Trophic ecology of a benthic marine hydroid, Campanularia everta

Abstract

The diet and prey capture rate were studied in Campanularia everta (Hydrozoa) in the western Mediterranean Sea during both a 1 yr period and a diel period. A sample of 20 colonies was collected from the thall~ of the alga Halirneda tuna every 2 wk, and the stomach content of 100 polyps was examined. The d ~ e t of C. everta consisted primarily (88%) of particulate organic matter between 30 and 80 pm in size (probably detritus). Zooplankton accounted for only 12% of the diet; the pre-dominant prey items were crustacean eggs, larvae, and fragments, and as well as other benthic invertebrate larvae. In terms of biomass, zooplankton contributed 54% of the diet. The percentage of prey-containing polyps was rather constant at around 70'X 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 is remained high throughout the year, with maximum values in autumn and minimum values in summer. Digestion t ~ m c was less than 2 h Prey capture estimates were slightly fewer than 4000 prey m-2 d-l in summer and 800 000 prey m-' d-l in winter. Accordingly, C. everta may ingest a mean of 1925 pg C m-2 d.' in particulate organic matter and 2260 pg C m-' d -' in zoo-plankton each year, representing a total of 1528 mg (: m-2 yrrl Each pg C of polyps (somatic biomass) ingests a mean value of 69.6 1-19 C of prey per year. These figures suggest that the contribution of hydroids to the transfer of energy from the plankton to the benthos in littoral systems may be much hlgher than that suggested by hydroid biomass values themselves