How do Blind cave fish find their way? It's in their bones...

 Blind cave tetra,  Astyanax mexicanus.

How the Blind cave tetra has managed to adapt a life of perpetual darkness could be explained in their bones.

Researchers from the University of Cincinnati have studied one biological adaptation that might help them to understand how Astyanax mexicanus navigate and find food without benefit of sight — asymmetry. Researchers examined juvenile and adult cavefish to understand how their skulls change during their lives.

Most fish are symmetrical — their left and right sides are virtually identical and streamlined to provide the most efficient locomotion in the water.

Cavefish are genetically similar to their symmetrical and keen-sighted cousins, Mexican tetras, found in nearby creeks and rivers on the surface. They’re so closely related that they easily interbreed and produce fertile young, even though the two species are believed to have diverged millions of years ago.

Cavefish start their lives with symmetrical features like other fish. But when they mature, their fragmented cranial bones harden in a visibly skewed direction, the study found.

UC’s researchers speculate that this adaptation helps the typically left-leaning cavefish navigate by using sensory organs called neuromasts to follow the contours of the cave as they swim in a perpetual counterclockwise pattern. This behaviour was observed among captive cavefish, which keep moving around the edges of their tanks while surface fish tend to stay motionless in the shadows of their tank or swim in haphazard ways.

“That was a real big piece of the puzzle for us,” said Joshua Gross, a UC biology professor and co-author. “It’s a mystery how they’ve been able to adapt. The amazing thing is that they’re not just barely surviving — they thrive in total darkness.”

The skulls of all but a couple cavefish UC studied bend to the left. They seem to be right-finned, swimming in a lazy counterclockwise pattern around their aquariums in the biology lab.“You could see how asymmetry might be an advantage in navigation,” said Amanda Powers, a UC graduate student and lead author of a study on Blind cavefish published in the journal PLOS One.

“They tend to swim in a unidirectional, circular motion around their tanks to explore their surroundings,” she said. “Having asymmetry in their skull we think is attributed to handedness. If their skull is bent to the left, they could be ‘right-handed.’ They’re feeling the wall to the right with their sensory structures.”

This kind of asymmetry is uncommon in nature. Think of the fiddler crab with its outsized claw. Owls have asymmetrical ears — one canal placed higher on the skull than the other — perhaps to help the night predators target the faint rustling of a mouse in the dark.