Diel changes in structure and trophic functions of motile benthic invertebrates on coral reefs at Rapa Nui (Easter Island)

Coral reefs are structurally complex habitats that provide diverse shelter for motile benthic invertebrates (MBI), including those with cryptic and nocturnal behaviors. In this study, diel changes in MBI structure (species composition, density, and biomass) and functions (trophic groups and isotopic diversity indices) were examined on coral reefs at Rapa Nui (27 degrees S, 109 degrees W). Diurnal and nocturnal surveys (March 2016) revealed that echinoderms were the dominant invertebrates throughout the diel cycle in terms of density, biomass, and their trophic role. The density and biomass of MBI were higher at night than during the day. Significant diel changes were detected in the structure of MBI and trophic groups, where the diurnal assemblages were mostly herbivores and the nocturnal assemblages were mostly detritivores. Moreover, the isotopic uniqueness index indicated that nocturnal MBI were trophically more redundant than their diurnal counterparts; consequently, the high nocturnal biodiversity did not necessarily increase trophic diversity. This time and trophic partitioning of MBI could have important consequences for energy transfer on the reefs and must be considered in future monitoring programs. Endemic taxa represented approximately a quarter of the species (26%), but had low density and biomass during both time periods, suggesting a modest influence on energy transfers on these coral reefs. However, their restricted geographic distribution and current low abundances could make these species vulnerable to extinction; therefore, management and conservation actions in the new multiple-use marine protected area of Rapa Nui should include these species.