Data and Code for: Biogeographic variation in mussel shell thickness and drilling predation on rocky shores

Abstract: 

Predator and prey species are often distributed over broad geographic ranges where reciprocal selection can shape the interactions between them. However, few studies have addressed how environmentally-driven variation in prey traits might create a landscape of selection that shapes the evolution of predator traits. We investigated whether variation in the drilling capacity of a predatory snail (the channeled dogwhelk, Nucella canaliculata) is associated with variation in prey defenses (mussel shell thickness) over ~1,000 km of coastline in the northeast Pacific. To test this hypothesis, we first hatched and raised snails from 6 populations under common laboratory conditions for a year. We then challenged these naïve snails with a series of mussels (Mytilus californianus) of increasing shell thickness to quantify their maximum drilling capabilities. Dogwhelks from California drilled mussels that were up to 3.4 times thicker than those handled by dogwhelks from Oregon. To quantify the spatial mosaic of mussel shell thickness, we analyzed M. californianus shells collected from the same six sites during three time periods (2000-2001, 2008-2009 and 2019). In the two earlier sampling periods, mussels from the central Oregon coast were consistently and substantially thicker than those from California. Thus, our findings are consistent with the hypothesis that prey defenses vary across the overlapping geographic ranges of these species, with the potential for strong prey-driven selection on the predator in California, and a co-evolutionary cold spot on the Oregon coast. However, relative to prior decades, mussel shells appear to be thinning, particularly on the Oregon coast. These changes are consistent with recent ocean acidification research in the northeast Pacific and may signal shifts in the selective landscape that has shaped the evolution of this dogwhelk-mussel interaction. Given the rapid pace of global environmental change, our study highlights the importance of studying species interactions within an explicit geographic context of shifting selection pressures.