ABSTRACT
A focus on nutrition is generally central to the study of trophodynamics and resource partitioning in terrestrial assemblages. In contrast, the study of trophic diversification in herbivorous fishes on coral reefs has focused on the effects of feeding, rather than its causes. The dietary substrata of parrotfishes are therefore understood largely in terms of where parrotfishes feed, not what they ingest. In this chapter an existing framework of parrotfish feeding is used to generate predictions on potential dietary resources, and evaluated these in terms of nutritional composition. The extent to which various resource targets matched predictions based on feeding behaviour and digestive mechanisms are then examined. Finally, the assimilation of nutrients by parrotfishes is assessed in light of data from biochemical analyses. All studied parrotfish appeared to utilise a diet that contained proportions of protein, lipid and carbohydrate inconsistent with either epilithic turf algae or macroalgae. Parrotfish have a specialised branchial feeding mechanism that appears designed to harvest and selectively retain microscopic particles in the pharynx. Biochemical data indicated a diet of bacteria and/or cyanobacteria distinct from that used by other large grazing fishes. The hypothesis that parrotfishes are microphages that target protein-rich epilithic, endolithic and epiphytic microscopic phototrophs provides a synthetic, unified explanation consistent with all available nutritional evidence.
