Seasonality and trophic diversity in molluscan assemblages from the Bay of Tunis (southern Mediterranean Sea)

Abstract

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.

Molluscs from Tidal Channels of the Gulf of Gabès (Tunisia): Quantitative Data and Comparison with Other Lagoons and Coastal Waters of the Mediterranean Sea

“…The dominant species found in our study are characteristic of areas of debris accumulation, associated with the presence of Zostera, Halophila, Cymodocea and Posidonia seagrass meadows. The molluscan fauna studied here is very similar to that of the shallow seagrass beds of Tunis Bay described by [16]. These authors reported a dominance of Varicorbula gibba, accompanied by most of the predominant species found in the current tidal streams of the Gulf of Gabès; in Tunis Bay, the temporal pattern of abundance is almost identical to that observed in the Gulf of Gabès, with maximum abundances in winter (January) becoming minimal in autumn (November).…”

“…Despite recent studies mainly in Tunisia waters [3,5,11,14,16,17] (see Figure 8), there is still relatively little published research on molluscs from the coastal waters of southern Tunisia, including the Gulf of Gabès [6,7,[28][29][30][31][32][33][34][35][36][37].…”

“…Therefore, studies of molluscan assemblages can be considered as representative for understanding the ecological status and dynamics of benthic communities in coastal Mediterranean areas [13]. Many ecological studies have been carried out on molluscan faunas along the Tunisian coastline [5,8,[14][15][16][17]. Other ecological studies have mentioned mollusc species in the context of bionomic studies [6,7,14,.…”

Assessing molluscs functional diversity within different coastal habitats of marine protected areas

“…The functional (trophic) diversity of the molluscan assemblages at each habitat was analysed through the Index of Trophic Diversity (ITD) (Help et al, 1998), which ranges from 0 to 1 (high and low trophic diversity, respectively), and is calculated according to the following equation: individuals. According to the modified version applied to molluscs (Antit et al, 2016), ITD was calculated as 1-ITD to have the highest trophic diversity with the greatest weight. Differences in the structure of mollusc assemblages both in Habitats and Sites (two fixed and orthogonal factors) were evaluated through a permutational analysis of variance (PERMANOVA, Anderson, 2001a).…”

Species diversity, trophic structure, and taxonomic distinctness of molluscan assemblages associated with Sargassum beds in a historically impacted bay

“…Trophic diversity (TrD) was calculated for each sample following Antit et al (2016): trophic categories were used instead of species for calculating the Shannon‐Wiener diversity index (Data S1), which is similar to the more intuitive inverse form of the Index of Trophic Diversity (ITD, Heip et al, 1998). Six trophic categories were used for estimating trophic diversity, based on Rueda et al (2009): (1) carnivores/scavengers, (2) herbivores; (3) micrograzers; (4) ectoparasites; (5) suspension feeders; and (6) omnivores.…”