Biogeographic variation in mussel shell thickness and drilling predation on rocky shores.

Although predator and prey species often interact over broad geographic ranges, little consideration has been given to whether environmentally-driven variation in prey traits might create a landscape of selection that shapes the evolution of predator traits. Here, we investigate whether predatory intertidal snails (the dogwhelk Nucella canaliculata) vary in their drilling capacity in association with prey defenses (mussel shell thickness) over ~ 1000 km of coastline in the northeast Pacific. We analyzed mussels (Mytilus californianus) collected over two decades and found that shells from central Oregon were historically ~ 20% thicker than those from California. We tested the drilling capacity of dogwhelks raised under common-garden laboratory conditions and found that dogwhelks from California could drill mussels 3.4 times thicker than those from Oregon. Thus, dogwhelks with stronger drilling ability are associated with thinner mussel shells. Although these findings appear counterintuitive at first, they are consistent with strong prey-driven selection on the predator in California, and a region of weak selection in Oregon where mussels may be too thick to favor drilling. Our results also suggest that this selective landscape may be changing; consistent with recent studies of ocean acidification, mussels collected in 2019 were thinner than in prior decades, particularly in Oregon. Overall, our study highlights the importance of studying species interactions within an explicit geographic context of shifting selection pressures.