Discordance between diet analysis and dietary macronutrient content in four nominally herbivorous fishes from the Southwestern Atlantic

Herbivorous fishes are an important component of coral reef systems worldwide, but their nutritional ecology is poorly understood, particularly the relationships between the taxonomic composition and the nutritional composition of their diets. We compared dietary composition with % carbon, % nitrogen and C:N ratios of diet in four species of nominally herbivorous fishes from the Southwestern Atlantic and used literature values to calculate proportional contributions of dietary items to total nitrogen intake. Both Sparisoma axillare (Labridae, Scarinae) and Acanthurus chirurgus (Acanthuridae) had a diet composed mainly of detritus, with contributions of red algae. However, the diet of S. axillare displayed higher %N and a lower C:N ratio, although animal material made only a slightly greater contribution to total nitrogen intake than in A. chirurgus. Kyphosus sectatrix (Kyphosidae) ingested mainly carbon-rich corticated algae, while Diplodus argenteus (Sparidae) had a varied, omnivorous diet. These results indicate that conventional diet analysis may not reveal important interspecific differences in nutrient intake and that a reassessment of the nutrient intake of different herbivorous fishes is required to fully understand their ecology. This finding highlights the fact that foods of nominally herbivorous fishes vary greatly in nutritional quality. Moreover, conventional dietary categories such as detritus may exhibit considerable heterogeneity in taxonomic and nutritional composition, suggesting a previously unrecognised level of dietary selectivity in this fish assemblage.