i g h t l y higher i n very shallow water. I t became evident, largely from work by Hargrave (1970a), that the two species had d i s t i n c t feeding habits, Hyalella being mainly a herbivore, Crangonyx a carnivore. Har-grave ' s evidence suggested that the d i s t r i b u t i o n of Hyalella was related to "that of i t s food supply. The aims of my study were : 1. to investigate more f u l l y the movement and feeding behaviour of the amphipods, with special emphasis on Crangonyx, the less well known of the two species. 2. To consider whether any differences in behaviour might account for the difference in depth d i s t r i b u t i o n . 3 MATERIALS AND METHODS Observations on Amphipod Behaviour at Marion Lake To compare the feeding behaviour and pattern of movement of the two species, I observed the movement of individual amphipods i n the lake from a glass-bottomed boat and also through a view-ing box with a plexiglass bottom. Unfortunately, i t was not possible to watch individuals for long in the lake because of their small size and the uneven surface of the mud, which often hid them from view. Therefore, I recorded paths of individuals i n c i r c u l a r wading pools 90 cm i n diameter near the lake. I made tracings on cellulose acetate placed over a plexiglass grid with c e l l s 5 cm a side and 0.7 cm deep. The grid enabled me to look d i r e c t l y down on the animal while following i t s movements. A metronome and stopwatch aided the timing of movements. Laboratory Conditions and Procedure Animals used i n laboratory experiments were collected from Marion Lake and held i n the laboratory at temperatures ranging from 16 to 22 C, and under a day length of 16 hr. They were kept on mud i n lake water or conditioned tap water. Crangonyx used i n searching experiments were kept i n the laboratory for several months. Crangonyx searching - food absent During an experiment on the d i r e c t i o n of paths taken by Crangonyx, each animal was kept i n a c i r c u l a r p l a s t i c box 25 cm 4 in diameter containing sieved mud 1 to 2 mm deep i n about 1 l i t e r of dechlorinated water. The mud was sieved to remove as much food as possible and to make i t more uniform. Five female Crang-onyx were used; " the animals were not fed during the experiment. Light sources included both diffuse overhead l i g h t and l i g h t sources at the side of the containers. Each animal was released from a central p l a s t i c cylinder, and i t s path was recorded u n t i l i t reached the edge of the container. The paths were traced on cellulose acetate over a glass plate as described i n the previous section. I made at least nine observations per animal and meas-ured the direction of each path by drawing a straight line between the point of release and the point where the animal reached the edge of the container. I followed the same procedure to test eight animals kept i n uniform l i g h t conditions. The containers, with their sides cov-ered i n black p l a s t i c to eliminate side l i g h t i n g , were placed on shelves l i t from above by fluorescent l i g h t s . If an animal was hidden i n the mud at the start of a t r i a l , I disturbed i t gently and recorded i t s path, unless i t swam d i r e c t l y to the edge. Its behaviour was considered normal i f i t swam. Each animal was ob-served 20" times. Crangonyx searching - food present Similar containers with black p l a s t i c were used during ex-periments on searching by Crangonyx where food was present. The 18 experimental animals were females ranging i n size from 1.22 to 1.48 mm head length. They were kept on the shelves i n separate 5 containers with about 250 ml of sieved mud and 1 l i t e r of dechlor-inated water. The experimental containers were the same as those i n which the animals were held. The food consisted of pieces of frozen Tubifex about 2 mm long, arranged i n the appropriate d i s -t r i b u t i o n just before each t r i a l (Fig. 1). The animal was i n t r o -duced at the edge of the container and i t s path traced u n t i l three pieces of food had been found or 30 min had elapsed. A piece of food was considered 'found' i f the animal handled i t but did not necessarily take i t away. Because they do not eat just before they molt, the animals were tested before every t r i a l to see i f they would respond to the food. If they were attracted to Tubifex juice introduced i n front of them, they were used i n the t r i a l . On days when an animal was not tested, i t was fed a piece of frozen Tubifex. In the Clumped Food Experiment, I used a balanced incomplete block design (Fisher and Yates, 1963) so that I could take into account any differences which might be associated with the time of day at which the animals were tested. I tested a group (block) of four animals i n the morning and a group of four i n the a f t e r -noon. I used 16 animals and made five replicates on each animal. In the Uniform Food Experiment, I used 10 animals and did five replicates on each. I did only two replicates i n the Straight Line Food Experiment, and tested the same 10 animals as in the Uniform Food Experiment. Bottom Samples The f i e l d study was carried out i n Marion Lake, B r i t i s h Fig. 1. Food distributions used i n laboratory experiments on searching by Crangonyx., Each black dot represents a piece of frozen Tubifex about 2 mm long. A. Food clumped; B. Food uni formly distributed; C. Food i n straight l i n e . 7 Columbia. Using a s t r a t i f i e d random sampling design, I took bottom samples at least twice a month with a 6-inch Ekman grab (area 225 cm ) at the 1- and 3-m depth zone along the east side of the lake (Fig. 2). I took 5 samples at each depth on each date, and recorded the temperature at the mud surface. I sieved the samples through a #30 mesh sieve (aperture 0.56 mm), sorted them l i v e , and preserved the amphipods i n 70$ alcohol. In a test of sorting e f f i c i e n c y , I sorted three l i v e samples from each of the two depth zones, then preserved the samples and sorted them several times more. Sorting e f f i c i e n c y was high on Crangonyx (88 to 100$) and on adult Hyalella (at least 77$), but was much more variable on immature Hyalella (42 to 76$); the consistency of the mud was important to the ef f i c i e n c y . I also carried out an analysis on data from a more extensive sampling program carried out under the di r e c t i o n of Dr. Efford. Twenty-five bottom samples were taken once a month from June 1968 to September 1969 and in December 1969. Two Hargrave samp-2 l i n g devices (Hargrave, 1969) were used, one with area 189 cm , 2 the other with area 225 cm.. A s t r a t i f i e d random sampling design was used because the numbers of several benthic organisms were known to change with depth. Fig. 2.shows the grid system d i v i d -ing the lake into 25 areas roughly homogeneous with respect to depth; every month, one sample was taken from each area. The samples were sieved (sieve aperture 0.8 mm), preserved i n 10$ formalin, and sorted later by hand. Sorting e f f i c i e n c y has not yet been calibrated. I cleared the Hyalella i n dilute potassium hydroxide and measured their head lengths. Dr. D. Johnson 100 M Fig. 2. Contour map of Marion Lake, B. C., showing sampling areas (A to T). 9 counted and measured the Crangonyx from the samples. Movement Studies For a movement study involving marking, releasing, and re-capturing large numbers of both species, the amphipods were marked with a v i t a l stain. Of several stains tested, Neutral Red, and Neutral Red V i t a l and Fluorescent were successful for Hyalella but not for Crangonyx. Methylene Blue, Methylene Blue Chloride, Trypan Blue, and Janus Green did not mark either species d i s t i n c t l y , and often k i l l e d them. However, Rhodamine B Fluorescent stain marked both species f a i r l y well; marked animals could be distinguished readily under an u l t r a - v i o l e t l i g h t . The amphipods were stained i n a solution of this stain at strength .5% diluted one part stain to nine parts lake water, as recommended by Hamilton (1969). No stain was found which would mark both species adequately without harming them and also l a s t for more than a day or two. For this reason, and because of the d i f f i c u l t i e s of processing large enough numbers of amphipods to ensure a high recapture rate, I could do only short term experiments i n which the animals were not allowed to disperse for long. Experiments designed to indicate both the extent and the direction of_movement of both species i n the lake were carried out i n July and August, 1969, i n areas L and M (Fig. 2). For these experiments, I collected mud containing large numbers of amphipods and sieved i t through a #20 mesh sieve (aperture 0.8 mm) to eliminate the smaller animals. The Hyalella used i n the 10 experiment ranged i n size, expressed as head length, from 0.35 to 0.82 mm, the Crangonyx from 0.58 to 1.02 mm. The amphipods were stained overnight. The next morning,, 1000 marked Hyalella and 165 marked Crangonyx were placed i n a p l a s t i c bag and l e f t for about 30 min to become acclimated at the mud surface i n the centre' of the 1-m experimental area. The amphipods were then released at the mud surface. The same procedure was followed i n the 3-m experimental area. Two hours la t e r , I sampled the 1-m area, and then the 3-m area, using a 9-inch Eckman grab (area 506.25 cm ). These samples were taken at the release point and at distances of 1 and 4 m away. I sampled i n four directions at 1 m from the release point, because I hoped to recapture enough marked amphipods i n each dir e c t i o n to be able to detect any ten-dency for Hyalella to move from the 3-m area towards shallower water. I hoped that the samples at 4 m from the release points would indicate how far the amphipods could move i n a given time. I sieved' the samples through a #20 mesh sieve, and recorded the number of marked and unmarked amphipods. The second time I performed this experiment, I carried out the mark-recapture procedure i n the morning on consecutive days, at one depth each day, to f a c i l i t a t e the processing of samples. In this experiment, I placed four small funnel traps around the release point i n an attempt to detect the direction of movement of the marked amphipods. I took samples after about 1 hr instead of 2 hr. 11 RESULTS ASPECTS OF BEHAVIOUR Locomotion Observations on Crangonyx and Hyalella i n the lake showed that there were obvious differences i n movement between the species. Crangonyx spent most of i t s time crawling over the mud surface, and pausing occasionally. It swam rarely, and when i t did, i t stayed close to the mud. In contrast, Hya1eI1a often swam well above the mud, pausing on rooted aquatics and on the mud; swim and pause times varied widely. I t crawled only b r i e f l y . Feeding Habits Gut analyses of female Crangonyx collected in the f i e l d and preserved i n 5% formalin showed that i t s diet includes chironomids. Much of the material i n the gut i s unidentifiable by microscopic examination. In the laboratory, Crangonyx eats food such as dead brine shrimp,,, l i v e and dead Tubif ex, Drosophila larvae, and chironomids. Crangonyx carries i t s food with i t s gnathopods, often eating as i t moves rather than remaining at the si t e where i t found i t . In the lake, Hyalella was found on the stems of rooted aqua-t i c s during the daytime, where i t was presumably grazing on epi-phytic material. In the laboratory, Hyalella ingested detritus and unidentified species of algae. 12 Approach to Food and Food Recognition In s t i l l water, Crangonyx may approach i t s food in various ways. It may start to turn i t s body from side to side more widely or more frequently than usual (klinokinesis); at a d i s -tance of about 2 cm from a food item i t may show a directed re-sponse i n which i t turns from side to side and moves toward the food ( k l i n o t a x i s ) ; i t pounces on the food from about 2 mm away. Alternatively, i t may pass by the food, loop sharply around i t from about 2 mm away, and then approach f a i r l y d i r e c t l y (tropo-t a x i s ) . Thus, i t shows the sequence of methods of orientation which i s t y p i c a l of response to chemical stimulation, as described by Fraenkel and Gunn (1961 ). As Crangonyx moves, i t waves its long f i r s t antennae ahead, above, and to the side i n the water column, while the short second antennae touch the mud surface. Sometimes, i t stops and rubs i t s f i r s t antennae with i t s gnathopods. A few simple tests of the importance of smell, taste, sight, touch and motion i n the finding and recognition of food were con-ducted on Crangonyx in glass bowls 10 cm i n diameter containing water and sieved mud. (1) Smell or chemical stimuli. When the f l u i d i n which brine shrimp had been soaked was introduced from an eye dropper into the container, the adult Crangonyx became very active, and occasionally rubbed their antennae with their gnathopods. Their speed of crawling increased, and then they started" to swim i n c i r c l e s i n the area where the juice had been injected. Several individual Crangonyx behaved i n the same way to f l u i d i n which 13 frozen Tubifex had been soaked. If an individual moved into an area "where the juice was very concentrated, i t quickly l e f t again. When plain water was introduced, the animals sometimes approached but showed no similar increase i n a c t i v i t y . Thus i t seems that Crangonyx's reaction involved chemoreception. Crangonyx can find pieces of frozen Tubifex buried 6 mm under the surface of the mud; tests where the food was deeper were not made. This observation provides further evidence for the importance of o l -factory stimuli. (2) Touch. To test the reaction of Crangonyx to a non-food item, I placed a small piece of gauze on the mud surface. The Crang-onyx did not react to the gauze u n t i l i t touched i t with i t s antennae. I t sometimes pounced on and sometimes merely crawled up to the gauze. After holding the gauze b r i e f l y i n i t s gnatho-pods, the Crangonyx dropped i t and moved away. On the next encounter, the Crangonyx repeated this response. It seems that the Crangonyx could not sense that the gauze was a non-food item u n t i l i t had grasped and perhaps chewed the gauze. Thus Crang-onyx may use t a c t i l e stimuli to distinguish the gauze from the mud, and chemical stimuli to distinguish food from non-food. (3) Sight. Pour small glass v i a l s were placed i n the bowl. The f i r s t contained l i v e Tubifex, the second dead Tubifex, the t h i r d a l i v e Crangonyx, and the fourth water only. The object was to see i f Crangonyx could detect dead and l i v e prey and another Crangonyx by sight alone. The v i a l containing only water served as a control for the reaction of Crangonyx to the v i a l i t s e l f . On approaching a v i a l , 14 the Crangonyx shoved no consistent difference i n behaviour towards the contents; i t occasionally stopped and waved i t s f i r s t antennae close to a v i a l , or attempted to dig underneath i t , but otherwise passed by with no obvious reaction. (4) Motion. In a test for rheoreception, a thread (comparable in size to prey such as a chironomid) was dangled about 5 mm in front of the f i r s t antennae of a motionless Crangonyx. The Crangonyx moved slowly forward, perhaps touched i t with i t s antennae, then pounced on the thread and held on for several seconds. Movement of an object seems to be s u f f i c i e n t stimulus to induce Crangonyx to approach i t . These observations suggest that smell, taste, and rheorecep-tion are a l l important i n food detection. The sense of touch may also be used, but t a c t i l e cues may not have been separated from chemical cues in these observations. Visual stimuli alone do not e l i c i t feeding behaviour, though they may aid an animal which has detected a prey by olfaction. H y a l e l l a 1 s approach to food consists of swimming i n a straight or s l i g h t l y curving path and then a l i g h t i n g on an object where food may or may not be present. Intra- and Inter-Specific Encounters ¥hen one Crangonyx had a piece of food i n i t s gnathopods, another hungry Crangonyx which had detected this food would pounce on the food and attempt to get some for i t s e l f . The f i r s t animal attempted to avoid the second by moving away quickly • 15 i n short bursts of swimming and crawling. In contrast, Hyalella aggregated i n large numbers on food, and did not show aggressive behaviour. In a p e t r i dish containing water only, two amphipods of the same species often reacted d i f f e r e n t l y upon meeting each other than did two individuals of opposite species. A Hyalella, upon meeting a Crangonyx, or very rarely another Hyalella, jumped up and to one side and swam away. If the Hyalella was motionless at the time of the encounter, i t might remain so. Crangonyx would sometimes attempt to seize a Hyalella but rarely another Crangonyx. Observations were made i n the f a l l , when the breeding season was over;' and thus the animals were not pairing. Results were similar when amphipods of equal size (juvenile Crangonyx and adult Hyalella) were used as when Crangonyx larger than the Hyalella (adults of both species) were used. On several occasions, Crangonyx was observed attacking Hyalella i n the lake by pouncing on i t and holding i t with i t s gnathopods. A crawling Crangonyx would suddenly leap forward onto a Hyalella and grasp i t . I do not know how frequently this occurs, or what the consequences are. In small lakeside contain-ers, Crangonyx ate newly released young Hyalella even when an acceptable alternate food (washed brine shrimp) was present. There i s no evidence that they do so i n the f i e l d . In the lab-oratory, Crangonyx which attacked a Hyalella often l e f t i t al i v e but maimed; Crangonyx also attacked young of both species. 16 Reaction to Light Crangonyx moves away from a strong l i g h t source. When each of f i v e animals was released repeatedly from the centre of a 25-cm container with l i g h t sources from the side, the Crangonyx moved away from these l i g h t sources more frequently than i n any other di r e c t i o n (Table I ) . However, under uniform l i g h t condi-tions, the eight Crangonyx tested moved i n random directions from the centre (Table I I ) . Observations on the behaviour of Hyalella when a l i g h t i s shone on one side of their container show that they are negative-ly phototactic. They become more active and move to the darker side of the container soon after the l i g h t i s turned on; they s h i f t to a new location whenever the l i g h t i s moved. SEARCHING- BY CRANGONYX  Food Absent To investigate the pattern of movement of a Crangonyx i n a lakeside tub, I measured the angle of turning from the previous path d i r e c t i o n for every one-second i n t e r v a l along the path. F i r s t I tested to see i f Crangonyx was turning uniformly at a l l angles from 0 to 360°, regardless of whether i t turned to the l e f t or to the right of i t s previous path. I compared row 3 of Table III with expected values of 8 and found that the d i s t r i -bution of path directions was not uniform (X = 35.54 with 5 df, p = 0.00); Crangonyx seldom turned back sharply. However, this test does not allow for the fac t that Crangonyx may have a bias to turn i n one direction. I compared the t o t a l number of turns 17 TABLE I Number of paths taken i n each direction by a Crangonyx moving out from the centre of a container where the dire c t i o n of the strongest l i g h t source was 353 to 7°. Angle from Light Source Number of Paths 0 - 30, 330 - 360 1 30 - 60, 300 - 330 0 60 - 90, 270 - 300 2 90 - 120, 240 - 270 3 120 - 1 50, 210 - 240 3 150 - 210 -10 7 Total 16 TABLE II Frequency of dir e c t i o n of paths taken by eight Crangonyx under uniform l i g h t conditions; animals were released in the centre of the container and the angle of their paths from a fixed compass direction measured. Animal Number Angle 1 2 3 4 5 6 7 8 Total 0 - 45, 315 - 360 6 4 3 3 10 3 4 3 36 45 - 90, 270 - 315 3 5 3 4 3 6 2 9 35 90 - 135, 225— 270 6 7 5 4 3 5 6 3 39 135 - 22.5 5 4 9 9 4 6 8 5 50 X 2 = : 25.62 with 21 df, P = 0.22 Total 1 60 18 Table III Frequency of angles turned from previous path di r e c t i o n by a Crangonyx at 1-sec intervals i n a lakeside tub where mud but no prey was present. Angle from Previous Path Direction 0T30 30-60 60-90 90-120 120-150 150-180 Total Right 1 5 3 2 1 0 0 21 Left 18 5 3 1 0 0 27 Total 33 8 5 2 0 0 48 19 to the right (21) and to the l e f t (27) with expected values of 24, and found that Crangonyx showed no bias to turn i n one d i r -ection (X = 0.167 with 1 df, p = 0.68). F i n a l l y , using rows 1 and 2, I tested to see i f there was an interaction between the dir e c t i o n Crangonyx turned and the angles i t turned. There was 2 no interaction (X =0T226 with 5 df, p = 0.998). Crangonyx tended to continue to move i n i t s previous direction and showed no bias to turn to one side or the other. In the uniform l i g h t experiment, Crangonyx moved i n random direction from the centre but paths of individual Crangonyx seemed to d i f f e r ; some animals turned more than others (Fig. 3). I used the measure just described, the number of times they turned at various angles from their previous path direction, this time at 4-sec intervals, to compare individual behaviour. I did not test for bias to turn l e f t or right, since the Crang-onyx already tested showed no such bias, and these animals did not seem to either. Some animals did turn more sharply than others (Table IT)'. Another measure of this behaviour was the time from release i n the centre of the container u n t i l the animals had crossed each of three concentric c i r c l e s of r a d i i 3, 6, and 9 cm. Anim-als which were turning back more would take longer to cross the c i r c l e s than animals with more direct paths. There was no d i f -ference between animals in time to cross the two smaller c i r c l e s , but some animals did take longer to cross the 9-cm c i r c l e than others (Table V). The length of time which had elapsed may have been too short for differences to show up before the animals 20 P i g . 3 . Paths of i n d i v i d u a l Crangonyx marked o f f i n 4-sec i n t e r -v a l s i n a container where food was absent. A. turner; B. non-turner. Numbers between demarcations r e f e r to the time i n t e r v a l between those demarcations when l e s s than 4 sec. 21 Table IV Angles turned from previous path d i r e c t i o n by eight Crangonyx i n uniform l i g h t c o n d i t i o n s . Angle of Path from Previous Path 1 2 3 4 5 6 7 8 T o t a l 330- 30 13 53 37 25 31 37 29 33 258 30- 60, 300-330 9 26 29 17 28 16 13 1 5 153 60- 90, 270-300 5 38 15 14 11 10 9 19 121 90-120, 240-270 2 11 4 '1 2 2 1 7 36 120-150, 210-240 3 12 2 1 •1 1 0 3 29 " 150-210 0 5 0 2 0 0 0 2 9 _ n 32 145 87 72 73 66 52 79 606 X 2 = 54.81 with 28 df p = 0.002 22 Table V Time taken by eight Crangonyx i n uniform l i g h t conditions to cross each of three concentric c i r c l e s ; individual animals were released i n the centre of the container. Animal Time ( sec) to Cross C i r c l e Number of Number Radius 3 cm Radius 6 cm Radius 9 cm Tracks 1 5.3 9.0 12.5 20 2 7.1 18.6 38.7 20 3 7.0 13.1 20.2 20 4 6. 2 10. 5 15.2 18 5 6.1 13.0 21 .0 15 6 6.4 13.4 18.3 20 7 6.6 10.0 15.0 19 8 6.0 10.1 19.0 20 Radius 3 cm Radius 6 cm Radius 9 cm Source of df Mean square F Mean square F Mean square F Variation Between Animals 7 6.77 .245 188.39 1.81 1301.78 3.20 Within Animals 144 27.66 104.33 407.10 p>.75 .05 <p<. 10 p<.005 23 reached the 9-cm c i r c l e . Eood Present Food clumped: To investigate the movements of Crangonyx when food was present, I observed 16 Crangonyx searching for food in a clumped d i s t r i b u t i o n . A l l animals were fed only a small amount of frozen Tubifex for a few days before the experiment and hunger levels should have been similar. Again, some animals seemed to behave d i f f e r e n t l y from others. Six 'turners' turned back and thoroughly searched the area where they had just found food; four 'non-turners' did not turn back, but continued moving as before. The other six showed an intermediate type or both types of behaviour, but the extremes were s u f f i c i e n t l y d i s t i n c t for one to recognize 'turners' and 'non-turners'. It seemed that i f individuals consistently behaved d i f f e r -ently, their e f f i c i e n c y of food-finding would depend on how the food was distributed. Animals which turn often would be expected to f i n d clumped food more e f f i c i e n t l y than evenly-spaced food; animals which turn less would be expected to find evenly-spaced food more quickly. To see whether individuals s t i l l d i f f e red i n search behaviour and whether their success at finding food differed, I observed their search on food distributed uniformly and then b r i e f l y on food distributed in a straight l i n e . To compare the behaviour of Crangonyx when food i s given i n different distributions, I attempted to quantify the observed differences in searching behaviour. Several measures were used 2 4 because no one measure a p p l i c a b l e t o the s e a r c h f o r food summar-i z e d i n d i v i d u a l d i f f e r e n c e s . The a n a l y s i s of v a r i a n c e of the s e a r c h times was c a r r i e d out by f i t t i n g c o n s t a n t s by means of a m u l t i p l e r e g r e s s i o n a n a l y s i s . E l i m i n a t i n g any e f f e c t of the day or time of day when the animals were observed d i d not change the r e s u l t s ; i t was thus p o s s i b l e t o compare the r e s u l t s of experiments done s e q u e n t i a l l y . (1) Search t i m e s . I t r a c e d the paths of the 16 Crangonyx f o r f i v e t r i a l s and r e c o r d e d the time from i n t r o d u c t i o n u n t i l they found each of t h r e e p i e c e s of f o o d . The o b j e c t was to see whether the animals d i f f e r e d i n time t a k e n t o f i n d the t h r e e p i e c e s when food was clumped. S i n c e the animals were c l a s s i f i e d as t u r n e r s and n o n - t u r n e r s on the b a s i s of t h e i r b e h a v i o u r i n t h i s e x p e r i m e n t , I c o u l d not compare the s e a r c h time of the two groups on clumped f o o d , but c o u l d o n l y t e s t f o r d i f f e r e n c e s among i n d i v i d u a l s , r e g a r d l e s s of t h e i r b e h a v i o u r . There was a d i f f e r e n c e between animals i n the time t a k e n to f i n d the f i r s t p i e c e of food and to f i n d t h r e e p i e c e s (Table V I ) . However, t h e a n a l y s i s of the time-spread between f i n d i n g the f i r s t and t h i r d p i e c e s of food shows no d i f f e r e n c e between a n i m a l s . One might expect the ti m e - s p r e a d t o be s h o r t e r f o r t u r n e r s , because once a t u r n e r found a p i e c e of f o o d , i t c o u l d r a p i d l y f i n d o t h e r p i e c e s i n the clump. A non-turner might spend as much time or more f i n d i n g each s u c c e s s i v e p i e c e of food as i t d i d the f i r s t , because i t s s e a r c h p a t h might take i t out o f the clump where i t found the f i r s t p i e c e . However, animals t u r n i n g back o f t e n found two of the t h r e e p i e c e s i n the 25 Table VI Analysis of search times for Crangonyx i n the Clumped Pood Experiment. Time i n Seconds Between Animals Mean Square df Residual Mean Square df F i\ P< Time to Find F i r s t 44,877.36 Piece of Food 1 5 16,384.86 62 2. 74 .005 Time to Find Three 175,558.31 Pieces of Food 1 5 48,810.10 62 3. 59 .001 Logarithm (Time to 1 .21 Find Three Pieces) 15 0.56 62 2. 16 .025 Time-Spread From 65,840.16 F i r s t to Third Piece 15 44,874.17 62 1 . 47 .25 Proportion of Time Taken to Find F i r s t 2.67 1 5 1 ,27 62 2. 11 .025 Piece of Food 26 clump, and then moved away. For the same reasons, turners might be expected to spend a larger proportion of th e i r search time finding the f i r s t piece than non-turners. This proportion di f f e r e d among animals (Table "VI), but the size of i t was not similar for animals whose search patterns looked similar. (2) Search patterns. The search patterns of the animals appeared to d i f f e r . Turners r e s t r i c t e d their movements to the area where they had just found food, whereas non-turners ranged more widely. However, because animals often came to the edge of the container and changed their path d i r e c t i o n for this reason, most measurements of search pattern were not useful. One measurement seemed to characterize the search pattern of the 10 animals chosen subject-i v e l y as turners and non-turners from inspection of their search patterns on clumped food. A count was made of the number of times that an animal found a piece of food in the same clump as before i n a t r i a l . These numbers were then summed for a l l t r i a l s and a l l animals. The same procedure was followed for the number 2 of times an animal found food i n a new clump. A X analysis of 2 these totals shows that the animals d i f f e r (X = 28.96 with 9 df; p < .005). Some animals found food in the same clump more often than others did. Also, the animals d i f f e r when grouped subject-ive l y from the pattern of their movements as turners and non-turners; turners found food i n the same clump more frequently than did non-turners (X 2 = 10.93 with 1 df, p < .005). A l l measures of search time but one suggested that animals d i f f e r from one another i n search time. 27 Food uniformly distributed; For this experiment, I selected the 10 animals that seemed to be either a turner or a non-turner i n previous t r i a l s . Six turners and four non-turners were chosen. I tested each animal at the same time each day. Again, I made several measurements of searching behaviour because I was unable to find any one which would distinguish the paths of a l l animals which appeared to behave d i f f e r e n t l y when searching. Some animals took longer than others to find the f i r s t piece and to find three pieces of food, as before (between individuals, Table VII). Time-epread between finding of the f i r s t and t h i r d pieces of food d i f f e r e d among animals, but there was no d i f f e r -ence in the proportion of time spent finding the f i r s t piece. Animals grouped as turners and non-turners i n the Uniform Food Experiment d i f f e r e d only i n time to find the f i r s t piece of food (turners vs non- turners , Table VII). The expected result was that they would d i f f e r in a l l measures, because turners would turn back around the f i r s t piece of food and be slow to find other pieces; non-turners, by following straighter paths, would be quicker to f i n d the spaced food. It may be that i f the food were spaced further apart this e f f e c t would show up but such an experiment was not attempted. One complication was that an animal c l a s s i f i e d as a non-turner after the Clumped Food Experi-ment changed i t s behaviour and turned back much more i n this experiment. Food in Straight Line: Two observations only were made on each of the same 10 animals used i n the Uniform Food Experiment. The Table VII Analysis of search times for Crangonyx i n the Uniform Food Experiment. Time to F i r s t Piece Time Spread From F i r s t to Third Source of Variation df Mean Square F P< Mean Square F P< Regression 9 35,790.4 5.16 .001 200,499.6 2. 59 .025 Turners vs Non-Turners 1 47,116.4 6.80 .025 95,017.9 1 . 22 .50 Between Individuals 8 34,374.6 4.96 . 00cl 213,684.8 2. 75 .025 Residual 36 6,928.8 77,755.6 Time to Three Pieces Proportion of Time to Find F i r s t Source of Variation df Mean Square F p<[ Mean Square F p< Regression 9 334,654.6 3.48 .005 .872 1.79 .10 Turners vs Non-Turners 1 275,953.6 2.87 .10 1.018 2.09 .25 Between Individuals 8 341,992.2 3.55 .005 .854 1 .75 .25 Residual 36 96,284.2 .487 0 0 29 o b j e c t was t o g e t an i n d i c a t i o n of t h e t i m e t o f i n d f o o d and g e n e r a l p a t t e r n of movement of the a n i m a l s on a t h i r d t y p e o f f o o d d i s t r i b u t i o n . Once a g a i n , a n i m a l s t u r n e d b a c k i n v a r y i n g d e g r e e s . Some f o u n d t h r e e p i e c e s v e r y q u i c k l y by l o o p i n g f r o m one p i e c e t o the n e x t ; o t h e r s f o l l o w e d r e l a t i v e l y s t r a i g h t p a t h s , and o t h e r s were i n t e r m e d i a t e . One a n i m a l changed b e h a v i o u r from n o n - t u r n i n g t o t u r n i n g d u r i n g t h i s e x p e r i m e n t , and f o u n d f o o d more q u i c k l y t h a n i t had d u r i n g t h e two p r e v i o u s e x p e r i m e n t s . T u r n e r s seemed t o t a k e l e s s t i m e t o f i n d t h r e e p i e c e s t h a n t h e y d i d on the u n i f o r m d i s t r i b u t i o n . Times of rmiost i n d i v i d u a l s were comparable t o t h e i r t i m e s on clumped f o o d l a t h e r t h a n on u n i f o r m f o o d . A c o m p a r i s o n of the r e s u l t s of t h e s e e x p e r i m e n t s where f o o d i s d i s t r i b u t e d i n t h r e e d i f f e r e n t ways shows t h a t , i n the Clumped and S t r a i g h t L i n e F o o d E x p e r i m e n t s , o v e r a l l mean t i m e s t o f i n d t h r e e p i e c e s , and t i m e - s p r e a d f r o m one t o t h r e e a r e l e s s t h a n t h o s e i n t h e U n i f o r m F o o d E x p e r i m e n t ( T a b l e V I I I ) . The s t a n d a r d e r r o r s i n c l u d e d i f f e r e n c e s between a n i m a l s . I n t h e S t r a i g h t L i n e Food E x p e r i m e n t , time t o f i n d t h r e e p i e c e s and t i m e - s p r e a d were s i m i l a r t o t h o s e i n t h e Clumped F o o d E x p e r i m e n t , a l t h o u g h two a n i m a l s ( n o n - t u r n e r s ) were t u r n i n g b a c k more t h a n t h e y had i n t h e Clumped Food E x p e r i m e n t . R e s u l t s o f t h e Clumped Food E x p e r -i m e n t a r e s i m i l a r when a l l 16 a n i m a l s or o n l y t h e 10 a n i m a l s u s e d i n l a t e r e x p e r i m e n t s a r e i n c l u d e d . The 10 a n i m a l s t e s t e d on a l l t h r e e d i s t r i b u t i o n s f o u n d clumped f o o d more q u i c k l y t h a n u n i f o r m l y d i s t r i b u t e d f o o d . S e a r c h t i m e s , e s p e c i a l l y t h o s e o f TABLE VIII Overall mean search times (sec) of a l l Crangonyx in Clumped, Uniform, and Straight Line Food Experiments, a. only animals used in later experiments included. b. a l l animals used i n Clumped Pood Experiment included. Clumped Uniform Straight Line Mean time (sec) Mean Standard Mean Standard Mean Standard error error error to f i n d f i r s t a. 97.6 25.81 84.4 16.62 110.0 21 .48 piece b. 70.1 16.77 to f i n d three a. 298.6 51.49 434.0 55.94 232.2 42.34 pieces b. 228.3 30.70 time spread a. 201 .0 43.29 349.5 47.1 6 122.2 35.03 one to three b. 158.2 25.05 proportion of time to find f i r s t a. b. 1.9 1.7 0.20 0.14 1 .3 0.11 5.1 2.10 U J o 31 non-turners, varied widely i n a l l three experiments. Searching by Hyalella For two reasons, similar experiments on searching by Hyalella were not carried out. F i r s t , techniques for providing discrete food sources of equal quantity and quality must be developed. Second, Hya1e11a may swim a foot or more at a time, and therefore di f f e r e n t recording methods are required. D i s t r i b u t i o n of Amphipods To investigate why the number of Hyalella decreases with depth whereas the number of Crangonyx changes l i t t l e , I docu-mented numbers of the two species for several months at two sp e c i f i c depths, compared the temperatures at each depth, and t r i e d to discover how any differences between the environments might affect each species. Fig. 4 shows the differences in r e l a t i v e abundance of the two species at 1-and 3-m depth i n my 1967 samples. Numbers of Crangonyx were similar at both depths throughout the sampling period, and similar to numbers of Hyalella at 3 m. Numbers of Hyalella at 1 m were at least twice as high as at 3 m. The young Crangonyx were recruited i n June and July; they appeared i n samples on June 26 at 1 m and on July 18 at 3 m. Fig. 5 shows that the midday temperature at the mud surface at 1 and 3 m depths di f f e r e d by no more than 4 C; wider f l u c t u -ations occurred i n the shallow water; By October, the temper-ature at the two depths was the same. Temperatures were similar 32 10000 _ CRANGONYX 0 CO CC UJ Q_ EE UJ 1000 1 100 1 10 1 5 IOOOO o 1000 100 10 - T f 3M 1 M + + 4 * I l / H Y A L E L L A —• f 1 M -f- -4 3 M JN JU AU S E OC NO 1967 Fig. 4. Density of Crangonyx and Hyalella at 1- and 3-m depths in 1967. V e r t i c a l lines represent standard errors. Each point is the average from 4-5 samples. 33 1 1 1 1 1 [—// 1 JN JU AU S E OC NO MA 1967 Fig. 5. Average midday temperature at the mud surface at 1 -and 3-m depths i n 1967. 34 at a l l depths from October u n t i l about mid-May (Efford, 1967; Hargrave, 1969). The sampling data for 1968 and 1969 provided by Dr. Efford d i f f e r s from data collected previously i n that i t covers a longer time period and includes samples from a l l parts of the lake. By using analysis of variance techniques, I compared the effect of stations (areas) and depths on the numbers of each species. The number of Hyalella d i f f e r e d among both months and stations (Table IX A). When,: the average depth of each station was estimated, a similar analysis showed that numbers were affected by depth (Table IX B). The small interaction term indicates that numbers of Hya1e11a at d i f f e r e n t depths varied the same way from month to month. These two analyses provided the information for the compar-ison of the r e l a t i v e effect of depth and station on Hya1e11a numbers. The sum of squares for stations was further partitioned into the part accounted for byi;depth and part remaining (stations within depths). The mean square for depth was much larger than that for stations within depths (Table IX C). Also, the stations within depths mean square was closer in size to the remainder and interaction mean squares i n TablelX B ? This indicates that, although numbers varied among stations, most of the v a r i a b i l i t y attributed to stations was absorbed when the difference i n their average depth was taken into account. Eig. 6 A shows the number of Hyalella in three depth zones i n 1968 and 1969. Numbers were similar i n the 0-1 and 1-2 zones but decreased at greater depths. Within each depth zone, there 35 Table IX Eff e c t of month, station, and depth on number of Hyalella. A. Analysis of variance by month and station. Source of Variation DF Mean Square F P< Months 16 6.31 2.00 .025 Stations (adjusted for months) 24 41 .15 14.30 .001 Months (adjusted for stations) 16 6.04 1 .91 .025 Stations 24 41 .33 13.09 .001 Month x station interaction 378 3.16. Remainder 0 0.00 B. Analysis of variance by month and depth Source of Variation DF Mean Square F P< Months 16 6.31 1 . 56 .10 Depth (adjusted for months) 3 233.89 57.32 .001 Months (adjusted for depth) 16 6. 14 1 .54 .10 Depth 3 234.78 57. 54 .001 Month x depth interaction 48 4.08 1.12 .50 Remainder 351 3.66 36 C. Comparison of v a r i a b i l i t y in Hyalella numbers among stations and depths. Source of Variation DF Sum of Squares Mean Square Stations (from Table A) 24 991.93 41.33 Depth (from Table B) 3 704.35 234.78 Stations v i t h i n depths 21 287.58 13.69 37 lcoon D E P T H = 1 - 2 M 1 0 0 0 9 i c o o o ^ D E P T H = 3+ M ID a loco 1 0 0 l J N J U AU S E DC NO DE J A F E MA AP MY J N J U AU 5 E DC NO DE 19GB _ . _ _ 19G9 D A T E 38 B IOOCO D E P T H = 0-1 M °- IOOCO D E P T H = 1-2 M x > Q 1 0 0 0 1 ID z < o: l o o i u LL • 1 0 cn LU m i 6 i 4 i 61 4 i 51 61 4 i 6 i41 6 i 41 7 i 41 9 i 7 \ 5 Z ) IOOOO = D E P T H = 3+ M • 1 0 0 0 1 0 0 . . 10 'i 2 i 4 i 31 2 i 4 i4 i 4 i 41 31 5 i 5 i 4 i 41 41 31 51 i i 71 J N J U AU SE DC NO DE JA FE MA AP MY JN J U AU 5 E DC NO DE • A T E Fig. 6. Density of Hyalella and Crangonyx at diff e r e n t depths i n 1968 and 1969. V e r t i c a l lines represent standard errors. A. Hyalella; B. Crangonyx. Numbers along the x-axis i n B repre-sent sample size for both A and B. 39 were discrepancies between years i n the trends of numbers (e. g. June and July at 1-2 m), perhaps because samples were taken only once a month and the population may have been i n a d i f f e r e n t phase i n the two years. The young Hyalella are recruited from June to September; they appear i n the population later in deep water than i n shallow (Fig. 7, June). These animals comprise the overwintering populations; most do not reach maturity u n t i l the next spring, and then decrease i n abundance over the summer. Sizes were similar i n the two years. The maximum period of growth occurred from May to July; l i t t l e growth took place over the winter. The Hyalella present i n A p r i l , when no young of either species had entered the population, were smaller i n deeper water (Table X). Since nearly a l l animals are adults at this time, the smaller size is not attributed to the presence of younger amphipods i n deep water. Numbers of Crangonyx varied with station (Table XI A) but not with depth (Table XI B). The stations mean square-was not reduced much after the depth of each station had been taken into account (Table XI C). The small interaction term (Table XI B) indicates that, from month to month, the numbers of Crangonyx at di f f e r e n t depths varied the same way. F i g . 6 B shows the density of Crangonyx i n three depth zones. In several months numbers were s l i g h t l y higher i n the 0-1 m zone than i n deeper water. Numbers were similar i n 1968 and 1969, except that in 1968, numbers at 3+ m were lower in June and higher in August than i n 1969. Peak densities were reached i n July and August, and lowest densities i n the spring. Toung Crangonyx are recruited SD-a MAY . 13S9 0-1 M N = 138 ALG •19E9 0 - i M N = 390 40 in 5 D * Q • »-a B -Ul IO.Q ^ 30-Q g « . Q fc H EO-Q B "~ a. o-aL. 1-2 M N = : 2-3 M N = IBS 3+ M N = 15 a . 30-a BO'Q tD-a h rn n. HEAD LEM3TH (kM) eo-a tD.Q 1-2 M N = 678 30-Q -rn U so-a. fc »-o H 80*0 rr LO-Q S-3 M N = 179 -da JX 90.CU 3* M N = 15 J X L 40-a EO-Q iD-a 0-Q . JLNE 19G9 0-1 M N - 1G1 40-Q 3D-Q eco lO'Q caL. SO-Of SD-Q EO-Q 2-3 M N = 75 90-0 eo-a ID-Q 3* M N = 15 rTh ri \Vm SEPT 19G9 0-1 M N s KCAD LEN3TH (»*» HEAD LENGTH (Ml) *>-a. JULY 13B9 0-1 M N s 104 r-TI m r i~rn. 90-CU O-Q eo-a io-a, O.QJ_ 1-a M N = 129 E-3 M N = 77 f-rrfl 30-Q «.a 30-Q eo-a M-0. 0-Q . 3+ M N = 9 HEAD LENGTH (•**) DEC 19G9 0-1 M N = 198 KM2 0.0 10 90,0 d ».o g BD.Q Ul 10-0. 5 *-a H 9CVO D eo-a ry 10.Q -n-rl 1-2 M N = 333 ,=dx4 a 2-3 M N = 234 rrn-m a. eo-o 3+ M N = 63 HEAD LENGTH (P*l) i g . 7. Size d i s t r i b u t i o n of Hyalella at dif f e r e n t depths rom May to September and i n December, 1969. 41 Table X Average size of Hyalella and of Crangonyx males at dif f e r e n t depths i n A p r i l , 1969. Hyalella Crangonyx Depth Head length (mm) 95% CI n Head length (mm) 95% CI n (m) - " -0 - 1 .57 .586 354 .82 .848 30 .574 .788 1 - 2 .53 .541 353 .86 .897 26 .521 .827 2 - 3 .53 .551 127 .76 .796 36 .518 .729 3 + .46 .517 10 .79 .860 14 .411 .724 42 Table XI Effect of month, station, and depth on number of Crangonyx. A. Analysis of variance by month and station. Source of Variation DP Mean Square F P< Months 16 6.59 2.39 .005 Stations (adjusted for months) 24 5.10 1.85 .025 Months (adjusted for stations) 16 6.48 2.34 .01 Stations 24 5.18 1.87 .025 Month x station interaction 378 2.76 Remainder 0 0.00 B. Analysis of variance by month and depth Source of Variation DP Mean Square F P< Months 16 6. 59 2.22 .01 Depth (adjusted for months) 3 5.44 1 .83 .25 Months (adjusted for depth) 16 6.45 2.17 .01 Depth 3 6.20 2.09 .25 Month x depth interaction 48 2.27 .765 >.75 Remainder 351 2.97 C. Comparison of v a r i a b i l i t y i n Crangonyx numbers among stations and depths. Source of var i a t i o n DF Sum of Squares Mean Square Stations 24 124.28 5.18 Depth 3 18.60 6.20 Stations within depths 21 105.68 5.03 44 i n June and July, as shown i n the size distributions (Pig. 8), and reach maturity by the next spring. The females die before the end of the summer, while some of the males l i v e on into the winter. The size of males present i n A p r i l showed no clear trend with depth (Table X). Too few females were collected to permit a comparison of their size at d i f f e r e n t depths. The average head length (mm) of a l l females collected i n A p r i l was 1.19 with a standard error of 0.255 (n = 20). Movement studies; To investigate the extent and d i r e c t i o n of movement of both species i n shallow and deep water, I carried out an experiment involving marking, releasing, and recapturing large numbers of amphipods i n the lake. I did two t r i a l s , one i n July and one in August, 1969. At each depth, I released 1000 marked Hyalella and 165 marked Crangonyx. In both t r i a l s , more marked Hyalella and Crangonyx were recaptured within a 1-m radius of the release point at 1 m than at 3 m (Table XII). One interpretation i s that the animals moved out of the 3-m sampling area faster than they moved out of the 1-m area, and that, therefore, on a short term basis, the 3-m environment i s less favourable to both species taken from the 1-m area. There i s no reason to believe that the animals were less susceptible to capture i n dredge samples taken i n the 3-m area. Further evidence that the 3-m habitat was unsuitable would be provided i f more marked amphipods were found moving into shallower water from the 3-m release point than into deeper water. However, i n s u f f i c i e n t numbers of marked amphipods 45 30 • 10 N = 110 June 30 • N=170 July 10 • . , r 30 10 30 10 • N=230 Aug xfJ N=145 S e p t N = 138 Oct 1 4 V 30 30 10 • XI N-139 Nov -i—i—i—i—r— N=136 Dec hm-H, 0.3 0.5 0.7 0.9 1.1 1.3 1.5 1968 30 30 10 30 10 30 • 10 • 30 • 10 30 30 10 30 10 • 30 • 10 30 10 -d N-120 - i Jan N=118 Feb —1 SU133 Mar T r i - ^ N»126 Apr T r r r ^ r r - r i I h r - m N-97 May N-103 June II N.192 July I-U227 Aug N=122 S e p t Tn-rn Oct 30 • — Nov 10-r i - "Tl 30 • N'98 • Dec 10 • ,,,,rT 0.3 0.5 0.7 0.9 1.1 1.3 1.5 1969 Head L e n g t h (mm) Fig. 8. Size d i s t r i b u t i o n of Crangonyx at a l l depths in 1968 and 1969. Dr. I. Efford kindly provided this graph, which was drawn by Mr. I. Yesaki. Dr. D. Johnson analyzed the data. 46 Table XII Total number of marked Hyalella and Crangonyx recaptured within a 1-m. radius of the release point at 1 - and 3-m depths during two movement studies i n Marion Lake. 1000 marked Hyalella and 165 marked Crang'onyx were released at each depth. Data for July are based on seven samples at each depth, 2 hr after the amphipods were released; data for August, on eight samples, 45 min after release. July 19 Aug 23-24 Hyalella Crangonyx Hyalella Crangonyx 1 m 19 20 26 22 3 m 10 7 2 1 Table XIII Density of unmarked amphipods per 506.25 cm i n a l l samples taken at 1- and 3-m depth for movement studies. Number of samples at each depth was 15 i n July, 16 i n August. July 19 Aug 23 -24 Hyalella Crangonyx Hyalella Crangony: Mean 1 m Standard; 20. 9 7. 5 116 .7 12 . 6 Error 1 . 68 0 . 87 14.26 1 . 61 Mean 18. 9 7. 7 9 .4 5. 7 3 m Standard Error 1 . 66 1 . 09 1.13 . o ; 73 47 were recaptured i n the grab samples or the traps to provide such evidence. A few of each species were recaptured at 4 m from the release point. Thus, both species are capable of moving at least 4 m i n an hour. When four traps were placed around the point of release i n one t r i a l , one marked Hyalella was caught within an hour i n each of three traps at 1 m but no marked animals were caught at 3 m. Unmarked animals were caught in the traps roughly i n proportion to their r e l a t i v e densities at the two depths (Table XIII). In July, the natural density of Hyalella appears similar at the two depths, because only the larger size classes were used. Differences i n number of young at the two depths were not evident i n Table XIII. Numbers at the two depths d i f -fered widely i n August, when many of the young were large enough to be retained i n the samples. Temperatures differed at the two depths: 21 C at 1 m and 15 C at 3 m i n July, and 18 and 16 C respectively in August. 48 DISCUSSION Differences between Crangonyx and Hyalella i n l i f e history, d i s t r i b u t i o n , and the effect of depth on energy flow have already been established (Mathias, 1971). Also, the feeding habits of the two are different, and i t seems l i k e l y that this difference accounts, at least in part, for the difference i n their d i s t r i -bution. Thus the two species seem to be distributed independently of one another, and the problem of explaining their d i s t r i b u t i o n concerns feeding behaviour rather than competitive interactions. D i s t r i b u t i o n of Amphipods Crangonyx numbers were similar at a l l depths, but varied among areas i n the lake; this patchiness may be related to food supply. Hyalella numbers varied more with depth than among areas. Hyalella numbers decrease with depth i n other lakes (Jackson, 1912; Gerking, 1962; Cooper, 1 965). Numbers .A also varied among areas within a depth zone. Cooper (1965) found Hyalella to be more numerous where aquatic plants were pre-sent. This could not be tested with the data presented here because samples were not taken s p e c i f i c a l l y from the weed beds. However, Hargrave (1970b) found no difference in Hyalella density or dry weight between beds of Potamogeton or Chara and open sediment. Generally, densities of amphipods were compar-able to those obtained by previous workers, although direct comparisons were not always possible because depth categories diffe r e d . 49 Food Supply for Crangonyx The main food source for Crangonyx i s probably l i v e prey; the guts of several hundred adult Crangonyx collected in shallow water during 1970 contained chironomids and cladocerans, and also a few r o t i f e r s , oligochaetes, copepods, and zygoptera (I. Tesaki, personal communication). The r e l a t i v e importance of these foods i n their diet i s unknown. The fact that no amphi-pods were found i n their guts suggests that the observed attacks by Crangonyx on Hyalella i n the f i e l d were incidental. Crang-onyx seems to ingest l i t t l e detritus, although another subspecies, Crangonyx richmondensis laurentianus Bousfield, i s thought to feed on organic detritus (Sprules, 1967). U n t i l -flae s p e c i f i c prey items of Crangonyx are known, one can only speculate that prey for Crangonyx may be equally abund-ant at a l l depths. Species of chironomids and oligochaetes are abundant i n deep water as well as i n the shallows. Food Supply for Hyalella Hya1e11a ingests algae and bacteria from the sediment, and grazes epiphytes (Hargrave, 1970a). When offered dead Crangonyx and Hya1e11a as the only food source, Hyalella from Marion Lake ingested none of this 'food' over a period of three weeks (Hargrave, 1970b). It finds i t s food mainly through chance c o l l i s i o n s , though i t can detect food at a short distance (not more than twice the length of the f i r s t antennae - Jackson, 1912). However, i t does recognize areas of abundant food (Hargrave, 1970b). Hargrave suggests that the d i s t r i b u t i o n of Hyalella 50 may be related to that of i t s food supply, and that decreased growth rate and size of Hyalella i n deep water may result from decreased temperature and food supply. Epibenthic algal production decreased with depth, though not as sharply as Hyalella numbers. The identity and d i s t r i b u t i o n of the different algal species is important, because, when offered d i f f e r e n t species of sediment microflora in the labor-atory, Hyalella digested them with d i f f e r e n t e f f i c i e n c i e s , and grew at di f f e r e n t rates (Hargrave, 1970 a, b). Also, Hyale 11a chose areas where food was abundant. When given a choice of sediments, Hyalella preferred sediment enriched with microflora to natural sediment; i t chose both of these over s t e r i l e sedi-ment. Gruendling (in press) found differences in the standing crop of various algal groups at d i f f e r e n t depths. Total algal standing crop was highest at 2 m, where Hyalella numbers have decreased. Efford (1970) suggests that grazing by the large number of Hyalella present i n July in shallow water could account for the decrease i n algal standing crop there; also the l'evel of production is lower than might be expected from the trend in deeper water. Grazing by Hyalella at similar densities i n cores caused a decrease i n production (Hargrave, 1970 c). Although food d i s t r i b u t i o n seems to be important, cause-and-effect relationships between the d i s t r i b u t i o n of the algae and that of Hyalella are not clear. Mathias (1971) also reaches the conclusion that the d i f f e r -ence i n d i s t r i b u t i o n i s probably linked to the difference i n feed-ing habits. He points out that egg production per individual Hyalella of a given size might be expected to be lower i n deep water, where food for females is lower. However, there was no 51 such difference i n egg production at different depths. Also, mortality of young Hyalella from these eggs was no higher i n deep water. He suggests that an equilibrium between amphipod density and food level may have developed, and that the effects of depth on the physiological processes of the amphipods are no longer evident. Movement Knowledge of the extent of amphipod movement, especially between depths, i s essential to our understanding of the reasons for their depth d i s t r i b u t i o n . Studies of the movement of marked animals suggest that both Hyalella and Crangonyx move out of the 3-m area faster than the 1-m area. This could simply be a t t r i b -uted to abnormal a c t i v i t y of the animals i f they were i n s u f f i c - . i e n t l y acclimatized to the physical conditions at 3 m. I t could also indicate that i n general the physical conditions at 3 m were not as suitable as at 1 m. If i t i s true that the amphipods taken from 1 m found the 3-m habitat unsuitable, then i t seems unlikely that either species would move d i r e c t l y from shallow to deep water by choice, at least not in a short time period. In future experiments, sampling e f f o r t should be concentrated on the areas towards shallower and towards deeper water i f enough marked amphipods are to be recaptured to allow comparison of the numbers moving i n each direction. To ensure that the amphipods are tested during their period of greatest a c t i v i t y , i n v e s t i -gation of movement should be carried out at night as well as during the day, and at times when the water temperature d i f f e r s . 52 Amphipods as a group are considered to be more active at night (Pennak, 1953). Mundie (1959) reports v e r t i c a l migration of Hyalella azteca at night in Lac la" Ronge, Saskatchewan. How-ever, plankton tows taken i n Marion Lake at night i n 1 968 con-tained no Hyalella; neither did zooplankton samples taken with a suction pump at various depths and locations both i n the day-time and at night from.spring to f a l l in 1966 and 1967 (D. J. McQueen, personal communication). Hyalella may be more active at night on or just above the mud. The use of traps for recap-turing marked animals would make i t much easier to carry out a movement experiment at night. Water temperature also influences amphipod a c t i v i t y . The proportion of amphipods on the mud surface at any one time i s po s i t i v e l y correlated with water temperature (Ware, 1971). Also, the a c t i v i t y of these exposed amphipods i s affected by temper-ature. Crangonyx, and perhaps Hyalella, were most active at 10 C; Hyalella was less active at temperatures above 10 C than at 10 C, but was not tested at lower temperatures. The movement studies have shown that the amphipods i n Marion Lake can move extensively i n the lake, but not whether they do move between depths. Mathias (1971) suggests from his.weekly samples that an increase in density of adult Hyalella from about 2 2 10/m to 60/m i n deep water (2.5 -5m) early i n August, 1966, reflected immigration from shallow water followed by emigration. However, there i s some indire c t evidence that l i t t l e movement takes place. The mean size of Hyalella and of Crangonyx males and juveniles was smaller i n deep water i n the summer, although 53 Crangonyx males did not d i f f e r i n size in the spring. As already mentioned, Hargrave (1970b) suggests that the size difference of Hyalella may be an effect of low temperatures and food supply i n deep water. To accept this explanation requires the assumption that the animals l i v e i n deep water long enough to be affected by the conditions there. The p o s s i b i l i t y that heavy size-selective predation i n deep water is a factor i n •the smaller size of Hya 1 e 11 a there awaits further investigation. There i s evidence that the number and size of f i s h feeding i n di f f e r e n t depth zones and their position i n the water column changes with temperature (K. D. Hyatt, personal communication). Light d i r e c t i o n is probably not important to the di r e c t i o n of amphipod movement i n the lake, although both species moved away from a l i g h t source i n the laboratory. Negative phototaxis has been demonstrated for Hyalella i n the present study and in those of Holmes (1901 ) and Phipps (1915); according to Pennak (1953) i t i s characteristic of amphipods as a group. The l i g h t i s diffuse and the di r e c t i o n the same i n a l l parts of the lake. However, the intensity varies with cloud cover, time of day, and depth, and l i g h t intensity may influence the proportion of amphi-pods, especially Hyalella, on the mud surface (Ware, 1971). As suggested by Hargrave (1970b), l i g h t may d i r e c t l y a f f e c t the v e r t i c a l d i s t r i b u t i o n of the amphipods within the sediment. It may i n d i r e c t l y affect the horizontal d i s t r i b u t i o n of Hyalella through i t s food supply. Less l i g h t reaches the mud i n deep water,, and epibenthic algal production i s lower there. 54 Searching by Crangonyx My studies of searching behaviour of individual Crangonyx arose from my investigation of factors influencing path direction and the consequences to amphipod d i s t r i b u t i o n i n the f i e l d . HowT ever, the studies of searching behaviour bear on predator-prey relationships rather than on depth d i s t r i b u t i o n . Much more work would be required for a thorough investigation of searching behaviour. However, neither Crangonyx nor Hyalella is well suited to a study of this sort. Both normally spend part of their time buried i n the mud out of sight. Good c r i t e r i a for comparing the movement of individuals and techniques for studying movement i n the f i e l d have yet to be developed. Under uniform l i g h t conditions, some individual Crangonyx turned more than others whether or not food was present. It seemed that animals which turned back would f i n d clumped food more e f f i c i e n t l y than evenly spaced food. Laing (1937, 1938) found that an insect parasite turned back more after examining or p a r a s i t i z i n g one host, and thus had a better chance of finding other hosts i n the area. Similarly, larvae of several insects which prey on the Citrus red mite c i r c l e back after feeding (Fleschner, 1950), as do larvae of Adalia 2-punctata, which prey on aphids (Banks, 1957). When I tested the Crangonyx on three food di s t r i b u t i o n s , a l l animals, p a r t i c u l a r l y non-turners, varied widely i n time taken to find food. However, time to f i n d food seemed to be a poor measure of individual differences i n movement. Also, the 55 food distributions may have been i n s u f f i c i e n t l y d i s t i n c t . Crangonyx as a group seemed to be quicker to fi n d clumped food than uniformly distributed food. There are several possible explanations. Different animals were used i n the Clumped and Uniform Pood Experiments. The same animals were used i n the Uniform and Straight Line Food Experiments, though i n each exper-iment, one animal noticeably changed i t s behaviour from non-turning to turning. However, when "these animals were eliminated from the comparison, the mean times to f i n d food i n the two experiments s t i l l d i f f e r e d . Experimental conditions or the physiological condition of the animals may have changed with time, but since the Straight Line Experiment was performed after the other two, any progressive change i n conditions should have had an effect as great as or greater than that i n the f i r s t two experiments. F i n a l l y , differences may be the result of the difference i n spacing of food. the clumped and straight l i n e distributions are r e a l l y two types of clumped d i s t r i b u t i o n . The fact thatthe results of both experiments are similar, and diff e r e n t from those of the Uniform Food Experiment, supports the suggestion that the food pattern i s the important factor influencing search success. It also suggests that Crangonyx i s quicker to find food in clumped d i s t r i b u t i o n . The advantage i s evident i f the d i s t r i -bution of the prey species i s clumped, but i t i s d i f f i c u l t to] find information about prey d i s t r i b u t i o n i n the f i e l d on a small enough scale. Northcote (1952) found the small-scale d i s t r i -bution of chironomids and oligochaetes i n Hatzic Lake, B. C. to 56 be random rather than clumped or uniform; he based his conclusion on data from 9-inch dredge samples divided into nine parts (area of each 9 square inches); he took 10 samples at depths of 15, 25, and 45 f t . Information on the species eaten by Crangonyx and their d i s t r i b u t i o n has not been gathered. Crangonyx would be able to take advantage of the clumped d i s t r i b u t i o n only i f the prey were small and could be eaten quickly, or i f the• Crangonyx could carry more than one at a time. As mentioned previously, several individuals which at f i r s t showed non-turning behaviour changed during the course of the experiments to turning behaviour. These changes seemed to bear no relationship to the food pattern, hunger le v e l of the animals, or stage of the molt cycle. Dixon (1959) found that well-fed larvae of a predatory C o c c i n e l l i d beetle searched more thoroughly than starved ones. In my experiments, a l l animals were similar i n size and were fed the same amount, so that hunger levels should have been comparable. Greze (1968) found that his marine amphipods had da i l y feeding rhythms. If this were true of Crangonyx, then motivation to feed might be different at d i f f e r -ent times of day. However,when the Crangonyx data were grouped to eliminate differences in time of day, the same results were obtained as when the data were not grouped; time of day was unimportant, at least over the range of times i n the experiments. 57 SUMMARY 1. The amphipods i n Marion Lake d i f f e r i n feeding habits. Crangonyx i s carnivorous; Hyalella is mainly herbivorous. Crangonyx finds i t s food by chemical stimuli and motion of the prey; i t seems to recognize food by touch or taste. Hyalella probably finds food by chance. 2. Crangonyx and Hyalella move in dif f e r e n t ways. Crangonyx crawls on the mud and rarely swims. Hyalella swims in the water column, pauses on the mud, and may crawl b r i e f l y . 3. When no food was present, individual movement patterns of Crangonyx diffe r e d . Some animals turned more than others. Both species moved away from a l i g h t source. 4. When food was present, the search patterns of Crangonyx appeared to d i f f e r . Some animals turned back after finding food and searched the immediate area; others moved away from the area without searching i t further; there was a range of i n t e r -mediate types of behaviour. Not a l l individuals were consistent. 5. In general, Crangonyx found food more quickly when i t was clumped than when i t was uniformly distributed. 6. The present study confirms that the amphipods d i f f e r i n r e l a t i v e abundance at di f f e r e n t depths. Hyalella numbers varied more with depth than with area i n the lake; numbers decreased sharply with depth. Crangonyx numbers varied with area but not with depth. 7. Of the differences i n behaviour, the di f f e r e n t feeding habits of the two species seems the most l i k e l y explanation for the 58 difference i n d i s t r i b u t i o n of their numbers. There i s no e v i -dence that interactions between the species influence survival or d i s t r i b u t i o n i n the lake. 8. Marked amphipods of both species were placed i n the lake at 1- and 3-m depths. Both species were recaptured i n samples taken an hour later at 4 m from the release point, and thus are capable of moving long distances in the lake. Fewer marked amphipods were recaptured i n the 3-m sampling area than in the 1-m area after the same time i n t e r v a l . It is unlikely that many amphipods move from one depth to another within short time periods. BIBLIOGRAPHY Banks, C. J. 1957. The behaviour of individual c o c c i n e l l i d larvae on plants. B r i t . J. Anim. Behav. 5: 12-24. Bousfield, E. L. 1958. Fresh-water amphipod crustaceans of glaciated North America. Can. F i e l d Natur. 72: 55-113. Cooper, ¥. E. 1965. Dynamics and productivity of a natural population of a fresh-water amphipod, Hyalella  azteca. Ecol. Monogr. 35: 377-394. Dixon, A. F. 1959. An experimental study of the searching behaviour of the predatory C o c c i n e l l i d beetle Adalia  decempunctata ( L i ) J. Anim. Ecol. 28: 259-281. Efford, I. E. 1967. Temporal and sp a t i a l differences i n phyto plankton productivity in Marion Lake, B r i t i s h Colum-bia. J. Fish. Res. Bd. Canada 24: 2283-2307. (In press). An interim review of the Marion Lake project. Proceedings of UNESCO-IBP Symposium on Productivity Problems of Freshwaters, Poland, 1970. Fisher, R. A. and F. Yates. 1963. S t a t i s t i c a l tables for b i o l o g i c a l , a g r i c u l t u r a l , and medical research. Sixth ed. Oliver and Boyd, Edinburgh, 138 p. Fleschner, C. A. 1950. Studies on searching capacity of the larvae of three predators of the citrus red mite. Hilgardia 20: 233-265. Fraenkel, G. S. and D. L. Gunn. 1961. The orientation of animals. Dover Publications, New York, 376 p. Gerking, S. D. 1962. Production and food u t i l i z a t i o n i n a population of b l u e g i l l sunfish. Ecol. Monogr. 32: 31-78. Greze, I. I. 1968. Feeding habits and food requirements of some amphipods i n the Black sea. Mar. B i o l . 1: 316-321 . Gruendling, G. K. (in press). Ecology of the epipelic algal communities i n Marion Lake, B r i t i s h Columbia. Hamilton, A. L. 1965. An analysis of a freshwater benthic community with special reference to the Chironomidae Ph.D. Thesis. Univ. B r i t . Columbia, Vancouver, B.C. 216 p. 60 1969. A method of separating invertebrates from sediments using longwave u l t r a v i o l e t l i g h t and fluorescent dyes. J. Pish. Res. Bd. Canada 26: 1667-1672. Hargrave, B. T. 1969. Epibenihicalgal production and community respiration i n the sediments of Marion Lake. J. Pish. Res. Bd. Canada 26: 2003-2026. 1970a. The u t i l i z a t i o n of benthic microflora by Hyalella azteca (Amphipoda). J. Anim. Ecol. 39: 427-437. 1970b. Distribution, growth, and seasonal abundance of Hyalella azteca (Amphipoda) in r e l a t i o n to sediment microflora. J. Pish. Res. Bd. Canada 27: 685-699. 1970c. The effect of a deposit-feeding amphipod on the metabolism of benthic microflora. Limnol. Oceanogr. 15: 21-30. Holmes, S. H. T901 . Phototaxis i n the amphipoda. Amer. J. Physiol. 5: 211-234. Jackson, H. H. T. 1912. A contribution to the natural history of the amphipod, Hyalella knickerbockeri (Bate). B u l l . Wise. Natur. Hist. Soc. 10: 49-60. Laing, J. 1937. Host-finding by insect parasites I. Observations on the finding of hosts by Alysia manducator, Mormoniella  v i t r i p e n n i s , and Trichogramma evanescens. J. Anim. Ecol. 6: 298-317. 1938. Host-finding by insect parasites I I The chance of Trichogramma evanescens finding i t s hosts. J. Exp. B i o l . 15: 281-302. Mathias, J. A. 1971. Energy flow and secondary production of the amphipods Hyalella azteca and Crangonyx richmondensis  occidentalis i n Marion Lake, B r i t i s h Columbia. J. Pish. Res. Bd. Canada 28: 711-726. Mundie, J. H. 1959. The diurnal a c t i v i t y of the larger invert-ebrates at the surface of Lac la Ronge, Saskatchewan. Can. J. Zool. 37: 945-956. Northcote, T. G. 1952. An analysis of v a r i a t i o n i n quantitative sampling of bottom fauna i n lakes. M.A. Thesis, Univ. B r i t . Columbia, Vancouver, B.C., 95 p. Pennak, R. W. 1953. Amphipoda (scuds, sideswimmers), p. 435-446. In R. W. Pennak, Fresh-water invertebrates of the United States. Ronald Press, New York. 61 Phipps, C. P. 1915. An experimental study of the behaviour of amphipods with respect to l i g h t intensity, d i r e c t i o n of rays, and metabolism. B i o l . B u l l . (Woods Hole) 28: 210-223. Sprules, ¥. G. 1967.. The l i f e cycle of Crangonyx richmondensis  laurentianus Bousfield (Crustacea: Amphipoda).Can. J. Zool. 45: 877-884. Ware, D. M. 1971. The predatory behaviour of rainbow trout (Salmo gairdneri). Ph.D. Thesis, Univ. B r i t . Columbia, Vancouver, B.C., 158 p. 

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