Predator diet and prey behaviour: freshwater snails discriminate among closely related prey in a predator's diet

Chemical cues are commonly used by prey to assess predation risk. Prey typically respond to predator cues by altering behaviour, but their response may depend on predator diet. There are few data on how predators feeding on prey types spanning a continuous gradient of relatedness affect prey behaviour. Here I present the results of a study evaluating the relationship between predator diet and prey behaviour. We presented two species of freshwater gastropods with caged crayfish fed one of five diets and compared them with a no-predator control. Diets included conspecific prey, congeners, prey in the same order, and two prey taxa in different phyla. We monitored behaviour and growth over 3 weeks. Predators feeding on conspecifics induced a large increase in the refuge use of both prey species. Refuge use was highly contingent on predator diet, as one snail species responded only to predators feeding on conspecifics and the other species responded only to predators feeding on conspecifics and congeners. Neither snail species responded to predators feeding on prey taxa in a different genus. Snail behaviour did not depend on the degree of ecological overlap with diet items, but behaviour was related to the degree of phylogenetic similarity. There was no evidence that diets of an intermediate relatedness induced an intermediate response. Instead, prey responded to the diet gradient in a threshold manner. Growth responses were generally concordant with behavioural responses. The overall effect of predators on prey in nature depends on how variation in predator diet translates into altered prey phenotypes.