A reappraisal of the chemical and physical defenses of Caribbean gorgonian corals against predatory fishes

Anti-predatory properties of crude organic extracts and calcitic sclerites from the tissues of 32 species of Caribbean gorgonians were examined at natural volumetric concentrations in laboratory feeding assays using the bluehead wrasse Thalassoma bifasciatum. The nutritional qualities of the gorgonian samples were analyzed by determination of protein content, total energy content, and ash mass. All of the species tested (100%) yielded predation-deterrent crude organic extracts, but the sclerites of only 2 species (6.3%), Pterogorgia citrina and the encrusting form of Briareum asbestinum, were deterrent. The mean NaOH-soluble protein content, total energy content and ash mass of the gorgonian species were 17 +/- 8 mg ml(-1), 4 +/- 2 kJ ml(-1), and 400 +/- 100 mg ml(-1), respectively. There was no apparent relationship between the nutritional quality of a species and its chemical or physical defense. Contrary to previous studies, results indicated that sclerites do not generally afford gorgonians protection against generalist fish predators, and there was no evidence that the presence of sclerites in gorgonian tissue decreases the nutritional value sufficiently to deter predation. As in sponges, secondary metabolites are the primary means of defense against fish predators for Caribbean gorgonians. Results confirm that feeding assays conducted on the basis of tissue volume yield different results than those conducted on the basis of tissue mass.