Herbivorous Fish Interspecifically Partition Diets on a Caribbean Coral Reef

Background/Question/Methods

: Herbivory is a key ecological process in shaping the benthic community of a coral reef. A robust population and high biomass of roving herbivores – large-bodied fish that diurnally forage on macrophytes as part of their diet – promote coral dominance by suppressing macroalgae overgrowth. However, the functional role each species fulfills within this broader ecosystem service is unclear. The functional role of roving herbivores is intrinsically linked to their diet. Yet, due to methodological limitations, their diets are poorly classified. Consequently, the interspecific dietary differences between roving herbivores that could lead to functional complementation are largely unknown. We used DNA metabarcoding and stable isotope analyses to elucidate dietary composition, dietary partitioning, and changes in diet across a reefscape. Fish were collected from four sites along the leeward forereef of Lighthouse Reef Atoll, Belize. At one site, we collected adult individuals of every species in the genus Acanthurus and initial phase individuals of every species in the genera Scarus and Sparisoma found within that site. At all four sites, we collected the three most common of the species across sites. We sampled muscle tissue for C and N stable isotope analyses and gastrointestinal contents for metabarcoding.

Results/Conclusions

: Our results show that roving herbivores interspecifically partition diets and that the fishes’ diets stay consistent across sites. Each species had a distinct %δ13C and %δ15N isotopic profile and dietary composition; only two species showed significant overlap in what they eat. Species from the genus Acanthurus had a higher percentage of δ15N in their muscle tissue and consumed food types more similar to congeners than species in Scarus or Sparisoma. Some of the species’ diets were broader than others, but we found no true generalists. Lastly, each species maintained similar diets across widely separated sites, suggesting that diets are more constrained by interspecific differences than by the environment. When interpreting these results in the context of ecosystem function, the data shows that each species removes a different subset of macroalgae taxa from the reef, suggesting that different communities of herbivorous fish could have different and complementary effects on shaping the benthic community. Furthermore, these findings provide evidence that, in addition to biomass, herbivore diversity is important for coral reef ecosystem resilience