Ocean-scale patterns of 'biodiversity' of Atlantic asteroids determined from taxonomic distinctness and other measures

We examine patterns of 'gamma' (within-region) and 'beta' (between-region) diversity from analysis of a presence/absence dataset for species of astero ids encompassing the whole Atlantic Ocean partitioned into 26 regions. Abso lute species numbers (a poor measure of biodiversity) in shallow coastal ar eas and the deep sea are the same, although species richness per area for t wo well-sampled regions suggests, qualitatively, that coastal areas ma): be more speciose. Taxonomic distinctness (Delta*), an index which is markedly less sample-size dependent than other common diversity measures, shows no significant association with geographic area and no clear pattern with dept h? suggesting an absence of latitudinal and coastal/deep trends. Cluster an alysis shows that distinctive faunal assemblages are most evident in shelf/ shallow waters, where six groupings separate recognizably acccording to ge ographical location. Three of these are the southernmost regions of the Atl antic (southeast of S. America, S. Angola/S. Africa & Tristan da Cunha/Goug h Island) and are characterized by their isolation and high levels of endem ism. As depth increases so does the amount of faunal similarity between reg ions. This indicates that beta diversity is highest in shelf regions and lo west in low er bathyal/abyssal regions. Our results may support the content ion which questions the emerging paradigm that the deep sea has exceptional ly high diversity. It is evident, however, that comparisons (e.g. between c oasts and the deep sea) are problematic and can depend very much on the ele ment(s) of biodiversity measured, sampling methods and the spatial scales ( e.g. alpha, beta or gamma diversity) over which assessment is made. Any wid er conclusions should therefore be drawn cautiously, particularly since ass essment is made of only one faunal group. Other findings include significan t correlation between the depth range of asteroids and their geographical r ange. The utility of low-resolution datasets is reviewed. It is concluded t hat within limitations they can be of value for determining broad (e.g. reg ional, ocean-scale and global-scale) patterns of diversity and community st ructure, especially when analysed using measures relatively uninfluenced by sample size. (C) 1999 The Linnean Society of London.