The Curious Case of <i>Eulimnogammarus cyaneus</i> (Dybowsky, 1874): Reproductive Biology of a Widespread Endemic Littoral Amphipod From Lake Baikal

Comparative studies of reproductive biology and formation of reproductive isolation need appropriate model systems, such as groups of related species. The amphipods (Crustacea: Amphipoda) of ancient Lake Baikal are an attractive group for such works, as they consist of several hundred species that radiated within the lake and have very different levels of intraspecific genetic diversity and reproduction timing. We have previously shown that one of the most widely distributed and best studied littoral species, Eulimnogammarus verrucosus (Gersfeldt, 1858), comprises cryptic species exhibiting a post-zygotic reproductive barrier. The object of this study was Eulimnogammarus cyaneus (Dybowsky, 1874), another widespread endemic littoral Baikal species, which has a surprisingly low genetic diversity within its large geographic range. The aim of this study was to check if the populations isolated by the Angara River source, which is approximately 120-thousand years old, are reproductively compatible. As neither prezygotic nor postzygotic barriers were found, at the moment these populations should be treated as belonging to a single species. At the same time, we found some noteworthy features of reproduction of E. cyaneus. They include successive reproductive cycles with amplexuses formed by females with juveniles in the brood pouch and deposition of unfertilized eggs by females. The former might mean that this species is a relatively promising object for a laboratory culture of Baikal amphipods. Taken together, these results contribute to the establishment of a research framework to look for the correlation between genetic divergence and biological species delimitation.