Predator-induced plasticity and morphological trade-offs in latitudinally separated populations of Littorina obtusata

Predation by shell-crushing predators is thought to be a principal force dr iving the evolution of gastropod shell form. However, recent evidence sugge sts that phenotypic plasticity in response to risk stimuli associated with predators may also be important. Thus, morphological coevolution between pr edators and their gastropod prey may be driven by natural selection on reac tion norms rather than genetically fixed phenotypes. In this study, I exami ned whether geographic variation in morphology and its plasticity in the in tertidal snail (Littorina obtusata) were associated with both historical an d present-day differences in the abundance of one its principal crab predat ors (Carcinus maenas). C. maenas has been well established in the southern Gulf of Maine for about 100 years, but has been present in the northern Gul f of Maine for at most 50 years. The shells of snails from the northern Gul f were thinner, weighed less and were weaker in compression than those of s outhern Gulf conspecifics. These geographic patterns in shell form may refl ect, in part, either selection on genetically fixed phenotypes or environme ntally induced phenotypes in response to geographic differences in C. maena s effluent concentrations. A laboratory experiment raising snails from the same field populations in t he presence and absence of C. maenas effluent was conducted to test whether differences in the duration of contact with C. maenas influences plasticit y in shell form. When raised in the presence of crabs, snails from all popu lations produced significantly thicker and heavier shells than conspecifics raised without crabs. These results support the hypothesis that geographic differences in shell form may partly reflect geographic differences in the abundance of C. maenas and, thus, the concentration of effluent indicating a risk of predation. In other words, the thinner shells of northern snails may reflect non-induced phenotypes, whereas the thicker shells of southern snails are an induced defence in response to C. maenas. Interestingly, des pite their different periods of contact with C. maenas, both northern and s outhern snails showed similar shell thickness plasticity in the laboratory suggesting that reaction norms in each region have evolved similar slopes. However, laboratory data coupled with comparisons of field populations sugg est that there has been an evolutionary shift in reaction norm intercept; s outhern snails, regardless of treatment, consistently produced thicker shel ls and showed less plasticity relative to their counterparts in the field. Predator-induced increases in shell thickness were accompanied by significa nt reductions in body mass (defined by soft tissue mass) and body growth. T hese trade-offs probably reflect geometric constraints imposed by shell for m on body mass and may explain the existence of micro- and macro-geographic variation and the evolution of inducible defences in marine gastropod shel l form.