Nutritional ecology of nominally herbivorous fishes on coral reefs

Nominally herbivorous acanthurids (surgeonfishes) and scarids (parrotfishes) have often been considered a 'homogeneous' functional group that consumes and digests algae. Recent work demonstrates that many of these fishes consume detritus. The objective of this study was to investigate the composition of dietary nutrients targeted by these and other fishes in terms of feeding behaviour, diet and short chain fatty acids (SCFA). We undertook a nutritional analysis of a range of species including detritivores, algivores, omnivores and planktivores from north eastern Australia. We calculated assimilation efficiencies for total protein amino acids (TAA), carbohydrate and lipid, and measured TAA in gut fluid along the intestine. Nutrients were assimilated similarly to their dietary proportions, with planktivores assimilating a high proportion of TAA, a moderate proportion of lipid and little carbohydrate. Omnivores assimilated moderate proportions of TAA and carbohydrate, and a low proportion of lipid. Algivores assimilated a low proportion of TAA and lipid, but a high proportion of carbohydrate. Detritivorous scarids and acanthurids differed significantly from algivores, assimilating a high proportion of TAA, a low proportion of carbohydrate and a moderate proportion of lipid. TAA levels in gut fluid of all species were highest in the anterior and lowest in the posterior intestine. Gut segments with highest TAA values were compared between dietary groups and followed a similar trend to TAA assimilation. Planktivores had high concentrations of TAA, while omnivores had intermediate, and algivores the lowest, concentrations. The highest gut fluid TAA concentrations were found in detritivorous scarids and acanthurids, and were significantly higher than in algivores. A significant negative correlation was found between anterior intestinal fluid TAA and posterior intestinal SCFA values. Detritivores had the highest levels of TAA but the lowest levels of SCFA. Planktivorous species had high levels of TAA and low to intermediate levels of SCFA. Omnivores had moderate levels of both TAA and SCFA. Algivores had low levels of TAA but high levels of SCFA. This indicates that there are major differences in the food resources targeted by detritivorous and algivorous fish species on coral reefs.