FLOW-DEPENDENT HERBIVORY AND GROWTH IN ZOOXANTHELLAE-FREE SOFT CORALS

The diet of Dendronephthya hemprichi and three other abundant soft cor als in the northern Red Sea consists mainly of phytoplankton, a food s ource so far unknown for cnidarians, We present a carbon budget for th is species, synthesizing field data on somatic growth, food intake, an d respiration. In situ rates of phytoplankton uptake and addition of p olyps were affected in a similar and nonlinear fashion by flow, with o ptimal values observed at similar to 15 cm s(-1). The size of polyps i ncreased with increasing flow between 1 and 32 cm s(-1). Zooplankton c apture was selective for weakly swimming bivalve and gastropod larvae and contributed <5% to the corals' carbon demand for growth and respir ation and about two to three orders of magnitude less than the carbon gained by phytoplanktivory. The ability of octocorals to feed on phyto plankton is probably related to the narrowly spaced pinnules on their tentacles as well as morphological and behavioral adaptations to livin g in strong flow. The utilization of phytoplankton, which has nearly a n order of magnitude higher biomass than zooplankton, is probably the principal mechanism allowing an azooxanthellate cnidarian to be highly productive in oligotrophic reef waters.