Selection of habitat has significant impacts on a variety of ecological factors including mate choice, recruitment, competition, and predation. For highly mobile aquatic invertebrates, habitat choice seems largely impacted by the combined forces of food availability and habitat structure (i.e., protection from predation). When considering habitat structure in isolation, an individual's choice may be mediated by its relative risk of predation which can be affected by size, sex, number of conspecifics, or the perceived threat of actual predators. This study investigated how choice of habitat structure in a common freshwater amphipod (Gammarus pseudolimnaeus) is influenced by group size, sex, and perceived predation risk. Single-sex groups ranging from one to sixteen amphipods were introduced into a container with four habitat choices: sand, gravel, rocks, and plants (plastic, to control for food availability). These choices reflected a range of available interstitial spaces from < 0.5 mm to > 15 mm. Trials were run with refrigerated spring water (no added predation cues) and repeated with refrigerated spring water in which predatory brook sticklebacks (Culea inconstans) had been housed until the experimental trial.
Female amphipods, which generally have smaller body sizes, were more likely to select substrates with small interstitial spaces (e.g., gravel), whereas males preferred rocks and were rarely found in sand. Both sexes significantly avoided the plastic plants. As group size increased, female amphipods were more likely to select gravel and less likely to select rocks, but no similar effect was seen in males. Both sexes tended to shift their preference toward sand from rocks in the trials with predator kairomones. These results support the hypothesis that amphipods are flexible in their choice of habitat and adjust their behavior in response to the relative risk of predation.