Plasticity and environmental heterogeneity predict geographic resilience patterns of foundation species to future change

Abstract

Although geographic patterns of species sensitivity to global environmental changes are defined by interacting multiple stressors, little is known about the biological mechanisms shaping regional differences in organismal vulnerability. Here, we examine large-scale spatial variations in biomineralisation under heterogeneous environmental gradients of temperature, salinity and food availability across a 30{degrees} latitudinal range (3,334 km), to test whether plasticity in calcareous shell production and composition, from juveniles to large adults, mediates geographic patterns of resilience to climate change in critical foundation species, the mussels Mytilus edulis and M. trossulus. We find mussels produced thinner shells with a higher organic content in polar than temperature regions, indicating decreasing shell calcification towards high latitudes. Salinity was the major driver of regional differences in mussel shell deposition, and in shell mineral and organic composition. In low-salinity environments, the production of calcite and organic shell layers was increased, providing higher resistance against dissolution in more corrosive waters. Conversely, under higher-salinity regimes, increased aragonite deposition suggests enhanced mechanical protection from predators. Interacting strong effects of decreasing salinity and increasing food availability on the compositional shell plasticity in polar and subpolar mussels during growth predict the deposition of a thicker external organic layer (periostracum) under forecasted future environmental conditions. This marked response potential of Mytilus species suggests a capacity for increased protection of high-latitude mussel populations from ocean acidification. Our work illustrates that mechanisms driving plastic responses to the spatial structure of multiple stressors can define geographic patterns of unforeseen species resilience to global environmental change.