Spatially Varying Selection Amplifies Intrapopulation Differentiation Among Phenotypic Traits in the Rocky‐Shore Mussel, <i>Mytilus californianus</i>

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 organism's ability to persist across diverse or new environments. We investigated the spatial variability of biological responses in the intertidal bivalve <i>Mytilus californianus</i> 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 a reciprocal transplant experiment with <i>M. californianus</i> individuals originating from the upper and lower intertidal measuring relevant phenotypic traits, followed by whole genome sequencing (WGS). We determined that morphological traits in individuals demonstrated phenotypic plasticity when moved to new environments, whereas physiological traits such as metabolism exhibited constraints in plasticity. Additionally, mussels from high intertidal zones, which experience greater heat and aerial exposure stress, maintained lower metabolic rates and showed 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 responded differently to spatially varying selection within the marine intertidal.