The cavefish, Phreatichthys andruzzii, has evolved for nearly two million years in the isolated darkness of caves beneath the Somalian desert.
Professor Nick Foulkes, of the Karlsruhe Institute of Technology in Germany, said that this particular species was chosen "because it was such an extreme example, having been isolated from a day-night cycle for so long".
In the course of its evolution it has lost its eyes, colouration and scales, having no need for them in the pitch-black of an underground cave system.
Regular feeding shows, however, that the fish will adjust its circadian rhythm for food.
Link -via BBC