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Takhteev V.V., Mekhanikova I.V.
Gammarids of the genus Pachyschesis Baz. are external parasites of the large nectobenthic Baikal amphipods, inhabiting mainly the depths from 100 m and more. Feeding habits, size and sexual structures, fertility, 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 pouch or gill cavity). The parasites feed on host eggs, have many fat inclusions in their body; they are resistant to deficit of the oxygen. The females belong to the spherical, slightlymobile life form. They are considerably larger than the males and have vastly growing brood pouch. There are more females then males in populations. Phenetic determination of sex or proterandric hermaphroditism is assumed for the species of the genus Pachyschesis. Males of the host species serve for the dispersal of parasites. Biocenotic role of the Pachyschesis species consists in some decreasing of productivity of animals in the Baikal deep-water zone.