Size structuring has been demonstrated consistently for temperate marine and freshwater food webs (Cohen et al. In these systems, an individual's size may serve as a better indicator of its role in the ecosystem than its taxonomic identity (Jennings et al. Size-structured food webs reflect size-based feeding by individuals (Cohen et al. Although gape allometry was not indicative of size structuring, it may have implications for ecosystem function: positively allometric gape size–body size scaling relationships in herbivores/detritivores suggests that loss of large-bodied individuals of these species will have a disproportionately negative impact on reef grazing pressure.īody size is a fundamentally important biological trait, influencing metabolic rates, energy flows and population abundances in organisms across taxa (Peters 1983 Werner & Gilliam 1984 Brown et al. Our results provide evidence that coral reef piscivores are size-structured and that gape limitation and ontogenetic niche shifts are the mechanisms from which this structure arises. Instead, relative gape sizes and mouth morphologies may be better indicators. Across species and functional groups, absolute and relative gape sizes were largest in piscivores as expected, but gape size–body size scaling relationships were not indicative of size structuring. There was, however, little evidence of size structuring in benthic invertivores. We found evidence of both mechanisms for size structuring in coral reef piscivores, with maximum prey size scaling positively with predator body size, and ontogenetic diet shifts including prey type and expansion of prey size. We then estimated gape size–body size allometric scaling coefficients for 21 different species from four functional groups, including herbivores/detritivores, which are not expected to be gape-limited. We used gut content analysis and quantile regressions of predator size–prey size relationships to test for evidence of gape limitation and ontogenetic niche shifts in reef piscivores ( n = 13 species) and benthic invertivores ( n = 3 species). Given the diversity of feeding modes amongst coral reef fishes, we also compared gape size–body size allometric relationships across functional groups to determine whether they are reliable indicators of size structuring. Here, we aimed to advance understanding of size structuring in coral reef food webs by examining the evidence for these mechanisms in two groups of reef predators. Little is known, however, about size structuring in coral reef fishes. Gape limitation and ontogenetic diet shifts are key mechanisms by which size structuring arises in these communities. Temperate marine fish communities are often size-structured, with predators consuming increasingly larger prey and feeding at higher trophic levels as they grow.

Fish cplot size surgeonfish