Different crab species influence feeding of the snail Nucella lapillus through trait-mediated indirect interactions

Indirect interactions are among the many important factors that influence the community structure of the rocky intertidal zone. Trait-mediated indirect interactions, in which the presence of a predator or competitor can influence the relationship between two other species, have emerged as vital for understanding community dynamics. This study examined the effect of different crab species on the feeding habits of an intertidal snail, Nucella lapillus. Crab species were defined as being sympatric predatory (Carcinus maenas and Cancer irroratus), sympatric non-predatory (Uca pugnax and Pagurus longicarpus), or allopatric predatory (Mithrax sculptus and Percnon gibbesi). Nucella lapillus were potentially exposed to risk cues from each of the crab species. Crabs were kept in perforated boxes, which allowed any chemical cues to be emitted but prevented direct contact. Nucella lapillus had significantly lower feeding rates in the presence of sympatric predatory crab species than N. lapillus exposed to either sympatric non-predatory crabs or allopatric predator), crabs. There was no difference in feeding rate between N. lapillus exposed to the sympatric non-predatory crabs and to the allopatric predatory crabs. Nucella lapillus in the presence of sympatric predatory crabs had a feeding rate of only 0.07 barnacles per snail per clay, whereas N. lapillus housed with non-predatory crabs and allopatric predatory crabs had rates of 0.11 and 0.12, respectively, suggesting that N. lapillus alter their behavior in response to chemical risk cues from local predators. These results Suggest that the ability to detect and respond to risk cues is a selectively evolved trait.