The specialised aquatic hypogean, i.e. stygobiotic, fauna has been recognis
ed in some regions moderately rich. Slovenia, the broader Dinaric region, a
nd Europe are particularly rich with about 7-8% of all Metazoa and about 40
% of Crustacea species being stygobionts. The hypogean biotic diversity is
in general predominantly a crustacean diversity. The high number of stygobi
ont Crustacea-Malacostraca species can be explained by the near absence of
Insecta as well as by their high endemicity and sometimes additional specia
lisation, i.e. the spatial and ecological partition of the environment by t
he species. Although one cave system may exceptionally shelter up to 40 sty
gobiont species, they are distributed there into separate associations. Amo
ng more than 2000 described stygobiontic Malacostraca species, which includ
e close to 950 Amphipoda, the species numbers within some genera are very h
igh (e.g. Niphargus with 275 spp.). With 10 orders represented the higher t
axonomic diversity of stygobiont Malacostraca matches that of fresh waters
or the sea. Comparison of some faunas shows that the limiting factors for b
iodiversity might be the lower ecological diversity of habitats and restric
ted food resources underground, both brought about to a high degree by the
darkness and absence of plants. Being K-strategists, stygobionts are endang
ered by any sudden changes in their environment. In the case of an increase
d food input by modest organic pollution, they can be outcompeted by energe
tically demanding but competitively stronger recent immigrants from surface
.