Importance of heterotrophic adaptations of corals to maintain energy reserves

We examined the ability of the two hard coral species Agaricia tenuifolia and Porites furcata to store lipids under natural conditions, under experimental starvation (weekly vs. daily feeding) and under heat stress. P. furcata fed more and accumulated greater lipid quantities than A. tenuifolia. Overall, lipid levels in situ showed an inverse relationship to turbidity and eutrophication with highest values at the least anthropogenically impacted site. Although zooxanthellae, chlorophyll a concentrations and heterotrophy increased in low light, the adaptive response was insufficient to maintain high energy acquisition levels. In the feeding experiment, corals fed weekly contained higher lipid levels than corals fed daily, suggesting that intermittent periods of starvation induce lipid storage. When transplanted to low light conditions P. furcata profited from feeding and were able to restore zooxanthellae and chlorophyll a levels after an initial reduction. Generally, lipid accumulation in both species was higher when fed with phytoplankton (≤1 μm), suggesting phytoplankton could be a more efficient food source than zooplankton (>180 μm). Temperature stress led to a reduction of lipids and a 100 % mortality of A. tenuifolia, while P. furcata exhibited the ability to rebuild lipid reserves through heterotrophy. This ability of P. furcata to rebuild energy for both host and symbiont resulted in lower mortality and strong fitness and resistance of this species, making it an important umbrella species to maintain reefs in areas strongly impacted by anthropogenic activities and increasing sea water temperatures.