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Herbivorous fishes are a critical functional group on coral reefs, and there is a clear need to understand the role and relative importance of individual species in reef processes. While numerous studies have quantified the roles of parrotfishes and surgeonfishes on coral reefs, the rabbitfishes (f. Siganidae) have been largely overlooked. Consequently, they are typically viewed as a uniform group of grazing or browsing fishes. Here, we quantify the diet and distribution of rabbitfish assemblages on six reefs spanning the continental shelf in the northern Great Barrier Reef. Our results revealed marked variation in the diet and distribution of rabbitfish species. Analysis of stomach contents identified four distinct groups: browsers of leathery brown macroalgae ( Siganus canaliculatus, S. javus), croppers of red and green macroalgae ( S. argenteus, S. corallinus, S. doliatus, S. spinus) and mixed feeders of diverse algal material, cyanobacteria, detritus and sediment ( S. lineatus, S. punctatissimus, S. punctatus, S. vulpinus). Surprisingly, the diet of the fourth group ( S. puellus) contained very little algal material (22.5 %) and was instead dominated by sponges (69.1 %). Together with this variation in diet, the distribution of rabbitfishes displayed clear cross-shelf variation. Biomass was greatest on inner-shelf reefs (112.7 ± 18.2 kg.ha −1), decreasing markedly on mid- (37.8 ± 4.6 kg.ha −1) and outer-shelf reefs (9.7 ± 2.2 kg.ha −1). This pattern was largely driven by the browsing S. canaliculatus that accounted for 50 % of the biomass on inner-shelf reefs, but was absent in mid- and outer-shelf reefs. Mixed feeders, although primarily restricted to the reef slope and back reef habitats, also decreased in abundance and biomass from inshore to offshore, while algal cropping taxa were the dominant group on mid-shelf reefs. These results clearly demonstrate the extent to which diet and distribution vary within the Siganidae and emphasise the importance of examining function on a species-by-species basis. [ABSTRACT FROM AUTHOR]

This study investigated the basis of pair formation in the abundant herbivorous rabbitfish Siganus doliatus on Orpheus Island, Great Barrier Reef. Pair formation was the most common social system in S. doliatus, with 67·4% of all individuals occurring in pairs. Pairs were stable ( i.e. individuals remained with the same partner throughout the study) and pair members were found within 5 m of each other 82·9% of the time. Of the examined pairs, 25% were homosexual resulting in a proportion of heterosexual pairs (75%) that was significantly lower than expected if pairs were formed solely for reproductive reasons. Therefore, although reproduction appears to be the main driver of pair formation in S. doliatus, other factors are likely to influence this behaviour. The high density of individuals on the reef crest (5·7 ± 0·9 individuals 200 m−2; mean ± s.e.) and extensively overlapping home ranges of pairs indicated that the defence of territories plays no role in pair formation. Instead, it appears that pair formation in S. doliatus is driven, in part, by other, non-reproductive, ecological factors. It is suggested that pair formation allows for increased vigilance against predation and enables S. doliatus to execute a novel feeding behaviour. [ABSTRACT FROM AUTHOR]