Nutrient acquisition in four Mediterranean gorgonian species

Carbon and nitrogen isotope abundance values (δ13C and δ15N, respectively) were measured for the first time in the soft tissue, axial skeleton, and spicules of 4 Mediterranean gorgonians, 3 asymbiotic (Leptogorgia sarmentosa, Paramuricea clavata, and Eunicella verrucosa) and 1 symbiotic with autotrophic dinoflagellates (Eunicella singularis). The isotopic composition of their diet, i.e. zooplankton, particulate organic matter (POM), and sedimentary organic matter (SOM), was also measured to understand gorgonian feeding ecology. (1) Carbon and nitrogen signatures of the symbiotic E. singularis tissue in summer differed significantly from the signatures of the other species; (2) carbon and nitrogen signatures of the axial skeleton were similar to those of the tissue, because the skeleton is primarily made of gorgonin secreted by the tissue; and (3) spicules had a high δ13C signature because they are made by a combination of 60 to 76% of respiratory CO₂ and of external CO₂, with a high δ13C signature. Comparison of the isotopic signatures of the gorgonian tissues and the food sources indicated that E. singularis and P. clavata had the same diet in both winter and summer, either zooplankton for E. singularis or POM and SOM for P. clavata. Conversely, L. sarmentosa and E. verrucosa shifted from zooplankton in winter to SOM in summer. These results bring insights into the feeding ecology of temperate gorgonians and explain their distribution, abundance, and role in the flow of particulate matter between the water column and the benthos.