Control of populations of the coral-feeding nudibranch Phestilla sibogae by fish and crustacean predators

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The aeolid nudibranch Phestilla sibogae (Bergh) is a specialist predator on corals of the genus Porites. Predation by this gregarious nudibranch causes extensive damage, ultimately mortal, to colonies of Porites compressa (Dana) kept in laboratory tanks at the Hawaii Institute of Marine Biology. In contrast, individuals of Phestilla sibogae are seldom found on Hawaiian reefs, even in areas of high coral cover by Porites spp., and evidence of predation by Phestilla sibogae, such as feeding scars on corals, is rarely encountered. We tested the hypothesis that predation by coral reef fishes and crustaceans on P. sibogae is an important factor controlling populations of this nudibranch under natural reef conditions. Survival of nudibranchs was determined in the presence and absence of two species of reef fishes, a carnivorous wrasse, Thalassoma duperrey (Quoy and Gaimard), and an omnivorous butterflyfish, Chaetodon auriga (Forsskal). Both species significantly reduced survival of P. sibogae. T. duperrey was a more efficient predator on large (1.5 to 3.0 cm length) than on small (0.7 to 1.4 cm length) nudibranchs, while C. auriga fed equally effectively on both size classes. These differences may be related to foraging behavior of the fish. Four species of crustaceans were found in abundance within colonies of Porites compressa, and fed readily on small nudibranchs. The largest and most abundant of these species, the xanthid crab Phymodius monticulosus, also consumed large nudibranchs. This combination of predation by fishes outside the coral colony and by crustaceans inside the coral colony may control populations of Phestilla sibogae. At low population densities, P. sibogae does not inflict excessive damage on its coral prey. In the absence of predators, P. sibogae populations have the potential to explode, resulting in extensive damage and mortality of coral colonies.