Symbiodiniaceae activity enhances larval energy reserves in the mediterranean Gorgonian Eunicella singularis

Corals and gorgonians often form symbiotic relationships with Symbiodiniaceae. Despite extensive research in adult colonies, the role of Symbiodiniaceae during the larval stage is still poorly understood, even though energy acquisition and storage at this early stage are critical for population recovery following disturbances. This study aimed to elucidate the contribution of Symbiodiniaceae to the larval energy dynamics of the Mediterranean gorgonian species Eunicella singularis (Esper 1791). Larvae were exposed to light and dark conditions to manipulate Symbiodiniaceae activity, and various biochemical parameters, including lipids, carbohydrates, proteins, chlorophyll-a content, and Symbiodiniaceae density, were measured at the beginning and end of the Pelagic Larvae Duration (PLD). Additionally, respiration rates and assimilation rates of isotopically labeled bicarbonate and ammonium were measured to quantify metabolic activity. We found a significant increase in carbohydrate content under light conditions, and a reduction in lipid consumption, result- ing in enhanced lipid reserves in larvae by the end of the PLD, which suggests active photosynthesis by the symbionts. Symbiodiniaceae thus contributed to larval energy reserves through both carbohydrate and possibly lipid production. These findings highlight the importance of Symbiodiniaceae in supporting energy dynamics during larval early development stages, enhancing their potential for dispersion, settlement and survival.