FEEDING AND PREY CAPTURE CYCLES IN THE APOSYMBIONTIC GORGONIAN PARAMURICEA-CLAVATA

A study of the stomach contents of a population of the Mediterranean gorgonian Paramuricea clavata (Risso, 1826) showed that zooplankton accounted for an important share of the diet. Nauplii, copepod eggs, other invertebrate eggs, and other small (100 to 200 mu m) prey items of low motility accounted for 78 % of prey items, though adult calanoid copepods (600 to 700 mu m) were also captured. The mean number of prey items per polyp ranged from 1.2 to 3.3. The prey capture rate was highly dependent upon the density and frequency of occurrence of planktonic prey in the habitat. Peak prey capture levels were recorded in spring and at the end of autumn, while prey capture rates fell off substantially in summer, when the proportion of colonies with contracted polyps was very high. There were significant differences in the prey capture rates between colonies, which was attributed to differences in the spatial distribution of the zooplankton. There were also large differences in prey capture rates within the same colony. Polyps located on the apical and peripheral branches captured twice as many prey as those located in the centre or at the base of the colony. It was estimated that the population, located off the Medes Islands (northwestern Mediterranean) at between 17 and 29 m depth with a mean density of 32 gorgonians m(-2), may capture between 1.52 x 10(5) and 1.06 x 10(6) prey items m(-2) d(-1), according to the time of year. The prey capture rates recorded during a diel cycle, when extrapolated to an annual cycle, suggest that gorgonians play an important role in littoral food webs and particularly in the flow of energy from the plankton to the benthos.