SOME DATA ON LIFE-HISTORY OF THE BAIKAL PARASITIC GAMMARIDS OF THE GENUS PACHYSCHESIS (AMPHIPODA, GAMMARIDAE)

Gammarids of the genus Pachyschesis Baz. are external parasites of the large nectobenthic Baikal amphipods, inhabiting mainly the depths fro m 100 m and more. Feeding habits, size and sexual structures, fertilit y, density of populations were studied in two species - P. branchialis (Dybowsky) and P. bazikalowae (Karaman). The species were found to be specialized to the host species and the localization place (brood pou ch or gill cavity). The parasites feed on host eggs, have many fat inc lusions in their body; they are resistant to deficit of the oxygen. Th e females belong to the spherical, slightly mobile life form. They are considerably larger than the males and have vastly growing brood pouc h. There are more females then males in populations. Phenetic determin ation of sex or proterandric hermaphroditism is assumed for the specie s of the genus Pachyschesis. Males of the host species serve for the d ispersal of parasites. Biocenotic role of the Pachyschesis species con sists in some decreasing of productivity of animals in the Baikal deep -water zone.