Blind cave fish evolved a shrunken brain to save energy

The Mexican tetra lost its eyes - as well as a significant portion of its brain - to ensure survival in a subterranean environment

At some point back in deep time, a group of fish were washed into a limestone cave somewhere in northeastern Mexico. With no way out and little more than bat droppings to eat, the fish began to adapt to their new troglodytic lifestyle. Unable to see other members of their group in the dark, they lost their colourful pigmentation. Then they lost their eyesight, their eyes gradually got smaller, and then disappeared altogether.

This was accompanied by a dramatic reduction in the size of the brain's visual system. Yet, the question of why the blind cave fish lost its eyes and a large part of its brain remains unresolved. Now, biologists in Sweden believe they have found the answer. In new research published today, they report that loss of the visual system saves the fish a substantial amount of energy, and was probably key to their stranded ancestors' survival.