COMPOSITION AND DIVERSITY OF CRUSTACEA ISOPODA OF THE SOUTHEASTERN AUSTRALIAN CONTINENTAL-SLOPE

The continental slope of southeastern Australia is a complex hydrological region where sediment is biogenic and largely derived from the nearby shelf. The isopod fauna of the slope between 200 and 3150 m depth has been investigated on four transects (32-degrees-S-42-degrees-S). Forty-six non-quantitative epibenthic sled, beam-trawl, corer and trawl samples captured 6870 isopod individuals. A total of 359 species belonging to 36 families is recognised, 10% of which can be identified to known species, a much smaller fraction than in previous comparable surveys; 69% are asellotes. Species estimates made using the rarefaction method suggest that the number of species on the southeastern Australian slope is greater than that found on several better-studied slopes in the Atlantic, Arctic or Antarctic. This seems to agree with the observation that species diversity in mid-latitudes in the Southern Hemisphere is higher than at similar latitudes in the Northern Hemisphere. The implication of this observation is that estimates of the number of species in the deep sea by Grassle and Maciolek may be too low. The number of species decreased less rapidly with depth at the more southern transects (off Bass Strait and Tasmania) than at the northern transect (off Sydney). Most species were very rare: only 32 occurred in five or more of the 46 samples; half occurred in only one sample. Classification and ordination of sites discriminated between shallow samples (200-520 m) and deeper samples (>400 m) with good characterisation of sample groups by species groups. Discrimination of transects was weak and gave little support for biogeographic regionalisation of the slope fauna.