How many species are there within the genus Acipenser?

In their paper in this volume Bemis et al. (1997) ask: 'How many valid species of Acipenser should we recognize?' Although a partial answer to this question is presented in their Table 5, we discovered in the course of preparing this volume that some additional commentary is needed. In fact, there are two questions: (1) how many species should be recognized? and (2) what scientific names should be used for some of the species? The sympatric distributions of most species of sturgeons set the stage for much confusion about species boundaries, but the situation is actually much more complicated. Confusion about the number of species of sturgeons living within the same basin can result from the often close morphological and meristic similarities of certain species of Acipenser, particularly during juvenile period. Moreover, we still have remarkably inadequate knowledge of the comparative anatomy of the species of Acipenser: no modern study has ever attempted a comprehensive examination of ail species, and it is impossible to rely on literature for the sorts of comparisons that must be made (for more on this general problem, see Grande & Bemis 1991,1997). Most classical descriptions and comparative anatomical studies relied upon small sample sizes. Voucher specimens of large sturgeons are especially rare in most historical collections, and type specimens (if available at all) are seldom prepared in ways that are suitable for making detailed anatomical comparisons (e.g., many skins are simply overstuffed with straw, so that all internal structures are lost). Intraspecific morphological and meristic polymorphisms occur in all species of acipenserids, and in most cases we have very poor knowledge of differences that develop during ontogeny, particularly changes in such features as the shape of the rostrum (Bemis et al. 1997). Another problem is the ease of hybridization between different species of sturgeons (reviewed in Birstein et al. 1997 this volume). In many of these cases, it is not easy to discriminate between parental species and the hybrids.