Black coral patches and reef fish diversity: An integrative multidisciplinary approach.

The black coral Antipathes galapagensis is a habitat engineer that may play critical roles for marine biodiversity but is still largely underexplored because typically found in the mesophotic zone. However, A. galapagensis occasionally colonizes shallow waters providing an excellent opportunity to assess its ecological role. Here, we compared the reef fish assemblages between rocky reefs with and without black coral in the Bay of La Paz (Mexico). To achieve a comprehensive understanding of the diversity in each habitat, we adopted an integrative approach that combined different methodologies tailored for specific groups, i.e., Underwater Visual Census for conspicuous, Passive Acoustic Monitoring for nocturnal soniferous, and environmental DNA for cryptobenthic species. Black coral habitats were found in isolated patches rather than extensive forests and demersal fish assemblages were largely similar between the habitats, except for Lutjanidae, with higher densities in the black coral habitat. However, small benthic fish species (5-20 cm) showed higher species richness, density, and distinct compositions in black coral. Historical fish surveys in reefs of the region confirmed these findings. Acoustic analyses revealed differences between the sites, although no relationship with black coral was detected. Although environmental DNA supported the role of black coral for Lutjanidae, it failed to capture the diversity of cryptobenthic fish, pointing to methodological challenges discussed in our study. Overall, our findings underscore the ecological significance of black coral for reef fishes, even in isolated patches, and highlight the need for further extensive studies on this underexplored and imperiled habitat.