Muir and Pichon 2019

scientific chapter |

Biodiversity of Reef-Building, Scleractinian Corals

Muir PR, Pichon M


Zooxanthellate scleractinian corals are moderately well-known for shallow reef habitats, but not for mesophotic depths (>30 m) that are relatively difficult to access. Mesophotic habitats are light-limited, with different hydrodynamics and sedimentation processes, which result in growth forms that are often difficult to classify using traditional schemes based largely on shallow reef specimens. We analyzed published data and museum records, using specimen-based records to minimize classification issues, finding 53 mesophotic species in the western Atlantic Ocean (85% of total species) and 338 in the Indo-Pacific (45%). Only four species were recorded exclusively below 30 m depth, while the great majority were common shallow reef taxa. Over 96% of western Atlantic and 82% of Indo-Pacific genera and most coral lineages were represented below 30 m depth. In the Indo-Pacific, species and genus richness varied widely between regions and were significantly correlated with shallow reef species richness. Overall, species richness decreased steadily with increasing depth, with little evidence for distinct faunal boundaries: 157 species occurred ≥60 m and 31 deeper than 100 m, with species occurrence only moderately related to phylogeny. Our knowledge of mesophotic biodiversity is rapidly changing as more regions are documented and new molecular techniques suggest taxonomic revisions and resolve deepwater cryptic species. We conclude that mesophotic scleractinian fauna are largely a subset of shallow scleractinian fauna, comprising a significant proportion of coral species and most genera, with the potential to play a significant role in lineage preservation and the future of coral reefs.