A behavioural polymorphism in the marine snail Nassarius pauperatus: Geographic variation correlated with food availability, and differences in competitive ability between morphs.

The snail Nassarius pauperatus (Lamarck) is common on intertidal sandflats in southern Australia. A behaviour described and defined as being a "Twister" was observed in some individuals. Upon contacting a member of their own species, Twisters gave a very specific response, rotating their shell from side to side in a way similar to the escape responses displayed by many gastropods to predators. However, some individuals, defined as "Non Twisters", never gave this response after contacting another N. pauperatus. Breeding experiments suggested that being a Twister was heritable. The frequency of Twisters varied between populations of N. pauperatus, and was negatively correlated with food availability (as estimated by measuring hunger) and positively correlated with the population density of juvenile N. pauperatus.It was found in the laboratory that Twisters were better than Non Twisters at competing for space to feed, when food was in short supply and distributed as discrete patches. There was also evidence that competition for space to feed occurred when food was in short supply in the field and it was postulated that Twisters thus had a selective advantage compared to Non Twisters in such situations.These results are discussed in terms of whether the behavioural differences between populations can be attributed to either selection or stochastic factors.