Native eelgrass Zostera marina controls growth and reproduction of an invasive mussel through food limitation

Abstract

In southern California, native eelgrass Zostera marina L. and the invasive non-native mussel Musculista senhousia have dynamic complex interactions. Although high densities of M. senhousia inhibit the growth of eelgrass, mussel survival and growth decline with increasing eelgrass shoot density and patch size. The correlation of these eelgrass attributes with local concentrations of chlorophyll a and water flow speeds suggested that the mussels, which feed on phytoplankton delivered by water currents, might suffer food limitation inside large eelgrass beds. By supplementing phytoplankton to M. senhousia living in eelgrass, we confirmed this hypothesis: mussels grew 50% faster when food availability was enhanced. Lab and field experiments investigating the effects of food limitation on growth and reproduction of M. senhousia showed that mussels respond by reducing shell growth, and subsequently fecundity. We found no evidence that mussels reallocated resources preferentially to reproduction when food was limited. Our results highlight how the effects of anthropogenic perturbations that currently threaten eelgrass populations directly could be magnified by interactions with a non-native species. Eelgrass habitat fragmentation and increasingly frequent phytoplankton blooms resulting from coastal development and eutrophication have well-described direct negative effects on eelgrass. By increasing phytoplankton availability to M. senhousia, such perturbations also indirectly affect eelgrass by acting to enhance mussel survival and growth, magnifying the negative effects of the mussel on Z. marina.