A molluscan community from coastal bioclastic bottoms in the Strait of Gibraltar area

The molluscan community of a soft bottom of bioclastic gravels and sand has been studied monthly over two years in four stations of the bay of Barbate, SW Spain at the Atlantic entrance of the Strait of Gibraltar, between 18 and 29 m depth. Species richness, dominance and frequency over the two years are recorded. Micromolluscs, less than 1 mm in size, were also recorded from the sample of April 1994. Over the two years, 203 species of macromolluscs were found, of which 174 species (115.104 individuals) were collected alive and quantified. A total of 25 species of micromolluscs were determined, some of which usually considered as rare species. Most species richness values range between 20 to 40 species per sample, which is comparable with, or higher than, the values reported in other studies from soft bottom communities of molluscs. The qualitative analysis based on Jaccard’s and Baroni-Urbani and Buser’s indices show that three stations are significantly similar (p < 0.01), whereas the lowest similarity was found between the station close to the harbour of Barbate and the other ones. The quantitative analysis based on the Bray and Curtis index is biased by the high number of individuals of Chamelea striatula but consistent with the qualitative indices when this species is not taken into account. The main community is similar to “Biocénose des fonds meubles instables (MI)” and “Biocénose des fonds detritique du large (DL)” of PÉRÈS AND PICARD (1964), but occurs shallower than in the Mediterranean. The concurrence of species from the Lusitanian, Mediterranean and Mauritanian regions results in an increment of the total number of species