Streit, Robert P., Hoey, Andrew S., and Bellwood, David R. (2015) Feeding characteristics reveal functional distinctions among browsing herbivorous fishes on coral reefs. Coral Reefs, 34 (4). pp. 1037-1047.

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Abstract

The removal of macroalgal biomass by fishes is a key process on coral reefs. Numerous studies have identified the fish species responsible for removing mature macroalgae, and have identified how this varies spatially, temporally, and among different algal types. None, however, have considered the behavioural and morphological traits of the browsing fishes and how this may influence the removal of macroalgal material. Using video observations of fish feeding on the brown macroalga Sargassum polycystum, we quantified the feeding behaviour and morphology of the four dominant browsing species on the Great Barrier Reef (Kyphosus vaigiensis, Naso unicornis, Siganus canaliculatus, and Siganus doliatus). The greatest distinction between species was the algal material they targeted. K. vaigiensis and N. unicornis bit on the entire macroalgal thallus in approximately 90 % of bites. In contrast, Si. canaliculatus and Si. doliatus avoided biting the stalks, with 80–98 % of bites being on the macroalgal leaves only. This distinctive grouping into 'entire thallus-biters' versus 'leaf-biters' was not supported by size-standardized measures of biting morphology. Rather, species-specific adult body sizes, tooth shape, and feeding behaviour appear to underpin this functional distinction, with adults of the two larger fish species (N. unicornis and K. vaigiensis) eating the entire macroalgal thallus, while the two smaller species (Si. canaliculatus and Si. doliatus) bite only leaves. These findings caution against assumed homogeneity within this, and potentially other, functional groups on coral reefs. As functional redundancy within the macroalgal browsers is limited, the smaller 'leaf-biting' species are unlikely to be able to compensate functionally for the loss of larger 'entire thallus-biting' species.

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