Blue light restores functional circadian clocks in eyeless cave spiders

Evolution in profound darkness often leads to predictable, convergent traits, such as the loss of vision. Yet, the consequences of such repeated evolutionary experiments remain obscure, especially regarding fundamental regulatory behaviors like circadian rhythms. We studied circadian clocks of blind cave spiders and their sighted relatives. In the field, cave spiders exhibit low per expression and maintain constant activity levels. Curiously, their clocks are not permanently lost; exposure to monochromatic blue light restores both circadian gene expression and behavioral rhythms. Conversely, blocking blue light in sighted relatives induces an arrhythmic "cave phenotype." Our RNA interference experiments suggest that clock genes regulate the rhythmicity of the huddle response, establishing a link between circadian gene networks and this behavioral rhythm. We demonstrate that circadian regulation is readily toggled and may play a latent role, even in constant darkness. Overall, our study expands understanding of circadian clock variations and paves the way for future research on the maintenance of silent phenotypes.