HETEROTROPHIC FEEDING BY GORGONIAN CORALS WITH SYMBIOTIC ZOOXANTHELLA

Gorgonians are one of the most characteristic groups in Caribbean cora l reef communities. In this study, we measured in situ rates of grazin g on pico-, nano-, and microplankton, zooxanthellae release, and respi ration for the ubiquitous symbiotic gorgonian coral Plexaura flexuosa. Zooplankton capture by P. flexuosa and Pseudoplexaura porosa was quan tified by examination of stomach contents. In nature, both species cap tured zooplankton prey ranging from 100 to 700 mu m, at a grazing rate of 0.09 and 0.23 prey polyp(-1) d(-1), respectively. Because of the g reater mean size of the prey and the higher mean prey capture per poly p, P. porosa obtained 3.4 X 10(-5) mg C pply p(-1) d(-1) from zooplank ton, about four times the grazing rate of P. flexuosa. On average, P. flexuosa captured 7.2 +/- 1.9 microorganisms poly p(-1) d(-1) includin g ciliates, dinoflagellates, and diatoms, but they did not appear to g raze significantly on organisms <5 mu m (heterotrophic bacteria, Proch lorococcus sp., Synechococcus sp., or picoeukaryotes). Zooplankton and microbial prey accounted for only 0.4% of respiratory requirements in P. flexuosa, but they contributed 17% of nitrogen required annually f or new production (growth and reproduction). Although the contribution of microbial prey to gorgonian energetics was low, dense gorgonian po pulations found on many Caribbean reefs may be important grazers of pl ankton communities.