Heterogeneous feeding in benthic suspension feeders:the natural diet and grazing rate of the temperate gorgonian Paramuricea clavata (Cnidaria:Octocorallia) over a year cycle

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Abstract
Paramuricea clavata Risso, 1826 (Cnidaria: Octocorallia) is a characteristic species of Mediterranean benthic fauna. The natural diet and prey capture rate of the species were studied in the field at the Medes Islands (NW Mediterranean Sea) using continuous flow incubation chambers. In situ grazing rates on DOG, detrital POC, and pico-, nano- and microplankton were examined over 1 annual cycle. P. clavata capture nanoeukaryotes, phytoplankton and ciliates as well as detrital POC. Colonies ingested an annual mean of 0.19 +/- 0.17 mu g C mu g(-1) C d(-1) from these food sources. No significant capture of DOC and picoplankton (heterotrophic bacteria, Prochlorococcus sp., Synechococcus sp, and picoeukaryotes) was observed. Carbon of detrital origin accounted for 86 +/- 14 % of the total ingested carbon and showed a marked seasonal pattern in which winter and spring were the seasons with the highest ingestion rates. Ingestion of live POC also exhibited a marked seasonal pattern, with the highest values occurring in winter. The results show that P. clavata has a broad and heterogeneous diet that ranges from 3.8 mu m (nanoeukaryotes) to 700 mu m (copepods). Capture of detrital and live POC showed minimum values during the summer period, which supports the hypothesis that trophic limitations may underlie the summer regression in activity of the species. The results, together with evidence from previous studies, suggest that the capture of detrital POC may be a common and important feature among gorgonian species. It was calculated that the species daily captured 1-22 % of diatoms, 1-9 % of nanoeukaryotes, 1-26 % of dinoflagellates, 2-99 % of ciliates, and 2-10 % of suspended detrital POC m(-3) of the water adjacent to the bottom. The estimated capture rate of the species suggests that, in littoral ecosystems, grazing by macroinvertebrates on planktonic communities and suspended detrital POC should not be disregarded.