Karyotype of the Baikal amphipod Polyacanthisca calceolata Bazikalova, 1937, (Crustacea, Amphipoda)

The karyotype of the endemic Baikal amphipod Polyacanthiska calceolata Bazikalova, 1937 was analysed for the first time. A diploid number (2n=52) and general chromosome morphology (with a median centromere) are typical amongst Gammaridea. A peculiar feature of the karyotype was its pronounced symmetry caused by the presence of metacentric (equal-armed) chromosomes insignificantly varying in size (from 5 to 8 μm). Sex chromosomes have not been distinguished. Supernumerary B chromosomes were regularly found in the metaphase plates of embryos P. calceolata, and may serve as an adaptive mechanism of this species to perform migrations to areas of gas hydrate seeps.