Among invertebrates, echinoderms are one of the main benthic taxa, especially in deep bottoms, where they constitute the most important group of mobile epifauna (Gage and Tyler, 1991). In addition, holothurians are one of the taxa dominating epibenthic communities in different North Atlantic basins (Billet, 1991), where they can form dense aggregations.
Despite the large literature on Atlantic echinoderms, the knowledge of this group in Northwest African waters primarily comes from the pioneering oceanographic expeditions carried out from late nineteenth century to mid twentieth century. However, the current knowledge of holothurians in Mauritanian waters is scarce, and only the works of Koehler and Vaney (1906), Hérouard (1929) and Massin (1993) were focused in this group and mainly on the fauna of the northern continental shelf.
This is a taxonomic and ecological study of the holothurians collected during the four multidisciplinary Spanish – Mauritanian surveys (Maurit) carried out from 2007 to 2010 on board R/V Vizconde de Eza in Mauritanian EEZ waters, between Cape Blanc (21ºN) and the Senegalese border (16ºN). A total of 291 stations were sampled between 80 and 2000 m depth, following a stratified random sampling methodology with and using a Lofoten commercial trawl (17.7 m horizontal opening, 5.5 m vertical opening and 35 mm mesh in the cod-end).
The holothurians collected in each trawl were sorted to morphospecies level, counted and weighed to obtain quantitative data on their abundance and biomass. Each specimen was photographed and a reference collection was preserved in 70% alcohol for further identification in the laboratory. The identification methodology included the study of the external and internal morphology by dissection and the microscopic study of the endoskeletal calcareous deposits.
Quantitative data (numerical abundance and biomass) collected during Maurit-0811, were standardized to 0.1 km2 and their respective matrices were calculated to enable an analysis of similarity between stations using the Bray Curtis index. Statistical treatment was performed using PRIMER software package v.6.
During the four cruises a total of 193,791 holothurians were collected in 128 of the 291 sampled stations, belonging to 15 species and 9 families. In terms of number of species, Synallactidae showed the highest specific richness (6 species), followed by Laetmogonidae (2 species). The others families were represented by only 1 species.
Enypniastes eximia Theel, 1882, the only representative of the family Pelagothuriidae, showed the greatest occurrence (52%) and was also the dominant species, both in numbers and biomass.
Paelopatides grisea Perrier, 1898 and Benthothuria funebris Perrier, 1898 were collected in about 40% of the stations and, due to their big size, they significantly contributed to the total biomass despite their low numerical abundance.
The multidimensional scaling (MDS) and the dendrogram resulting from the multivariate analysis applied to biomass matrix clearly separated stations located in the deep-shell and upper slope (<300 m) from those located at greater depths along the continental slope. The dendrogram evidenced five groups, two of them corresponding to shelf and upper slope stations (80–300 m) and the others including stations beyond 1000 m depth. Holothurians were almost absent between 300 and 1000 m in Mauritanian slope, which is in agreement with the results obtained by Billet (1991) in Porcupine Seabight (North Atlantic), where the abundance of holothurians was low in the break upper slope area (200 to 1000 m) but clearly increased beyond.
References
Billet, D.S.M. 1991. Oceanography and Marine Biology: An Annual Review, 29: 259-317.
Gage, J. D. & Tyler, P. A. 1991. Cambridge University Press, 504 pp.
Hérouard, E. 1902. Résultats des Campagnes Scientifiques Prince Albert I de Monaco, 21: 1-83.
Koehler, R. & Vaney, C. 1906. Act. Soc. Linn. Bordeaux, 60: 59-67.
Massin, C. 1993. Zoologische Mededelingen, 67:397-429.MAVA (contract 12/87 AO C4/2012
