Biodiversity and assemblages of molluscs (Gastropoda and Bivalvia) from coastal and deep-sea soft-bottoms off Mauritania

Abstract

The collections as well as quantitative faunistic and environmental data obtained from 384 bottom-trawling stations during the four Spanish Maurit and two FAO–CCLME expeditions have allowed us, for the first time, to establish the biodiversity distribution patterns and assemblage structures of Mauritanian molluscs. The six surveys prospected the soft-bottoms of the shelf and continental slope between Cape Blanc (20°50′N) and the Senegalese border (16°04′N), from a depth of 19 to 1860 m. Almost 13,000 live specimens collected from 150 stations (40% of the total) belonged to 104 species (37 bivalves and 67 gastropods). Nassariidae and Xenophoridae were the most frequently found families, and the gastropod Nassarius wolffi (Knudsen, 1956) was the most frequent mollusc species in the Mauritanian soft-bottoms. Naticidae and Marginellidae, followed by Muricidae and Pectinidae, were the most diverse families.Depth and depth-related variables, besides the carbonate content of the sediments, are the major environmental factors responsible for the distribution patterns of the ecological descriptors as well as separation of the three main assemblages identified in the shelf, slope and deep-slope. Besides their dissimilarities (β-diversity), the three faunistic assemblages showed significant quantitative differences linked to similar and marked bathymetric trends of diversity, abundance and biomass of both bivalves and gastropods, which strongly decline from the shelf along the slope. Although the mean value of species richness seems uniform along the coast, abundances and biomass are the lowest in the southernmost zone (17°N–16°N), with the highest values, particularly for gastropods, in the Banc d’Arguin region.