The molluscan assemblage inhabiting photophilous algae from artificial rocky shores in the Bay of Tunis) was studied from February 2009 to February 2010. Five monthly replicates were taken with a quadrate of 25 × 25 cm. A total of 16105 individuals of molluscs, belonging to 93 species, were collected. Gastropods were the most species-rich group, with 66 species (70.96%), followed by bivalves with 26 species (27.95%). Gastropods were also the most abundant with 11529 individuals (71.60%), followed by bivalves with 4575 individuals (28.41%). Nevertheless, Mytilaster minimus was the dominant species overall during the studied period (15.6% of total abundance), followed by Tricolia miniata (11.6%), Bittium reticulatum (11.5%), Gibbula racketti (10.4%) and Rissoa similis (9.8%). The high abundance of juveniles of soft (e.g. Venerupis aurea, Nassarius corniculum) and hard bottom species (e.g. M. minimus, Fissurella nubecula) highlight the important role played by this habitat as a nursery for species from nearby habitats. The presence of the alien molluscs Mitrella psilla (2.5% dominance) and Arcuatula senhousia stresses the importance of algae as a vehicle and/or receptor habitat for alien species. Species richness and abundance displayed significant seasonal changes, with maxima in winter 2009, but the Shannon-Wiener diversity index showed similar values throughout the year. Multivariate analyses also indicated the presence of significant seasonal differences for this molluscan assemblage.
Soft bottom molluscan assemblages from the Bay of Tunis have been studied in order to analyse their seasonality and trophic diversity in relation to environmental variables. A total of 147 species of molluscs was identified, with gastropods displaying the highest species richness and bivalves the highest abundances, and including five non-indigenous species such as the dominant ectoparasite Polycerella emertoni. Carnivorous and scavenger gastropods were among the most frequent species, reflecting a diverse benthic community. Seasonal changes were significant, being more acute at 3-4 m than at 10-15 m depth, and were driven mainly by seawater temperature and percentage of organic matter in the sediment. The high affluence of tourists in summer was coincident with high decreases in species richness and abundance of molluscs, together with a strong siltation of the sediment. Nevertheless, most trophic groups remained and the trophic diversity was relatively high. Significant relationships were found between the index of trophic diversity and Shannon-Wiener diversity and evenness indices, suggesting that the identity of the species with its particular trophic trait, together with the good distribution of the individuals among the species (density) would be the drivers for the maintenance of the molluscan food web under environmental stress. The abundance of P. emertoni altered the trophic structure of the molluscan assemblage, with the ectoparasite trophic group reaching an unusual punctual higher dominance. Soft bottom molluscan assemblages of the Bay of Tunis should be taken into account in monitoring programs for anthropogenic impacts and non-indigenous species trends throughout the Mediterranean basin.
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