Food fights: aggregations of marine hermit crabs (Pagurus samuelis) compete equally for food- and shell-related carrion

Competition for limiting resources drives animal aggression. Aggression in hermit crabs has been well studied in shell acquisition circumstances, yet less is known about hermit crabs' competitive behavior in other contexts, especially as they relate to feeding. As active foragers, competition for food resources may be a major determinant of hermit crab aggression and fighting behavior, particularly in intertidal marine environments, where protein-rich carrion may be limiting. Here, we measured aggressive, competitive interactions between hermit crabs [Pagurus samuelis (Stimpson, 1857)] that were presented with carrion in the laboratory, immediately after they were collected from the field. Aggregations of three crabs were provided with either mussel or gastropod flesh. Both forms of carrion naturally indicate food availability, though only the latter may correlate with potential shell availability. We found that levels of aggression rose significantly in response to both carrion conditions, but not in response to a control condition involving the introduction of a non-eatable object. Larger individuals displayed the highest levels of aggressive behavior, but levels of aggression did not differ between crabs provided with the gastropod and the mussel flesh. These results reveal that food-independent of a shell resource-can be a powerful motivator for competitive behavior in hermit crabs. Further studies of food-related aggression between different hermit crab species could shed light on how interspecific competition might lead to potential specializations on different carrion or prey resources.