The role of light in shallow subterranean habitats

Abstract

Absence of light is the defining feature of subterranean environments. Different shallow subterranean habitats have different spatial relationships with light. Epikarst and calcrete aquifers are entirely aphotic environments—there is no light, even at the boundary of the habitat; the same holds for lava tubes except near an entrance. For the other four, there is a light–dark gradient at the boundary of the habitat. For the milieu souterrain superficiel and the hyporheic, the gradient may be less abrupt. There are also twilight habitats, especially leaf litter. Most eyeless and depigmented animals are rarely found in photic habitats. Some reports in the literature of surface occurrences are actually occurrences from SSHs that were unrecognized as such. In those cases where they do occur in surface habitats, competitors and predators are rare or missing. Surface-dwelling populations of the amphipod Niphargus, a subterranean genus sometimes found in surface habitats, retain close connections to SSHs where they also occur. The converse, the presence of eyed species in SSHs, is a common occurrence, as is the occurrence of eyed species in deep subterranean habitats. The retention of some visual apparatus may even be advantageous when organisms are in a situation where they need to avoid light as well as to retain circadian rhythms. All eyeless and depigmented species can be grouped into the cryptozoa, which occur in aphotic and, in some cases, dimly-lit habitats.