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The rare species Polyacanthisca calceolata Baz., 1937 inhabits the maximum depths of the central and southern parts of Lake Baikal. Several specimens were collected together with Ommatogammarus albinus (Dyb., 1874), the typical necrophilous species, in 1991 using a trap with a lure. That fact, as well as several morphological features, shows, that P. calceolata is a scavenger, and the capture of the crustacean by plankton nets is an evidence of its bcntho-pclagic mode of life. The same features are characteristic to Ommatogammarus spp., which arc much more abundant in Lake Baikal and inhabit broader range of depths. All of collected in the trap specimens of P. calceolata were females (13 ex.), whereas among the specimens caught by plankton nets were animals of both sexes. The parallelism in the formation of ecologically identical groups of bcntho-pclagic scavengers in the abyssal fauna of Lake Baikal and that of ocean is noted.
Timoshkin O.A., Mekhanikova I.V., Takhteev V.V.
The side-line organs in Baikal amphipods (analogous to the corresponding system in lower vertebrates were studied using SEM. The arrangement of the side-line organs on the body surface, intraspccific and ontogenetic variability of these organs were studied in planktonic Macrohectopus branickii (Dyb.). The side-line system is formed by the groups of microtrichs designated by us as «clavi» (derived from the Latin word «clavus» meaning «a little nail»). The length of clavi in M. branickii varies from 4 to 11 mcm. Their arrangement on the body surface is species - specific. Clavi are found in all the body segments (except the second urosomal one) and telson. On the first five urosomal segments, only the lateral groups of clavi are present, on the urosomal segments - only the dorsal ones. The number of clavi depends on the sex and age of an animal. The size of clavi is constant throughout poslembrional development. Their size is nearly equal in dwarf males (the size of mature individuals is up to 5.5 mm) and large females (the body size is up to 38 mm). The direct correlation between the extent of the development of the groups of clavi on the body segments and the functional significance of the limbs of these segments under locomotion is revealed.
Mekhanikova I.V., Takhteev V.V., Timoshkin O.A.
The interspecific variation of the side-line organs in 34 species of Baikal amphipods from 21 genera belonging to different phylogcnetic lines, ecological groups and life forms, in the freshwater palaearctic Ganunarus lacustris Sars and marine Stegocephalus inflatus Kroycr was studied. The size of clavi in Baikal amphipods varies from 2.4 to 24.6 mem. Long and slender microtrichs are characteristic for nectobenthic species and planktonic M. branickii, short and thick ones were observed in burrowing species, parasitic species (Pachyschesis) and necrophages (Ommatogammarus). Clavi are arranged in rows or fields of irregular contours, which are highly variable. The asymmetry (sometimes very significant) in the shape and arrangement of the right and left groups of clavi was revealed. Their number in Baikal amphipods usually was no more than 10-15 per group. The maximum number was observed in nccrophage O. albinus (Dyb.) (up to 85 clavi in a group). Probably, the side-line in amphipods is a multifunctional system (scismosensitivc, hydrodynamical and chemoscnsitivc). The side-line organs in amphipods represent a further interesting example of the parallel and independent development of similar systems in vertebrates and Arthropoda.