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

Abstract

Two benthic amphipod species (Crangonyx richmondensis var occidentalis Hubricht and Harrison and Hyalella azteca Saussure) live together in Marion Lake, but, as previous workers have shown, differ markedly in the distribution of their numbers. The aim of the study was to compare the behaviour of the two species and to see if any differences might account for the differences in distribution. Particular attention was paid to movement and feeding behaviour. Crangonyx was equally abundant on the bottom at all depths in the lake; Hyalella was abundant in the shallows (1m) and scarce in the deeper waters ( 3 m). Numbers of each species differed among areas in the lake. Crangonyx usually moved by crawling over the mud, and seldom swam. It seemed to find food by its smell or motion or both, and to recognize food by touch or taste. It attacked living prey by pouncing on it and grasping it with its gnatho-pods. In contrast, Hyalella moved mainly in a series of swims and pauses. Hyalella fed by grazing on the mud and rooted aquatics, and ingesting detritus. Both species moved away from sources of light. When food was present in various distributions, the search patterns of Crangonyx differed: some animals turned back and thoroughly searched the area where they had just found food; others crawled along a relatively straight path; yet others behaved in an intermediate manner. In general, Crangonyx seemed to be 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 in 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 in the deep area than in the shallow area, indicating that they left the deep area faster. Of the behavioural differences noted, only the difference in feeding habits between the species seems likely to account, at least in part, for the difference in the distribution 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 chose areas of abundant food. Its assimilation efficiency and growth differed when it fed on different sediment microflora. Dr. Gruendling found that the abundance of the algal groups in Marion Lake differed with depth. The distribution of Hyalella may be related to the distribution of certain species of algae in its diet. The distribution of food for Crangonyx is unknown.

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