UC Santa Barbara

Marine heatwaves (MHWs) are becoming an increasingly pervasive threat to marine ecosystems. Projected increases in the frequency and intensity of future MHW events highlight the need to assess the adaptive capacity of marine organisms to such extreme events. While the massive mortality caused by MHWs is frequently observed, the sublethal consequences for reproduction and development serve as understudied areas of research. Phenotypic plasticity, the ability of an organism to change its physiology and/or morphology without alteration to the genome, presents an avenue in which the effects of MHWs can be transduced across life history stages. In this dissertation, I explored the impact of elevated temperatures associated with MHW events on the fertilization process and early development of two species of sea urchin found in the Santa Barbara Channel (SBC).First, I used the purple urchin species, Strongylocentrotus purpuratus, to measure the effects of male thermal history on fertilization and early development. In the lab, male urchins were acclimated for one month to either non-MHW or MHW-like temperature conditions, as determined by temperature data collected in the SBC during past MHW events. Fertilization trials and offspring rearing were then conducted at both temperatures to assess sperm and developmental performance. While sperm appeared robust to elevated temperatures during fertilization, sperm produced by MHW-acclimated males had overall diminished fertilization capacity as compared to those acclimated to non-MHW temperatures. For early developmental stages, the effects of paternal temperature were masked by the influence of developmental thermal conditions, where elevated temperatures resulted in larger offspring. Alternatively, the thermal physiology of offspring was significantly influenced by the interaction of paternal and developmental temperatures. Here, offspring had a higher thermal tolerance when their developmental temperature matched that experienced by their sire, possibly hinting at an adaptive paternal effect. The influence of fertilization temperature on later developmental success was also explored in the summer-spawning sea urchin species, Lytechinus pictus. For this experiment, fertilization trials were performed under one of three temperatures representing: ambient, current MHW, and future MHW temperature. The resulting embryos from each fertilization treatment were then split and raised under the three temperatures. Here, elevated temperatures during fertilization had largely negative effects on fertilization success and the performance of later developmental stages. The number of successfully hatched and gastrulated embryos, representing key stages of development in echinoderms, was significantly lower in embryos produced from fertilizations at both MHW conditions than those from fertilizations under ambient conditions. Overall, the results of this dissertation highlight the importance of the reproductive response to extreme events like MHWs. The carry-over effects within and across generations seen here, show that the stress response of individual life history stages should not be studied in isolation, and that the negative effects of MHW events may persist long after these events have passed.