Spatially varying selection amplifies intrapopulation differentiation among phenotypic traits in the rocky-shore mussel Mytilus californianus
Strong ecological gradients along heterogeneous environments play an important role in shaping population differentiation across species ranges. Thus, the selective pressure of environmental variation on phenotypic variation strongly affects an organisms ability to persist across diverse or new environments. We investigated the spatial variability of biological responses in the intertidal bivalve Mytilus californianus to highlight the costs and trade-offs of local adaptation and phenotypic plasticity across various functional traits in a dynamic environment, the marine intertidal. To test this, we performed Whole Genome Sequencing (WGS) and a reciprocal transplant experiment with M. californianus individuals originating from the upper and lower intertidal measuring relevant phenotypic traits. We determined that morphological traits in individuals demonstrated increased phenotypic plasticity when moved to new environments, whereas physiological traits such as metabolism had reduced plasticity. Additionally, mussels from high intertidal zones, which experience greater heat and aerial exposure stress, maintain lower metabolic rates and show increased frequencies of non-synonymous mutations in functionally relevant heat shock proteins when compared to low intertidal mussels. These results suggest that morphological and physiological traits respond differentially to spatially varying selection within the marine intertidal.