Scotia Arc deep-water bivalves: composition, distribution and relationship to the Antarctic shelf fauna

Abstract

Agassiz trawl, epibenthic sledge and large box core samples were made in the Scotia and northern Weddell Seas during the 2002 ANDEEP expeditions with R.V. Polarstern at 21 locations (774–6348 m depth). The deep-water bivalve fauna sampled was species rich; in total 40 species of bivalves belonging to 17 families were found. At least seven of these species are new to science. Per location species richness varied from 5 to 15 species. Kelliella sirenkoi was the most common and abundant species, occurring at 17 locations (1121–6348 m), followed by Genaxius sp.1 occurring at 12 locations. The epibenthic sledge hauls were characterised by low abundances, varying from 1 to 68 individuals 1000 m⁻² in depths greater than 1000 m. In comparison with the shelf fauna of the Scotia Arc, the deep-water bivalve community showed similar species richness. This indicates that there is no diversity cline with depth in Antarctic bivalves, but it does provide evidence for underestimated species richness in deep water because of lack of sampling.

Introduction

Since the early days of Antarctic exploration bivalves were collected and studied by malacologists (Smith, 1885; Pelseneer, 1903). Most of the early sampling took place on the shelf and around the sub-Antarctic islands (e.g., Smith, 1885; Pelseneer, 1903; Lamy, 1906, Lamy, 1910), but deep-water samples were collected as well (Thiele, 1912; Thiele and Jäckel, 1931). In the late 20th century intensive investigations on the Antarctic benthos were carried out, especially in the Weddell Sea (R.V. Polarstern cruises EPOS and EASIZ, Hain 1990; Linse, 2003), in the Davis Sea (Egorova, 1982, Egorova, 1985), and the Ross Sea (Dell, 1964, Dell, 1990). Most of these studies focused on the shelf and slope fauna. Nowadays bivalves are one of the best-known taxa in Antarctica; they are well studied and relatively well sampled. The distribution of many species has been investigated and detailed aspects of ecophysiology have been researched in selected species (e.g., Davenport, 1988; Berkman, 1990; Peck and Bullough 1993; Brey and Mackensen, 1997; Ahn and Shim, 1998; Pörtner et al., 1999). The most recent diversity report (Clarke and Johnston, 2003) records 110 bivalve species in Antarctic waters; defining Antarctic waters as south of the Polar Front. They stated that bivalves are low in biodiversity in Antarctic waters compared to non-Antarctic regions. Only 5.5% of the globally known species live in an area that comprises of 9.6% of the world's ocean (Clarke, 2001). In comparison with other classes in the Antarctic, it is accounted to be the fifth most species-rich taxon (Dell, 1972). Typically, Antarctic bivalves are considered to be small in size (often smaller than 10 mm) and show eurybathy (Hain, 1990).

The intensity of deep-sea research has grown since the 1960s when first evidence emerged that species richness increased with water depth while faunal abundance decreased (Sanders et al., 1965; Hessler and Sanders, 1967; Sanders and Hessler, 1969). Knudsen (1970), examining bivalves collected in the South Atlantic with R.V. Galathea, suggested that Antarctic bivalves are related to those in the deep sea. Further studies showed that deep-sea diversity can be influenced by environmental factors such as organic carbon fluxes, current velocities and sediment type (Levin et al., 2001). The apparently homogeneous soft-sediment environment of the deep sea was interrupted by geologically complex regions such as hydrothermal vents, cold seeps, distinct trenches, mid-ocean ridges, and seamounts (e.g. Van Dover, 1990; Rex et al., 1997). The vent and seep habitats have particularly received much research focus: several studies have now described the bivalve communities of these systems, with respect to their ecology and phylogeny (e.g. Gebruk et al., 2000; Smith et al., 2000; Southward et al., 2001; Goffredi et al., 2003). Apart from the hydrothermal systems, comparatively little research had been done on deep-water bivalves. Most studies have focused on the systematics, morphology and reproduction of typical deep-water taxa such as Protobranchia, Kelliellidae, Thyasiridae and the mytilid Dacrydium (Payne and Allen, 1991; Tyler et al., 1992; Allen, 1998, Allen, 2001) or described single species found in deep-sea samples (Egorova, 1998, Egorova, 1999; Allen, 2000). Until now just a handful of diversity studies which included Mollusca, or more specifically bivalves on non-vent deep-sea fauna have been published. Two studies are particularly notable. Rex et al. (1993) reported a latitudinal decrease in species richness for bivalves from the equator towards the poles in the Atlantic. Cosson et al. (1997), studying the macrofaunal community structure and spatial patterns at three deep-sea stations in the tropical northeast Atlantic Ocean, found them to be dominated by the Polychaeta, Tanaidacea, Isopoda and Bivalvia.

Our knowledge on Antarctic deep-sea bivalves in regards to community structure, density and possible latitudinal gradients in diversity is still patchy, for example there are areas (such as the Bellingshausen Sea) where little is known. The expeditions ANDEEP I & II targeted the deep slope and abyssal depth of the Scotia Sea and north western Weddell Sea (Fütterer et al., 2003). This paper presents the results on the ANDEEP bivalves and sets them into the context of our knowledge on Antarctic bivalves.