Macroalgal grazing selectivity among herbivorous coral reef fishes

Despite growing interest in examining functional groups among coral reef species, few studies have examined the relative functional impacts of individual herbivorous fish species in coral reef ecosystem processes. We investigated potential functional roles within an herbivorous reef fish assemblage by assessing the feeding selectivity of fishes on macroalgae. Transplanted multiple-choice algal assays and remote stationary underwater digital video cameras were used to quantify the feeding selectivity of 6 herbivorous reef fish species: Siganus doliatus and S. canaliculatus (Siganidae); Chlorurus microrhinos, Hipposcarus longiceps, and Scarus rivulatus (Labridae); and Pomacanthus sexstriatus (Pomacanthidae). The siganids strongly selected Sargassum sp. (Phaeophyta) and, to a lesser extent, Hypnea sp. (Rhodophyta), but strongly avoided calcified macroalgae. However, S. doliatus displayed strong selection for Hypnea sp., whereas S. canaliculatus exhibited stronger selection for Sargassum sp. In contrast, the parrotfishes exhibited similar selectivities, with all 3 species selecting heavily calcified Halimeda spp. (Chlorophyta), but strongly avoiding Galaxaura sp. (Rhodophyta) and other red and brown macroalgal species. The pomacanthid was the least selective of the 6 fish species, exhibiting no clear selection or avoidance. These results emphasize the potential variation among species within presumed functional groups, and the need for care when defining functional groups at the family or the genus level. The extent of specialization, with strong selection and avoidance, suggests that functional redundancy among herbivores may be less than previously assumed.