The Baikal fauna is genetically heterogeneous. The question of its sources, according to present-day data, is within the general hypothesis of the fresh water faunas origin owing to paleo-, meso- and neolimnic elements. The most ancient fauna was formed in the shallow Mesozoic basins and is adapted to the changeable environmental conditions. The successors of this fauna populate common continental lakes. In the Baikal the paleocomplex representatives live mostly in the coastal-sor zone and make the so-called Siberian element within it.
The mesolimnic fauna found in its fossil condition in the upper mesozoic and tertiary sediments of Central Asia is adapted to existence in the waters with stable conditions: low temperatures, waters saturated with oxygen and stable gas conditions. The mesolimnic fauna was the predecessor of the endemic faunas of the world’s great continental lakes in general and started the richest endemic water and ground fauna of the open Baikal. The oldest representatives of the mesolimnic fauna are the Lyubomirskiides sponges, higher crustacea (gammarides, gasteropods) as well as dendrocellides worms, oligochetes of the Lumbricullides family and others.
The neolimnic complex of organisms is also sufficiently represented in the Baikal fauna. They are recent tenants from the present-day side seas. In the Baikal these are relatively young elements which came to the lake from the north. Many representatives of this organism group are widespread in the lake and numerous, their form-making processes are very active which is registered at the level of species (the Baikal seal, or nerpa), sub-species (the white and black graylings) and ecological and morphological groups and populations (the omul).
One of the most important peculiarities of the Baikal animal world is its deep water fauna. According to present-day conceptions the deep water fauna of the Baikal invertebrates is young. It is being formed owing to the coastal one and has not lost its connection with it though it is specific. The characteristic features of the Baikal deep water inhabitants are bigger dimensions, the receptive systems changes (loss or reducing of sight organs, sense of touch organs development), different character of moving.
The research made with the help of deep water devices of the Pisces type broadened the conception of the areas of some Baikal species spreading and of their way of life. It has been stated, for example, that the Lyubomirskiides sponges live not only in the coastal parts of the lake but also at the depth of about 1,000 m. In such conditions they are colourless, lose their green symbionts (green unicellular algae) but they are well developed and are able to reproduce.