Abstract
The impact of invasive predators during the early stages of invasion is often variable in space and time. Such variation is expected to initially favor plasticity in prey defenses, but fixed defenses as invaders become established. Coincident with the range expansion of an invasive predatory crab in the Gulf of Maine, we document rapid changes in shell thickness-a key defense against shell crushing predators-of an intertidal snail. Field experiments, conducted 20 years apart, revealed that temporal shifts in shell thickness were driven by the evolution of increased trait means and erosion of thickness plasticity. The virtual elimination of the trade-off in tissue mass that often accompanies thicker shells is consistent with the evolution of fixed defenses under increasingly certain predation risk.