These fish are native to the limestone caverns found deep beneath the Somalian desert. As you might imagine, these are creatures of eternal darkness, and thus the terms "night" and "day" are meaningless. And that's what makes their biological clocks so bizarre.
The Somalian cave fish, or Phreatichthys andruzzii, has an internal biological clock just like we do. Ours, however, is set on a 24-hour cycle, in keeping with the passage of day and night. That's pretty much the standard operating procedure for any organism that has any contact with the Sun. But since the cave fish lives without these concerns, it's been free to develop its own unique sense of time, one that stretches each day out to 47 hours.
This particular cycle appears to be determined by the fish's food cycle. Recent research probed into the fish's biological clock to see whether it was completely "broken." They found that the cave fish had absolutely no response to light and dark cues in laboratory conditions, but when the researchers spliced in some key timekeeping proteins from zebra fish, the cave fish started responding just fine, indicating the basic mechanisms are still in place.
That would suggest that the cave fish are still in the process of evolving away from a light-based biological clock. It's believed that they have been completely cut off from light for the last 2.6 million years, and it might be a million years more before they completely lose the ability to respond to day and night.
Via Science News. Image by Saulo Bambi.