Marine heatwaves (MHWs) – prolonged periods of unusually warm surface seawater temperatures - have increased in intensity and frequency in US coastal waters. Recent MHWs on the US west coast have resulted in changes in species distributions, loss of biodiversity and closed fisheries due to harmful algal blooms. The frequency of MHWs is predicted to increase in the future, and these events will likely impact natural marine ecosystems and important resources from the ocean (e.g., fishing and aquaculture). The goal of this project is to better understand how MHWs will impact kelp forests, an important coastal marine environment. There are many gaps in our knowledge about the expected impacts of MHWs, and one key missing piece of information is how early life history stages (the embryonic and juvenile forms), the stages that disperse and maintain healthy populations of marine organisms, would fare in response to these anomalously high ocean temperatures. In this project, we plan to study the embryos and larvae of two sea urchin species, the red and purple urchin, from thekelp forests of coastal California. Overall, we hope to understand more about the response and resilience of these marine invertebrates to MHW stress. In addition, from a wild fisheries perspective, data on the red sea urchin (the roe is sold as uni) will contribute to management of this important species. Lastly, the project will train several early career researchers, undergraduates and graduate students, and will contribute to training our future scientists in the science of MHWs and ocean global change biology. The goal of this project is to gain insight into the consequences of anomalously high temperatures that occur during MHW events at a time when many marine species are reproductive or developing in the plankton. In our coastal California system, marine invertebrates of the kelp forest ecosystem are reproductive when MHWs tend to occur and intensify. The core goal of this proposal is to examine how maternal and paternal effects, that play out in a MHW context, influence the thermal tolerance of early developmental stages of two sea urchin species from the temperate kelp forest communities of coastal California, the purple (Strongylocentrotus purpuratus) and red (Mesocentrotus franciscanus) sea urchins, the latter being an important wild fisheries species. Results from this project will address the climate-change relevant aspects of larval ecology, and examine how phenotypic plasticity contributes to adaptive capacity of marine organisms. We plan to assess a series of traits for each species: (1) fertilization kinetics, (2) successful developmental progression, (3) thermal tolerance (via thermal tolerance trials and biochemical indices of thermal stress), and (4) the effects of parental effects. We will also profile gene expression using RNA sequencing in order to capture the influence of maternal effects, here asking whether females that experience MHW temperatures in situ during gametogenesis produce progeny with greater tolerance of temperature via changes of the larval transcriptome. From a Broader Impacts and education standpoint, our goal is to provide research experiences and mentoring to under-represented undergraduates at UC Santa Barbara, a Hispanic-Serving Institution since 2015. In addition, the project directly supports the PhD dissertation research of three women UCSB Ph.D. graduate students from under-represented groups in STEM.