Zooplankton provisioning does not enhance thermotolerance and survival of Favia fragum settlers

Abstract Reef-building corals can shift from autotrophic to heterotrophic feeding during episodic heatwaves, which may have mitigating effects on thermal stress. It is yet unclear to what extent this could occur during early life stages of corals, and how this affects survival. We exposed settlers (primary polyps) of the Caribbean Golf ball coral Favia fragum to a lab-controlled heatwave lasting 48 days under two feeding regimes (36 versus 3600 Artemia salina nauplii L −1 ), starting from 28 °C and peaking at 32.4 °C with a daily increase of 0.19 °C. Thermal stress, measured as declining effective photosystem II yield, became apparent at 31.8 °C (or 7.5 Degree Heating Weeks, DHW). Growth of heat-exposed settlers ceased between 31.0 and 31.8 °C (4.1–7.5 DHW), regardless of feeding regime. As a consequence, their size was 27% lower compared to controls at 31.8 °C (7.5 DHW), irrespective of feeding regime. All heat-exposed settlers had died at 14.4 DHW. Mortality was preceded by a 41–64% loss of symbiont densities and a 46–59% reduction of chlorophyll a fluorescence at 32.4 °C (9.4 DHW). No beneficial effect of feeding on thermotolerance was observed. This was likely due to significant reductions in prey capture during the heatwave of 94–98% as compared to controls at 32.4 °C (9.4–10.7 DHW). Our findings identify a potential coupling between disruption of autotrophy and heterotrophy in F. fragum settlers.