Latitudinal differences in Mytilus californianus thermal physiology

Abstract

The California ribbed mussel Mytilus californianus (Conrad 1837) is a dominant intertidal species that ranges from Alaska (USA) to Baja California (Mexico) along the west coast of North America. Despite its broad latitudinal range, which results in populations living at widely different temperatures, multiple studies have concluded that this species is genetically homogenous. This may be due to high gene flow, which would inhibit local adaptation, and lack of strong post-settlement selection pressure, which would not favor shifts in allelic frequencies among sites. We investigated several physiological traits that show temperature-adaptive variation in other Mytilus species. We sampled populations of M. californianus at 7 sites across 33° of latitude to test for the possibility of local adaptation or phenotypic plasticity. Individuals acclimated to common conditions were examined for possible differences in whole-organism thermal tolerance. The most northern population (Tatoosh Island) had a significantly lower survival rate following an acute heat stress event than the other populations. Additional physiological processes were examined that affect thermal optima and limits: critical thermal maximum (Hcrit) of cardiac function and metabolic capacity (as indexed by tissue activity of an indicator enzyme, malate dehydrogenase). Tatoosh mussels had a significantly lower Hcrit than other sites, indicating that this population may be adapted to cooler conditions compared to other populations. Metabolic activity did not correlate with latitude of collection. Phenotypic plasticity at the adult stage did not explain differences in thermal tolerance. Thus, genetic differences (local adaptation or balanced polymorphism), developmental plasticity, maternal effects, or within-site thermal heterogeneity may be responsible for the observed variation among populations. © Inter-Research 2012.