Associated mollusc communities of a Posidonia oceanica meadow in Cap Zebib (off North East Tunisia)

Abstract

Mollusc communities associated with the seagrass Posidonia oceanica were investigated in Cap Zebib (North-Eastern Tunisia). Samples were collected monthly from May 2007 to May 2008, in two stations located at 3 and 12m depths using a corer of 25cm diameter. A total of 753 individuals belonging to 47 species were identified. The species richness (S) and Shannon–Wiener diversity index (H′) showed significant variation in relation with depth and sampling period, with higher values at 3m depth. The community structure at 3m depth varied considerably from one month to another, while 12m depth did not show any significant temporal variability. The results of similarity percentage (SIMPER) and Factorial Correspondence Analysis (FCA) showed that the species Clanculus cruciatus, Cantharus dorbignyi, Columbella rustica and Cardita calyculata contributed most to the distribution pattern of the mollusc community at 3m depth. In contrast, at 12m depth, the highest contributions were given by the species Jujubinus exasperatus, Smaragdia viridis and Tricolia pullus. Taking into consideration a possible relationship between the faunal composition and plant (P. oceanica) features, a negative correlation was found between the number of mollusc species, Shannon–Wiener diversity and water depth. In contrast, a positive correlation was noted between the number of species and meadow shoot density, with high values in shallow area and more dense stands. For the abiotic parameters, a positive correlation was observed between Shannon–Wiener diversity and the dissolved oxygen. The Leaf Area Index was also positively correlated with temperature and dissolved oxygen.