UC Santa Barbara

The rapid pace of global climate change is challenging marine species worldwide. Increases in average seawater temperatures, combined with extreme climatic events such as marine heatwaves (MHWs), are pushing organisms near or past their physiological tolerance limits. To predict how these stressors will impact species and ecosystems, knowledge of thermal limits and the capacity to adjust these limits via acclimation is critical. This is especially true for species living near their thermal limits or species that vary in sensitivity to thermal stress across their life history.In my dissertation I examined how changes in ocean temperature are impacting two ecologically important species in coastal waters of the Northeast Pacific, the California mussel (Mytilus californianus) and the purple sea urchin (Strongylocentrotus purpuratus). For both study organisms, I began by examining how the thermal experience of adult animals was altered during MHW events using data collected by long-term monitoring programs. Here, I found that across the biogeographic range of M. californianus, mussels differed in thermal exposure during MHW events where exposure of the Oregon and California populations differed in the onset of elevated temperatures and the proximity to optimal performance thresholds. Regarding S. purpuratus, I found that MHWs coincide with reproductive life history stages that may have implications for gametogenesis, spawning, and larval development. For early life history events in my study organisms, I examined transgenerational and intragenerational plasticity as an acclimatory response mechanism to warming oceans. In a field-to-laboratory experiment using intertidal mussels, I investigated transgenerational plasticity in nature by examining how parental exposure to differing thermal regimes that exist across tidal height and season impact offspring phenotype. To further examine the influence of developmental temperature on larval performance, offspring were reared at two developmental temperatures. At the veliger stage, physiological performance of offspring was assessed via measurements of larval body size, respiration rate, and thermal tolerance. Results from this study show that seasonal acclimatization of adult M. californianus and larval developmental temperature had prevailing effects on offspring performance. Lastly, to study whether MHWs might induce a transgenerational response in S. purpuratus, adult urchins were acclimated to MHW and non-MHW conditions in the laboratory during their period of gametogenesis. Following the adult acclimation, offspring from each parental treatment were reared in reciprocal conditions. To assess for transgenerational and intragenerational effects throughout ontogeny, thermal tolerance and body size of each offspring treatment was measured at critical developmental stages. Embryos from MHW-acclimated females were more thermally tolerant with higher LT50 values compared to progeny from non-MHW-acclimated females. Additionally, there was an effect of female acclimation state on offspring body size at two stages of development - early gastrula and prism. To examine maternal provisioning as a means to also alter embryo performance, I examined egg size and biochemical composition. Here I found that eggs from MHW-acclimated female had higher total protein concentrations. Overall results from my dissertation research highlighted natural temperature variation experienced by M. californianus and S. purpuratus, and how this thermal exposure was influenced during heatwave events. Further, using a combination of field and laboratory experiments I provided evidence for transgenerational and intragenerational acclimation to alter performance of early developmental stages.