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

Paramuricea clavata Risso, 1826 (Cnidaria: Octocorallia) is a characteristi c 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 Med iterranean Sea) using continuous flow incubation chambers. In situ grazing rates on DOG, detrital POC, and pico-, nano- and microplankton were examine d 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 c apture of DOC and picoplankton (heterotrophic bacteria, Prochlorococcus sp. , Synechococcus sp, and picoeukaryotes) was observed. Carbon of detrital or igin accounted for 86 +/- 14 % of the total ingested carbon and showed a ma rked seasonal pattern in which winter and spring were the seasons with the highest ingestion rates. Ingestion of live POC also exhibited a marked seas onal 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 PO C showed minimum values during the summer period, which supports the hypoth esis that trophic limitations may underlie the summer regression in activit y of the species. The results, together with evidence from previous studies , suggest that the capture of detrital POC may be a common and important fe ature among gorgonian species. It was calculated that the species daily cap tured 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 wat er adjacent to the bottom. The estimated capture rate of the species sugges ts that, in littoral ecosystems, grazing by macroinvertebrates on planktoni c communities and suspended detrital POC should not be disregarded.