Food preferences of two sandy beach scavengers with different foraging strategies

Beaches are interesting ecosystems on which to test foraging ecology because the resident fauna mostly depend on unpredictably supplied allochthonous food resources. This study tests the food selection of two beach scavengers with different foraging strategies. Ghost crabs (Ocypode ryderi) patrol along the shore in search of food; it was predicted that, if given equal choices, they would select the most calorie-rich food. In contrast, swash-riding plough snails (Bullia spp.) need to make immediate decisions whether to emerge from the sand to feed on a beach-cast food item before it is washed away. It was predicted that they would emerge in response to all food equally because there is no guarantee of a better meal in their immediate future. These predictions were tested in two different in situ experiments that simulated natural foraging conditions for each scavenger. Ghost crabs consumed fish (highest calorific value) significantly more often than other foods. Similarly, significantly more snails responded to fish (higher calorific value) than jellyfish (lower calorific value); a result that was consistent with snail size, but was not affected by food-cue concentration. Beach scavengers are well adapted to the challenges of the ecosystem, and although food is supplied unpredictably, they still select for high-quality food, supporting optimal foraging theory. It is hypothesized that the patchy distribution of resident macrofauna on sandy shores may (partly) reflect their foraging ecology, sensu the ideal free distribution theory.