Variation of lipid and free fatty acid contents during larval release in two temperate octocorals according to their trophic strategy

This study investigated the energetic investment in larval release of 2 Mediterranean gorgonians with different trophic strategies: Eunicella singularis (mixotroph) and Corallium rubrum (heterotroph). Both are internal brooders, releasing larvae within a few weeks in summer. A biochemical approach, based on the analysis of stable isotopes, total lipid and free fatty acid (FFA) content, was applied in combination with the quantification of sexual products. Stable isotopes showed that food source varied seasonally for E. singularis, while it remained constant throughout the year for C. rubrum, suggesting a higher trophic plasticity of E. singularis, likely due to its mixotrophic feeding strategy. Although total lipid and FFA content were higher in E. singularis than in C. rubrum, both species showed low energetic investment in reproduction, probably linked to their low fecundity and reproductive output, compared to other gorgonians (e.g. Paramuricea clavata). The higher FFA content in E. singularis compared to C. rubrum can be explained by a higher metabolic demand and metabolite exchange between the host and its symbiotic algae. The higher inter-annual variability in total lipid and FFA content in C. rubrum suggests that this species is more sensitive to food constraints than E. singularis. Indeed, the inter-annual consistency in the trophic ecology (showed by the stable isotopes) means that C. rubrum is more affected by changes in food availability. Conversely, the mixotrophy makes E. singularis less sensitive to stress conditions caused by starvation or thermal stress. These physiological differences could partly explain the wider distribution of E. singularis compared to C. rubrum.