Dogwhelks are intertidal snails that feed by drilling through the shells of sessile prey. In particular, Nucella spp. dogwhelks on the west coast of North America are important predators that consume large quantities of a habitat-forming, dominant space competitor, the California mussel. These predators have very low dispersal and are expected to adapt differentially to their local environments. I am exploring the extent of local adaptation among populations of Nucella to mussel phenotypes. I expect varying environmental conditions along the coast, such as pH, temperature, and seawater productivity, would affect their choice of what to drill. My main questions are: (1) Do different populations of dogwhelks drill mussels with different traits (in terms of attributes important to drilling predators, i.e. shell size and thickness)? (2) Given what is available, are populations of dogwhelks selectively drilling mussels based on size or thickness? (3) What environmental conditions covary with dogwhelk size and thickness preferences? Mussel availability and dogwhelk predation were measured at 7 sites by examining mussel shells with and without drill holes. Dogwhelk selectivity in terms of mussel size and thickness were compared with pH, temperature, and chlorophyll-a at each site to test for correlations.
Results/Conclusions
There were significant differences among sites in the mean size and thickness of drilled mussels, as well as mean size and thickness selectivity, and such trait variation is the first criterion for local adaptation. At sites where dogwhelks were selective, they preferred larger or thinner mussels, which is expected if they are foraging optimally. At the sites where dogwhelks were not selective, selection to choose the most rewarding prey may not be strong. The environmental variables best correlated with length and thickness selectivity included measures of low pH events and median chlorophyll-a. This implies that dogwhelk prey selectivity is more sensitive to ephemeral but extreme drops in pH rather than changes in the mean, potentially due to physiological thresholds. This result also indicates that environmental drivers are affecting dogwhelk selectivity indirectly through their prey since chlorophyll-a is a measure of seawater productivity that directly influences mussel growth. Whether these drilling differences among populations are evolutionary or plastic, they can have implications for community diversity, since California mussel beds are habitats for a large variety of species. Future studies will examine how dogwhelk prey selectivity affects community diversity in the mussel beds.