Warming but not Food Limitation Alters Metabolism During Larval Development in Crown-of-Thorns Sea Stars (Acanthaster...

Marine ectotherms are vulnerable to physiological challenges wrought by climate change-driven ocean warming (OW). It is important to understand the impacts of OW on fundamental processes in keystone species such as the corallivorous crown-of-thorns sea star (CoTS; Acanthaster cf. solaris), which poses a key threat to coral reefs. The planktonic life phase of CoTS may benefit from OW through faster development, but increasing temperature can be metabolically costly. We tested whether CoTS larvae can maintain a positive energy budget under warming to offset increased energetic demands within their thermal tolerance range. We reared fed and unfed CoTS larvae at four temperature treatments (control: 26°C; warm: 28°C, 30°C, 32°C) based on current and projected OW and measured metabolic rate (oxygen consumption, $\dot{V}{O}_2$ ) across development to the late larval stage. As predicted, warming increased metabolic rates, with oxygen consumption higher at 30°C and 32°C, than at the control 26°C. High mortality occurred at 32°C in late bipinnaria (14 days), likely due to cumulative heat stress over time. In the 28°C and 30°C treatments oxygen consumption of bipinnaria (14-21 days) and brachiolaria larvae decreased, potentially due to metabolic suppression. The resilience of unfed larvae (to 21 days) and the similar metabolic rates of fed and unfed larvae to Day 14 indicate that the energetic requirements for larval maintenance and growth were provided by maternal (egg) nutrients, a resilience trait of CoTS. Our results indicate that under increased temperature, CoTS larvae can maintain positive energy budgets. While the presence of food is not critical for development to late bipinnaria, reduced growth in warm treatments in the absence of food may have fitness consequences. Our results highlight the need to understand the energy budget across the entire life cycle of CoTS.