These freshwater fish have lost their eyesight over time because they don’t need to see inside dark caves, but being blind has other implications and has actually helped this fish survive.
The behaviour of most animals is coordinated by light and other external cues that tune the internal clock. Light tells some animals to wake up and tells others it’s time for bed, but if an animal cannot see light then it is unaffected. This fish has managed to stop its internal clock completely so it is frozen in time.
The blind cave fish is exempt from light cues, and actually uses 30 percent less energy than other fish its size. This is because energy that would have been used for sight can be used for metabolism and body repair. In fact, its DNA tells the fish that it is experiencing constant light, so it behaves as if it is light all the time. It sleeps less that fish that live in light and is active throughout the night.