Episymbionts as possible anti-fouling agents on reef-building hydrozoans

Barnacles (Crustacea; Cirripedia) of the subfamily Pyrgomatinae are all obl igatory epizoans of stony corals and hydrozoans, living within the skeleton of their host organisms and filtering plankton using their thoracic cirri. The nature of the symbiotic relationship between these barnacles and their hosts has received some attention over the last decade, concentrating most ly on trophic aspects. Cook et al. (1991), and Achituv and Mizrahi (1996) s uggested that phosphorus and nitrogen excreted by barnacles could be absorb ed by the host's endosymbiotic zooxanthellae, thus enabling greater photosy nthetic activity and growth in nutrient-poor waters. Inversely, Achituv et al. (1997) suggested (using stable-carbon isotope ratios) that several cora l-inhabiting barnacles feed mainly on organic matter abraded from the host and endosymbiotic zooxanthellae expelled by the host. In the present study, we tested whether the host Millepora dichotoma was protected from fouling by their barnacle symbionts.