Biological Rhythmicity in Subterranean Animals: A Function Risking Extinction?

In this chapter, we discuss evidence of regression of circadian locomotor activity in exclusively subterranean species (troglobites), having fishes as models, by comparing such findings with observations on related epigean (surface) species, that may also form self-sustained subterranean (troglophilic) populations. These results favor the hypothesis of regression of a function which may have lost its adaptive value for species permanently isolated in hypogean habitats for many generations—regression similar to the reduction of eyes and dark pigmentation, typical of troglobites in general. Recent data on feeding behavior of blind catfish compared to epigean congeners suggest a process of partial regression, affecting locomotion but not feeding, due perhaps to the persistence of regular food availability in the otherwise continuously dark cave environment. Among non-troglobitic subterranean animals, trogloxenes present regular, cyclical movements between hypogean and epigean habitats, whereas troglophiles may move between these habitats, promoting genetic connectivity between surface and subterranean populations, but without following well-defined rhythmic patterns