TROPHIC ECOLOGY OF A BENTHIC MARINE HYDROID, CAMPANULARIA-EVERTA

The diet and prey capture rate were studied in Campanularia everta (Hy drozoa) in the western Mediterranean Sea during both a 1 yr period and a diel period. A sample of 20 colonies was collected from the thalli of the alga Halimeda tuna every 2 wk, and the stomach content of 100 p olyps was examined. The diet of C. everta consisted primarily (88 %) o f particulate organic matter between 30 and 80 mu m in size (probably detritus). Zooplankton accounted for only 12 % of the diet; the predom inant prey items were crustacean eggs, larvae, and fragments, and as w ell as other benthic invertebrate larvae. In terms of biomass, zooplan kton contributed 54 % of the diet. The percentage of prey-containing p olyps was rather constant at around 70 % over both the annual and diel cycles. Prey capture rates were quite high overall (between 0.6 and 2 .7 prey items per polyp) and likewise the number of polyps with prey i s remained high throughout the year, with maximum values in autumn and minimum values in summer. Digestion time was less than 2 h. Prey capt ure estimates were slightly fewer than 4000 prey m(-2) d(-1) in summer and 800 000 prey m(-2) d(-1) in winter. Accordingly, C. everta may in gest a mean of 1925 mu g C m(-2) d(-1) in particulate organic matter a nd 2260 mu g C m(-2) d(-1) in zooplankton each year, representing a to tal of 1528 mg C m(-2) yr(-1). Each mu g C of polyps (somatic biomass) ingests a mean value of 69.6 mu g C of prey per year. These figures s uggest that the contribution of hydroids to the transfer of energy fro m the plankton to the benthos in littoral systems may be much higher t han that suggested by hydroid biomass values themselves.