Chemically mediated avoidance of Hemigrapsus nudus (Crustacea) by Littorina scutulata (Gastropoda): effects of species coexistence and variable cues

Some gastropods can respond to waterborne chemicals released from predators and injured conspecifics with avoidance behaviour. We examined the avoidance response of the intertidal snail Littorina scutulata sensu lato to chemical cues from the predatory crab Hemigrapsus nudus, both from the Pacific coast of Canada, using two-choice laboratory experiments. Littorina scutulata s. l. from a site with H. nudus (Ross Islets) avoided water containing chemical cues from the predator, but snails from a site without H. nudus (Prasiola Point) did not. The avoidance behaviour of L. scutulata s. l. from the ‘crab site’ strengthened with increasing perceived predation threat. We found a risk hierarchy, with risk increasing from control sea water (no risk), to predator cues (low risk), to injured-conspecific cues (intermediate risk), to a combination of predator and injured-conspecific cues (high risk). This risk gradient seems logical, because the sole presence of predators does not necessarily represent an immediate predation threat, whereas predator and injured-conspecific cues together may indicate that predation is actually occurring nearby. These results are important for understanding the effects of chemically mediated predator-prey interactions on intertidal distribution patterns of littorinids.