At the edge of darkness and light: macrozoobenthic assemblages of streams in the entrance zone of caves in the Western Caucasus

A study was conducted on the macrozoobenthos fauna of 15 watercourses flowing through the caves of the Duripshski, Gumistinsko-Panavskiy and Dzhalski speleo districts in Abkhazia (Western Caucasus). Assemblages inhabiting the inner part of caves, entrances and surface biotopes are described. A total of 84 species of aquatic invertebrates were identified in the surveyed streams, with 23 species belonging to the ecological group of stygobionts, 17 classified as stygophiles, and 44 stygoxenes. The species richness and abundance of stygoxenes exhibited a decline from surface stations to stations situated deeper within the caves. Stygobionts were found to be most diverse and abundant within subterranean cavities. The number of stygophile species was found to be highest in the ecotone zone. The similarity of the species structure of macrozoobenthos assemblages was the highest in watercourses flowing from neighboring caves and was mainly determined by the distance factor between sites. At the taxonomic level of families, on the contrary, a large proportion of structural variation in assemblages was attributed to the light factor. Ecotone assemblages, although having their own specificity, occupied a dependent position in relation to surface and subterranean. It is substantiated that illuminance is the primary regulatory factor in the inhibition of the penetration of surface and subterranean faunas into habitats that are not typical for them. The active and passive ways of the invasion of stygobionts into the surface biotopes and of stygoxenes into the subterranean cavities are discussed.