RESEARCH ARTICLE J. Howard Choat Æ William D. Robbins Kendall D. Clements The trophic status of herbivorous fishes on coral reefs II. Food processing modes and trophodynamics Received: 9 May 2003 / Accepted: 15 January 2004 / Published online: 8 April 2004 Ó Springer-Verlag 2004 Abstract Estimates of feeding rates, alimentary tract structure and temporal patterns of food processing ob- tained from twelve species of nominally herbivorous fishes on the northern Great Barrier Reef were com- pared. These included members of the families Acan- thuridae, Scaridae and Kyphosidae. Based on an analysis of diet and short-chain fatty acid (SCFA) pro- files from a previous study we initially partitioned the twelve species into four dietary categories, as follows: (a) Category 1: herbivores with a diet of macroscopic brown algae and high SCFA profiles in the hindgut region (Naso unicornis, Kyphosus vaigiensis); (b) Category 2: herbivores feeding on turfing and filamentous red and green algae with moderate SCFA profiles in the hindgut region (N. tonganus, K. cinerascens, Zebrasoma scopas, Acanthurus lineatus); (c) Category 3: zooplankton feed- ers with moderate SCFA profiles (N. vlamingii, N. brevirostris); (d) Category 4: species feeding on detrital and sedimentary materials with low levels of SCFA (Chlorurus microrhinos, Scarus schlegeli, Cteno- chaetus striatus, A. olivaceus). The purpose of this comparison was to determine whether measures of feeding activity, alimentary tract structure, and food processing were concordant with diet. A dichotomy in feeding rates was observed. Species with a diet of algae and zooplankton (categories 1–3) had slower feeding rates than those feeding on detrital aggregates and sed- iment (category 4). The pattern of food processing also followed the same dichotomy with species of categories 1–3 retaining food in the alimentary tract overnight and commencing the feeding day with substantial amounts of food in the intestine and hindgut. Category-4 species commenced the feeding day with empty alimentary tracts suggesting a rapid turnover of gut contents. Within the herbivorous and zooplankton-feeding species neither alimentary tract structure nor food processing mode were predicted by diet or SCFA profiles. A hindgut fermentation chamber was present in K. vaigi- ensis but not in N. unicornis, a species with high levels of SCFA in the hindgut region and a diet of brown mac- roscopic algae. In contrast N. vlamingii, with a diet dominated by animal matter, retained large amounts of food material in a hindgut chamber over the entire feeding cycle. In tropical perciform fishes, herbivory and fermentation are not associated with the alimentary tract structures that characterise herbivorous terrestrial ver- tebrates. Estimates of the abundance of the different groupings of nominally herbivorous fishes indicated that the dominant elements in the reef grazing and browsing fauna were consumers of detrital and sedimentary materials. These could not be classified as herbivores. Members of this group were dominant in all habitats investigated. Explicitly herbivorous taxa were a minority component in all habitats investigated. Introduction A prevailing assumption in reef-fish trophodynamics is that the abundant grazing fishes included in the families Acanthuridae, Scaridae and Kyphosidae are herbivores (Choat 1991; Horn 1992; Polunin 1996). This implies that the dominant dietary items are the ‘‘photosynthe- sizing parts of living attached multicellular plants’’ (Vermeij and Lindberg 2000). This assumption was evaluated (Choat et al. 2002) through an investigation of the diets of 17 species of grazing and browsing fishes from the above families. The study concluded that only 7 of the 17 species could be classed as herbivores. On the basis of this dietary analysis (Choat et al. 2002), four Communicated by G.F. Humphrey, Sydney J. H. Choat (&) Æ W. D. Robbins School of Marine Biology and Aquaculture, James Cook University, 4811 Townsville, Australia E-mail: john.choat@jcu.edu.au K. D. Clements School of Biological Sciences, University of Auckland, Private Bag 92019, Auckland, New Zealand Marine Biology (2004) 145: 445–454 DOI 10.1007/s00227-004-1341-7