Mexican Cave Tetra

Extreme environments often call for extreme measures when it comes to survival, and when you’re living in perpetual darkness, having a few extra tricks up your sleeve can sometimes give you the edge you need.

The cave-dwelling form of the Mexican tetra (Astyanax mexicanus) is one such little fish that has made the best of its situation, adapting itself over time to a completely lightless environment whilst providing scientists with a superb example of regressive evolution in action. 

Not only does it have unpigmented skin and a complete lack of eyes, but thanks to the taste buds spaced all over its head, it has a better sense of smell and taste than its surface-dwelling cousins and is able to store four times more energy as fat, allowing it to cope with irregular food supplies more effectively. Despite being blind, it can even find its way around with ease thanks to its lateral lines, which are highly sensitive to fluctuating water pressure.

As impressive as these changes are, however, perhaps the most amazing adaptation is the fact that it has ditched its own circadian rhythm in order to better conserve energy in its pitch-black habitat. While most animals have a clear day/night rhythm, the absence of a day and night cycle in the cave-dweller's metabolism means that it exists in a much more steady metabolic state that lies somewhere between “asleep” and “fully active”, thus using significantly less energy over a 24-hour period compared to its surface-dwelling counterpart. By wisely saving energy in its food-limited environment, the tetra has survived and thrived across millennia to intrigue scientists and aquarists alike today.