Increasing body size unlocks gelatinous planktivory in a key coral reef fish species

Abstract Coral reefs depend on the assimilation of off-reef nutrients, particularly from zooplankton, to fuel on-reef productivity. Planktivorous reef fishes thus play a critical role in sustaining a key ecosystem function as the major consumers of plankton. The importance of gelatinous zooplankton to the nutritional ecology of larger-bodied planktivorous fishes is becoming increasingly apparent. Yet, the extent to which feeding on gelatinous plankton within these taxa is a product of ontogeny has not yet been addressed. It has been hypothesised that Caesio cuning , a key planktivorous species that contributes substantially to reef fish biomass on Australia’s Great Barrier Reef, undergoes an ontogenetic diet shift from predominantly non-gelatinous prey as juveniles to a diet composed mainly of gelatinous zooplankton as adults. We examine high-taxonomic resolution DNA metabarcoding of gut contents, with morphometric analyses, to investigate whether changing body size and habitat are linked with a purported dietary transition in C. cuning . We reveal clear evidence of a shift towards increased feeding on gelatinous plankton with increasing body size in C. cuning . Across the sampled habitats, small- and large-sized individuals occupied distinct dietary spaces. The increase in size and ontogenetic dietary shift in C. cuning was also correlated with a transition towards morphological traits associated with feeding and movement in the water column. These results emphasise the importance of gelatinous zooplankton in shaping the distribution, feeding ecology, and productivity of C. cuning on coral reefs and underscore the close connection between large body size and exploitation of gelatinous plankton by reef fishes.