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

Recent reports suggest that 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 31.0-31.8°C (4.1–7.5 DHW), regardless 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, likely because settlers were unable to feed on zooplankton during the heatwave: at 32.4°C (9.4–10.7 DHW), prey capture was reduced by up to 98% as compared to controls. Our findings identify the coupling between symbiont and host performance under thermal stress in F. fragum settlers, and help explain low recruitment on Caribbean reefs.