Natural heterotrophic feeding by a temperate octocoral with symbiotic zooxanthellae: a contribution to understand the mechanisms of die-off events
Coma, R.; Llorente-Llurba, E.; Serrano, E.; Gili, J. M.; Ribes, M.
Octocorals are among the most emblematic and representative organisms of sublittoral communities in both tropical and temperate seas. Eunicella singularis is the most abundant gorgonian in shallow waters and the only gorgonian with symbiotic zooxanthellae in the Mediterranean Sea. We studied the natural diet and prey capture rate of this species over an annual cycle and characterized prey digestion time over the natural temperature regime. The species captured zooplankton prey between 40 and 920 µm. A mean content of 0.14 ± 0.02 prey polyp−1 was observed throughout the year. The strong pattern of decrease in digestion time with temperature increase (from 25 h at 13 °C to 8 h at 21 °C) allowed us to estimate that the prey capture rate was 0.017 ± 0.002 prey polyp−1 h−1 (mean ± SE); the ingestion rate exhibited a seasonal pattern with higher values in spring (0.007 µg C polyp−1 h−1). Feeding on zooplankton had a low contribution to the respiratory expenses of E. singularis except in early spring. Then, heterotrophic nutrition in the natural environment seems unable to meet basal metabolic requirements, especially in summer and fall. This result, in conjunction with the documented collapse of photosynthetic capacity above a warm temperature threshold, indicates the occurrence of a resource acquisition limitation that may play a role in the repeated summer die-off events of the species.