Complementary assessment of fish biodiversity across the upper/lower mesophotic interface in a subtropical coral reef using eDNA metabarcoding and baited cameras

Mesophotic Coral Ecosystems (MCEs) occur in the middle to lower photic zone ([~]30-150 m) of tropical and subtropical regions, are often extensions of shallow reef communities, and generally hold great importance for local commercial fisheries. Compared to their shallower counterpart, MCEs have been traditionally understudied, primarily due to their inaccessibility with traditional monitoring methodologies. In this study, we aim to provide an interdisciplinary assessment of the biodiversity of Bermudan reef fish communities from the upper/lower mesophotic interface (60 m) by utilising a combination of environmental DNA (eDNA) metabarcoding and baited remote underwater videos (BRUVs). In total, 155 species from 137 genera were detected by eDNA metabarcoding whilst a total of 85 species from 53 genera were detected by BRUVs. The combined species detections totalled 182 species with approximately half of those detections unique to this study when compared to previous studies. Both methodologies found differences in -diversity between study locations, with each method independently detecting the highest species richness at the same location. The species assemblage at each location was dominated ([~]80%) by species known to occur throughout the shallow reef system and the upper mesophotic, whilst species only known to inhabit mesophotic ecosystems accounted for [~]6% at each location. These findings suggest a high level of species continuity with the adjacent shallower reef systems. The complementary nature of eDNA metabarcoding and BRUVs allows for a more accurate characterisation of fish biodiversity at the upper/lower mesophotic interface, which can lead to a more comprehensive understanding of ecosystem structure and more informed management decisions.