Lipid biomarkers reveal the trophic plasticity of octocorals along a depth gradient

Symbiotic octocorals are important reef inhabitants, from shallow to mesophotic depths. However, information regarding their nutritional ecology along the depth gradient, and in particular the changes in the autotrophic and heterotrophic acquisition of nutrients, is limited, despite the fact that nutrient acquisition is a fundamental process in explaining the distribution of reef organisms. Here, the abundance of three lipid biomarkers, specific for autotrophy and heterotrophy, was investigated in the tissue of four octocoral species and one scleractinian sampled in shallow and upper mesophotic reefs of the oligotrophic Northern Red Sea. Our findings show functional mixotrophy for all dinoflagellate‐bearing species, with a significant input of heterotrophic feeding on herbivorous crustacean zooplankton at both depths. The relatively high level of the heterotrophic marker in all symbiotic species in shallow conditions does not corroborate the common idea that corals rely mostly on autotrophy in shallow waters. The increase in heterotrophic capacity with depth was however species‐specific, likely related to physiological and morphological characteristics. In addition, octocorals maintained similar concentrations of autotrophic markers with increasing depth, while the scleractinian species exhibited a decrease in these markers with depth. These results are in agreement with previous measurements of photosynthetically‐fixed carbon acquisition, suggesting that autotrophy remains stable in Red sea soft corals, whereas it decreases in scleractinian corals. This study highlights the importance of heterotrophy across the euphotic‐upper mesophotic depth gradient and brings relevant advances on our understanding of the ecological significance of feeding for reef corals.