Evolution of the circadian clock in fish: lessons from adaptation to extreme environments
During the Cambrian explosion animal body plans evolved very rapidly and image-forming eyes and visual systems emerged. However, in the pre-Cambrian era early organisms already evolved photoreceptors that were capable of light detection to mediated simple behavioral responses as the phototaxis. For this reason, extra-retinal photoreceptors represent the most basal form of light reception. Fish represent the most fascinating model to study deep brain photoreception because they colonized all aquatic habitats characterized by different photic environments including the subterranean waters in perpetual darkness. Comparative studies on epigean and hypogean fish species could help to shed light on key genetic and physiological mechanisms whereby animals respond to light.