Flow‐dependent herbivory and growth in zooxanthellae‐free soft corals

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Abstract
The diet of Dendronephthya hemprichi and three other abundant soft corals in the northern Red Sea consists mainly of phytoplankton, a food source so far unknown for cnidarians. We present a carbon budget for this species, synthesizing field data on somatic growth, food intake, and respiration. In situ rates of phytoplankton uptake and addition of polyps were affected in a similar and nonlinear fashion by flow, with optimal values observed at ∼15 cm s‒1 . The size of polyps increased with increasing flow between 1 and 32 cm s‒1. Zooplankton capture was selective for weakly swimming bivalve and gastropod larvae and contributed <5% to the corals’ carbon demand for growth and respiration and about two to three orders of magnitude less than the carbon gained by phytoplanktivory. The ability of octocorals to feed on phytoplankton is probably related to the narrowly spaced pinnules on their tentacles as well as morphological and behavioral adaptations to living in strong flow. The utilization of phytoplankton, which has nearly an order of magnitude higher biomass than zooplankton, is probably the principal mechanism allowing an azooxanthellate cnidarian to be highly productive in oligotrophic reef waters.

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