Conspecific interactions as drivers of activity and mortality in a prominent sandy beach amphipod

In harsh habitats, community structure is believed to be controlled primarily by physical factors while biological interactions are considered to be of little relevance. However, evidence of avoidance behaviors between the life stages of some species suggests otherwise. A long-standing hypothesis from sandy beaches suggests that avoidance between adults and juveniles dictates some aspects of their ecology, but this has not been explicitly tested. Here, we used a field survey to document the diel activity of adults and juveniles of the talitrid amphipod Americorchestia longicornis . We then used laboratory experiments to assess the influence of adults on the activity of juveniles and to measure juvenile mortality rates in the presence and absence of adults, with and without food. In the field, we found clear differences in the diel activity of adults and juveniles. Adults were active throughout the night but not in daylight, whereas juvenile activity peaked at dusk and dawn. Similar patterns of activity were recorded in laboratory tanks that included both stages. However, when adults were excluded, juveniles shifted towards night activity, suggesting that the pattern of activity observed in the field is a mechanism to avoid larger conspecifics. In separate trials, predation by adults was a significant driver of juvenile mortality, regardless of initial juvenile density and food availability. Altogether, these results suggest that segregation between amphipod life stages is driven by the avoidance of direct interactions, specifically, cannibalism. This likely influences the role played by these amphipods, both as prey and as processors of stranded seaweeds.