Rhythmic response of cave animals to external cycles

Caves are quite stable environments where organisms are usually described as lacking biological cycles. However, recent evidence is emerging on the rhythmicity of cave-dwelling animals, with experimental, observational and genetic analyses. There are still many uncertainties, as some organisms may have regular or irregular rhythms, and some others seem to lack them. Moreover, it is not known whether these cycles are species-specific, ancestral characters, or a plastic behaviour linked to environmental conditions. In this study, we analysed the activity of groundwater-dwelling species: the olm (Proteus anguinus) and two crustacean, Monolistra pavani and Spealeomysis bottazzii. These species are found both in caves and springs. We used Bayesian hierarchical models to test the relationship between the activity of these species and classical biological cycles: circadian, circalunar, and circannual. We noticed a general pattern in the seasonality of cave-dwelling species, both in springs and caves, as well as a circadian rhythmicity in springs. Moon cycles shown contrasting results. Our results suggest that depigmented cave-dwelling organisms are vulnerable to sunlight and tend to avoid it, actively exploiting the spring environment during the night, when ecological conditions are more similar to caves. Higher activity in springs may as well be related to trophic availability. Long term seasonal rhythms, despite the constant climate in deeper parts of cave systems, may be linked to external variations in the rainfall regime and primary production of vegetation. Lastly, circalunar cycles require further careful studies. In order to clearly disentangle the nature of these rhythmicity, a molecular approach may be necessary. In conclusion, cave animals may exhibit a rhythmic behaviour both in springs and caves.