Natural feeding of the temperate asymbiotic octocoral-gorgonian Leptogorgia sarmentosa (Cnidaria: Octocorallia)

Abstract

Octocorals are common components of sublittoral benthic communities in temperate, tropical and polar areas. However, their natural diets and feeding rates are poorly known. In this study, we examined natural feeding and respiration of the ubiquitous temperate species Leptogorgia sarmentosa (Cnidaria: Octocorallia) in the NW Mediterranean. We measured in situ grazing rate on dissolved organic carbon (DOC), detrital particulate organic carbon (POC) and live particulate carbon <100 μm (i.e. pico- and nanoplankton, dinoflagellates, diatoms and ciliates) using continuous flow incubation chambers. L. sarmentosa captured nanoeukaryotes, dinoflagellates, diatoms, ciliates, as well as detrital POC, but no significant capture of DOC, nor of prey items smaller than nanoeukaryotes (mean 3.6 μm), was observed. Feeding rate on detrital POC and live carbon <100 μm was 402 ± 35 and 67 ± 6 μg C g ash-free dry mass (AFDM)-1 h-1, respectively. Detrital POC and zooplankton previously studied - were the 2 main food sources, while live carbon <100 μm accounted for less than 10% of the total ingested carbon. Respiration rate was 0.55 ± 0.24 mg O2 g AFDM-1 h-1 (mean ± SE), which is equivalent to a requirement of 155 ± 67 μg C g AFDM-1 h-1. On the basis of estimated feeding and respiration rates, the respiratory demand can largely be met. The spectrum of food sources and the contribution of different food sources to the diet is consistent with previous feeding studies and suggests 2 general patterns in the diet of octocorals. First, octocorals feed on a wide spectrum of food sources ranging from ca. 4 μm (nanoeukaryotes) to seston particles measuring several hundred microns. Second, despite the wide spectrum of food sources, microplankton and detrital POC constitute the bulk of the heterotrophic diet of octocorals. The contribution of detrital POC to the diet of octocorals suggests that there may be important seasonal variations in energy intake, which may affect the seasonal dynamics of temperate octocorals due to the marked seasonal variation in the availability of this particulate carbon source in temperate ecosystems like the Mediterranean.