The structure of the molluscan assemblages of sea-grass beds in the Maltese Islands

Abstract

Macrobenthic molluscs associated with meadows of the sea-grasses Posidonia oceanica and Cymodocea nodosa were sampled between November 1993 and November 1995 at about bimonthly intervals. Samples were collected from the Posidonia meadow at four depths (6m, 11m, 16m and 21m) using a suction sampler and a hand-towed net. Samples were collected from the Cymodoce.&meadow at two depths (4m and 8m) using a suction sampler. Phenology of both sea-grasses was studied, and sediment granulometry and other sediment parameters were measured. Species composition, abundance, and biomass of Mollusca were determined for each sample, and community parameters (species richness, diversity, evenness, and dominance) were estimated. The molluscan assemblages were analysed using classification and ordination, analysis of similarities (ANOSIM), and similarity percentages (SIMPER). The structure of the P. oceanica and C. nodosa meadows appeared to be influenced by water depth, resulting in distinct types of meadows at different depths. Shoot density of Posidonia decreased steadily with depth. The number of leaves per shoot varied more with season than with depth. Leaf length and width changed significantly with season but not with depth. The leaf area index (LAI) and leaf standing crop (LSC) showed significant seasonal changes; higher values were recorded in the warmer than in the cooler months. Only LAI showed significant variation with depth. For Cymodocea, shoot density varied significantly with depth, but not with season; the LAI and LSC were higher at the shallow (4m) station than at the deeper (8m) one. Seasonal changes in LSC were significant between but not within stations. Sediment granulometry of the substratum of both Posidonia and Cymodocea meadows was very similar at all depths, and the near absence of mud and of very fine sand indicates that the areas appear to be under the influence of considerable water movement, and little influenced by the presence of the sea-grasses. The organic carbon content of the sediment was not significantly different between stations for either meadow. The Mollusca of the Posidonia meadow were differentiated into foliar, and rhizome layer/matte assemblages. Altogether, five polyplacophorans, 99 gastropods, and 28 bivalve species, represented by 7559 individuals, were collected. Of this, about 65% of the total individuals were inhabitants of the rhizome layer/matte. Only 16 species of molluscs were widespread and abundant, most of them gastropods of the leaf stratum. Molluscan species richness, abundance, and distribution along the depth gradient were influenced mainly by the structure of Posidonia meadows and by water movement. The biomass of Mollusca (totalling 22878.3mg AFDW) was essentially contributed by gastropods (72-99.8%). Significant changes in biomass generally occurred with season and with depth. The use of multivariate analyses delimited the molluscan taxocene into: (1) a shallow water (6m and 1 lm) assemblage; and (2) a deep water (16m and 21m) one. A comparison between the two sampling methods used showed that the suction sampler collected twice the number of species and half the number of individuals of the handtowed net. The suction sampler is probably a better method for qualitative and quantitative sampling of the molluscan assemblages associated with Posidoma meadows. [...]