Heterotrophic feeding by gorgonian corals with symbiotic zooxanthella

Abstract

Gorgonians are one of the most characteristic groups in Caribbean coral reef communities. In this study, we measured in situ rates of grazing on pico‐, nano‐, and microplankton, zooxanthellae release, and respiration for the ubiquitous symbiotic gorgonian coral Plexaura flexuosa. Zooplankton capture by P. flexuosa and Pseudoplexaura porosa was quantified by examination of stomach contents. In nature, both species captured zooplankton prey ranging from 100 to 700 µm, at a grazing rate of 0.09 and 0.23 prey polyp−1 d−1, respectively. Because of the greater mean size of the prey and the higher mean prey capture per polyp, P. porosa obtained 3.4 × l0−5 mg C polyp−1 d−1 from zooplankton, about four times the grazing rate of P. flexuosa. On average, P. flexuosa captured 7.2 ± 1.9 microorganisms polyp−1 d−1 including ciliates, dinoflagellates, and diatoms, but they did not appear to graze significantly on organisms <5 µm (heterotrophic bacteria, Prochlorococcus 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 for new production (growth and reproduction). Although the contribution of microbial prey to gorgonian energetics was low, dense gorgonian populations found on many Caribbean reefs may be important grazers of plankton communities.