Predator-induced defenses appear to be ubiquitous in marine systems yet little attention has been paid to how they may shape large-scale patterns of phenotypic variation. Here I consider how the invasion of the green crab (Carcinus maenas) into the Gulf of Maine has led to the evolution of geographic differences in shell thickness and shell thickness plasticity in the marine snail Littorina obtusata. Because the historical impact of Carcinus maenas in the Gulf of Maine changes with latitude, I examined how geographic differences in shell thickness plasticity may have evolved in response to different risk cues. I also examine how trade-offs in snail body mass, which influence snail fecundity, correlate with the magnitude of induced defense employed by the snail. Finally, theory predicts that natural selection should minimize the impact of trade-offs associated with an inducible defense, thereby making them difficult to detect experimentally. I explore this issue by explaining (1) why such trade-offs are indeed readily detected in marine snails, and (2) why the scaling of such trade-offs shows no geographic differentiation when one would expect otherwise.