Composition and structure of the molluscan assemblage associated with a Cymodocea nodosa bed in south-eastern Spain: seasonal and diel variation

The molluscan taxocoenosis associated with a Cymodocea nodosa seagrass bed was studied throughout 1 year in Genoveses Bay, in the MPA Parque Natural Cabo de Gata-Nijar (south-eastern Spain). A total of 64,824 individuals were collected and 54 species identified. The molluscan fauna was mainly composed of gastropods (99.56% of individuals, 43 spp.). The families Rissoidae (72.98%, 11 spp.) and Trochidae (16.93%, 7 spp.) were the most abundant and diversified in terms of number of species. Rissoa monodonta (47.1% dominance), Rissoa membranacea (25.1%) and Gibbula leucophaea (11.6%) proved the top dominant species in both diurnal and nocturnal samples. Bivalves (0.41%, 10 species) and cephalopods (0.03%, 1 species) represented only a low percentage of the molluscan taxocoenosis. The molluscan assemblage was mainly composed of species with a wide geographical distribution in Europe, followed by strictly Mediterranean species. The abundance was significantly higher in the cold (December, March) than in the warm months (June, July). Species richness (S) was higher in nocturnal than in diurnal samples, reaching maximal values in diurnal samples of March and June. Shannon-Wiener diversity (H') values were generally higher in nocturnal samples than in diurnal ones, displaying minimum values in December and June, respectively. Evenness was similar in diurnal and nocturnal samples, with maximum values in July in both groups. S and H' were also significantly different between diurnal and nocturnal samples. Multivariate analyses based on both qualitative and quantitative data showed a significant seasonal and diel variation. Diel changes revealed to be more distinct than seasonal ones.