The enigmatic Caspian Sea Russian sturgeon: how many cryptic forms does it contain?

TheRussian sturgeon,Acipensergueldenstaedtii, is closely related to three other sturgeon species (A. persicus, A. naccarii and A. baerii), with populations in the Caspian Sea containing a cryptic lineage with an A. baerii-like mtDNA profile. Using morphological evidence (morphometrics, meristics) and additional genetic analysis (cytochrome b gene and control region sequencing), cryptic lineages within the Russian sturgeon and their relation to other closely related species of sturgeons are further examined. These data indicate that three genetic forms exist within what is presently known as A. gueldenstaedtii. These forms include the pure A. gueldenstaedtii and A. baerii-like individuals plus a third rare genetic form whose mtDNA is similar to the mtDNA of the Adriatic sturgeon, A. naccarii. Morphological comparison of the three forms and the Yenisei River A. baerii indicates that although the three forms of A. gueldenstaedtii are not different from one another, all three significantly differ from theYenisei RiverA. baerii. Competing explanations, including translocationandcentreoforiginhypothesesareconsidered.The threegenetic forms of A. gueldenstaedtii likely colonized different geographic areas during different geological periods, and subsequently evolved in these regions independently into the species currently recognized as A. gueldenstaedtii, A. naccarii and A. baerii.