Deep reef fishes in the world’s epicenter of marine biodiversity

The Philippines is often highlighted as the global epicenter of marine biodiversity, yet surveys of reef-associated fishes in this region rarely extend beyond shallow habitats. Here, we improve the understanding of fish species diversity and distribution patterns in the Philippines by analyzing data from mesophotic coral ecosystems (MCEs; 30–150 m depth) obtained via mixed-gas rebreather diving and baited remote underwater video surveys. A total of 277 fish species from 50 families was documented, which includes thirteen newly discovered and undescribed species. There were 27 new records for the Philippines and 110 depth range extensions, indicating that many reef fishes have a broader geographic distribution and greater depth limits than previously reported. High taxonomic beta-diversity, mainly associated with family and genus turnover with depth, and significant effects of traits such as species body size, mobility and geographic range with maximum recorded depth, were observed. These results suggest that MCEs are characterized by unique assemblages with distinct ecological and biogeographic traits. A high proportion (60.5%) of the fish species are targeted by fishing, suggesting that Philippine MCEs are as vulnerable to overfishing as shallow reefs. Our findings support calls to expand conservation efforts beyond shallow reefs and draw attention to the need to explicitly include deep reefs in marine protected areas to help preserve the unique biodiversity of MCEs in the Philippines.