Underground habitats are characterised by a lack of light and the alternation of day and night, a lack of seasons, a very high and stable relative air humidity close to complete saturation, a relatively stable air temperature, a low and stable water temperature, and a relatively small amount of food coming from above-ground habitats, and, consequently, a very short food chain dominated by scavengers.
Animals that inhabit the underground have evolved from their above-ground ancestors, adapting partially or completely to the demanding conditions of the habitat over a long series of years, developing new characteristics with the simultaneous loss of others through regressive evolution. Adaptations to life underground include the reduction of organs for vision, loss of pigment, thinning of the integument, the lengthening of body parts, and the development of other senses, a slowed metabolism, increased longevity, a low degree of reproduction, accumulation of fat reserves, reduction of aggressiveness, the cessation of a day/night rhythm of activity, and a cessation of seasonal changes.
Individual animals only go underground for various needs, such as avoiding unfavorable conditions on the surface, hibernation, or reproduction.
About one hundred and seventy taxa have so far been recorded in speleological structures in the Park area, many of which are endemic to the Dinarides and to Croatia, while four are endemic to the Krka National Park (stenoendemic): certain freshwater cave snails (Dalmatella sketi and Lanzaia skradinensis), a cave centipede (Eupolybothrus cavernicolus), and the Marguš cave springtail (Verhoeffiella margusi). Recent research has uncovered several taxa where preliminary examination has confirmed that they are new species, but they have not yet been described scientifically.
The best-known underground inhabitant of the Park is the olm (Proteus anguinus), endemic to the Dinarides, the only true underground vertebrate in Europe. The underground waters of the Park are also inhabited by the cave-dwelling tube worm (Marifugia cavatica), also endemic to the Dinarides. Bats also find shelter in the caves of the Park: they hibernate in them and/or create birthing colonies.
