Shallow- and deep-water mollusc distribution at Terra Nova Bay (Ross Sea, Antarctica)

The aim of this work is to improve the knowledge of the mollusc fauna of Te rra Nova Bay (Ross Sea), on the basis of more than 100 stations sampled fro m 25 to 1100 mm depth, during Italian Antarctic Expeditions (austral summer s 1987/1988, 1989/ 1990, 1993/1994, 1994/1995, 1995/1996 and 1997/1998). In shallow waters, gastropod fauna is represented by 31 species, among which Neobuccinum eatoni, Onoba gelida, Powellisetia deserta, Philine cf. apertis sima and Austrodoris kerguelenensis are the most abundant, while among biva lves (25 species) the commonest species are Adamussium colbecki, Yoldia eig htsi, Laternula elliptica and "Montacuta" nimrodiana. At present, most of t he Antarctic mollusc species are known as having circumantarctic distributi on and are considered eurybathic. Moreover, in general, only scarce data ar e available on their substrate preferences, because of their occurrence in scattered sampling static,ns with different sediment features. In the prese nt study carried out at Terra Nova Bay, based on a relatively high number o f stations, a clear zonation of mollusc assemblages is shown, according to depth and sediment features. While gastropods characterise the superficial algal belts dominated by the red algae Iridaea cordata and Phyllophora anta rctica, bivalve distribution is wider. Adamussium colbecki is the dominant species in the upper 100 m, both on soft and hard bottoms, if the slope is suitable (density up to 40-60 ind./m(2)). On coarse sands, this species is frequently accompanied by L. elliptica (density <20 ind./m(2)), while Y. ei ghtsi characterises organic-enriched bottoms (density 70-80 ind./m(2)). Bel ow 200 m depth Adacnarca nitens, Limatula hodgsoni, "Montacuta" nimrodiana and Mysella gibbosa are commonly found on relatively coarse sediments, whil e Thyasira dearborni and Yoldiella ecaudata prefer muddy bottoms. In order to identify bivalve assemblages, multivariate analyses were applied to stat ions grouped into depth ranges, sediment features and location. While shall ow bathymetric ranges (25-200 m) are clearly defined, deeper clusters of st ations are more influenced by the high degree of eurybathy of most of the s pecies and by the scattered distribution of the sediment particle sizes.