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 organism’s 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 a
reciprocal transplant experiment with M. californianus 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 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, 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.