A Sixth Sense for Darkness
In the subterranean pools of Tamaulipas, Mexico, lives a fish that has abandoned sight for a more useful sense. The Mexican tetra, Astyanax mexicanus, exists in two forms: a silver, eyed version that swims in sunlit surface rivers, and at least 30 distinct cave-dwelling populations that are blind and pale. These cavefish navigate their pitch-black world using an exquisitely sensitive lateral line, a system of sensory organs that detects minute vibrations and pressure changes in the water.
The lateral line is common to most fish, but in the cave-dwelling Astyanax, it is a supersense. The system is composed of hundreds of sensory organs called neuromasts, which contain tiny hair cells that respond to water displacement. Cavefish have evolved a greater number of these neuromasts, particularly on their heads and around their empty eye sockets. Individual neuromasts in the suborbital region are also larger and about twice as sensitive as those on their surface-dwelling relatives. This enhanced array allows the fish to construct a detailed, three-dimensional hydrodynamic "image" of its surroundings. It can detect the shape of stationary objects, the movement of prey, and navigate complex terrain without ever bumping into an obstacle. Some cavefish even use the waves of suction created by their own mouths to actively probe their environment, a behavior similar to echolocation.