Cold-water coral reefs and mounds off Mauritania and associated fauna

The objective of this study is to describe the associated fauna of the two cold water coral (CWC) ecosystems occurring off Mauritania; the chain of mounds and the live reefs.

The fauna inhabiting the coral mound has been described through a comparative analysis of trawl catches from coral mounds (400-600m) and catches from adjacent off-mound areas (200-350m) conducted from 1982 to 2022 by four research vessels, while the fauna associated with CWC reef habitats was described through the analysis of 11 video dives undertaken by using ROV as part of two seafloor habitat mapping surveys conducted by the R/V Dr. Fridtjof Nansen off Mauritania in 2020 and 2021.

Analysis of 47 trawling station data carried out on coral mounds off Mauritania revealed significant diversity. Overall, 282 taxa were identified, these species belong to the 5 taxonomic group, Fish (187), Crustacea (56), Mollusca (31), Cnidaria (5) and Echinodermata (3). The most ten species inhabiting coral mound was: Helicolenus dactylopterus, Laemonema laureysi, Merluccius polli, Coelorinchus caelorhincus, Hoplostethus cadenati, Malacocephalus occidentalis, Gephyroberyx darwinii, Hoplostethus mediterraneus, Galeus polli and Malacocephalus laevis. Results showed also that the values ​​of the diversity indices (richness, Shannon and Simpson index, accumulation curve) were higher on-mound than off-mound.

The analysis of 11 ROV dives allowed to identify eighth species of CWC to species level, the most abundant was the gorgonian coral Acanthogorgia cf. hirsuta followed by Desmophyllum pertusum, Swiftia phaeton (a new octocoral species of Swiftia), the sea fan Thesea talismani, Madrepora oculata, the black coral Tanacetipathes cf. spinescens, Anthomastus cf. grandifloras and Clavularia borealis.

The results showed that the reef offers habitat for many species. In effect, 120 taxa belonging to 68 families have been identified divided into 11 groups, the group of fish was the most represented with 47 taxa, followed by the group of cnidaria (18 taxa), crustacea (17 taxa), Mollusca and Echinodermata with 11 taxa for each group and Porifera with 7 taxa. In terms of occurrence, H. dactylopterus was the most observed for the fish group, with over 371 individuals. The cnidarians are dominated by the lesser cylinder anemone Synarachnactis cf. lloydii with more than 13000 individuals. For the crustaceans, the African spider shrimp Nematocarcinus africanus is the most common with more than 900 individuals. As far as the group of mollusks is concerned, the bivalve Gigantidas mauritanicus is the most observed with more than 1800 individuals. The sponge Cladorhiza corallophila was common for the group of Porifera. Several of these species are known to be associated with CWC from other studies even though the reefs are in an oxygen minimum zone they offer habitats for many of the same associates.

The results of our study provide insight to the importance of these CWC habitats to the Mauritanian deep-sea ecosystem, information that is essential for the development of a national management plan for use of marine resources and their ecosystems, especially in the face of ongoing environmental challenges (e.g. oil and gas activities, deep sea fishing etc.).