Proteomic changes associated with predator-induced morphological defences in oysters.

Inducible prey defences occur when organisms undergo plastic changes in phenotype to reduce predation risk. When predation pressure varies persistently over space or time, such as when predator and prey co-occur over only part of their biogeographic ranges, prey populations can become locally adapted in their inducible defences. In California estuaries, native Olympia oyster (Ostrea lurida) populations have evolved disparate phenotypic responses to an invasive predator, the Atlantic oyster drill (Urosalpinx cinerea). In this study, oysters from an estuary with drills, and oysters from an estuary without drills, were reared for two generations in a laboratory common garden, and subsequently exposed to cues from Atlantic drills. Comparative proteomics was then used to investigate molecular mechanisms underlying conserved and divergent aspects of their inducible defences. Both populations developed smaller, thicker, and harder shells after drill exposure, and these changes in shell phenotype were associated with upregulation of calcium transport proteins that could influence biomineralization. Inducible defences evolve in part because defended phenotypes incur fitness costs when predation risk is low. Immune proteins were downregulated by both oyster populations after exposure to drills, implying a trade-off between biomineralization and immune function. Following drill exposure, oysters from the population that co-occurs with drills grew smaller shells than oysters inhabiting the estuary not yet invaded by the predator. Variation in the response to drills between populations was associated with isoform-specific protein expression. This trend suggests that a stronger inducible defence response evolved in oysters that co-occur with drills through modification of an existing mechanism.