Quantifying surgeonfish functions in coral reef sediment dynamics: an ecological and evolutionary perspective

Fish-driven ecosystem functions directly contribute to sediment dynamics on coral reefs, shaping how reefs operate in the present-day and potentially through evolutionary time. However, our understanding of these biologically-mediated sediment processes and their underlying evolutionary history remains poorly resolved for surgeonfishes (Acanthuridae), despite their known role in reef sediment dynamics. To fill this knowledge gap, we combined detailed field sampling and phylogenetic comparative methods to quantify the functional role of surgeonfishes in the past and present. Phylogenetic generalised linear mixed models revealed a positive association between body size and the mass of sediment ingested across all surgeonfish functional groups. At an assemblage-level and individual body-mass-standardised scale, brushers and sediment suckers – united by their possession of gizzard-like stomachs – ingested the most sediment relative to other functional groups. However, the larger overall body size of sediment-sucking surgeonfishes means that they ingest the highest sediment masses, individually and relative to all other functional groups across reef habitats at Lizard Island. Surprisingly, grain-size composition did not differ substantially between the gizzard-like stomachs and the intestines of brushers and sediment suckers, suggesting that grain-size modification is not a major function performed by surgeonfishes. Finally, we detected a shift towards greater sediment ingestion in the Miocene (~15–17 million years ago) that temporally aligns with the evolution of gizzard-like stomachs in detritivorous surgeonfishes (i.e. sediment suckers and brushers) and the early formation of modern reefs. These findings showcase the extent to which surgeonfishes may have been, and continue to be, key contributors to reef sediment dynamics.