Back from the underworld: the exploitation of spring habitats by stygobiont species

Stygobionts, namely animals with strong adaptations to subterranean environments that are unable to complete their life cycles outside groundwater, can be observed in spring ecotones, but their occurrence is generally considered accidental. The aim of this paper is to assess if stygobiont occurrence in springs is linked to specific environmental conditions or if it is random, irrespective of their features. For three years, we surveyed 59 spring sites recording the occurrence of vertebrate and invertebrate stygobiont species and assessing if spring features were related to their distribution. Moreover, we recorded the escape reactions of two easily identifiable stygobiont species. We detected six taxa usually considered as strictly stygobiont based on their troglomorphic features. Two of them were quite widespread: the salamander Proteus anguinus and the shrimp Troglocaris planinensis. Environmental characteristics were significantly related to the distribution of stygobionts. Hydroperiod and occurrence of flooding were the factors that played the strongest role in affecting occurrence. Our study suggests that the occurrence of stygobionts in springs is linked to specific habitat features rather than being a random mechanism and that the exploitation of ecotones can be important for the lifecycle of some species usually assumed to be strictly associated to caves.