Morphological analysis of two sympatric ecotypes and predator-induced phenotypic plasticity in Acanthina monodon (Gastropoda: Muricidae)

Some marine gastropod species are characterized by having marked polymorphism in the shape, colour and thickness of their shells. This variation can be induced by physical or biological factors such as wave exposure and predation. We examined shell variation in the direct-developing Acanthina monodon. In a field survey, we collected and analysed two snail ecotypes: a thick-shelled ecotype from environments with intense predation by the crab Homalaspis plana, and a thin-shelled ecotype from environments where predation was negligible. When we controlled for shell length, the thick-shelled ecotype had a heavier and thicker shell, but lower body weight, than the thin-shelled ecotype. However, only shell thickness and body weight showed a significant difference between habitat types. In order to determine the nature of shell variation in A. monodon, we conducted a laboratory experiment with juvenile snails from sites where the predation was low or absent. Seawater aquaria containing crabs were connected to aquaria containing snails via a cylinder with flows that enabled the transport of seawater from crabs to the snails aquaria. The snails treated with seawater-borne stimuli (predation treatment) developed thicker shells than the snails raised in the absence of the stimulus (control treatment). We suggest that adaptive phenotypic plasticity may account for part of the shell variation surveyed in the field. We discuss the metabolic and reproductive costs of maintaining anti-predator defence strategies that involve adaptive morphological changes to a particular environment.