As we've seen before, evolution has a way of helping life adapt to the most extreme of environments. It's helped ocean-dwelling microbes evolve into superbugs (gee, thanks) and given Sherpas their superhuman mountain climbing skills. The Mexican cavefish, though, seems to have gotten the short end of the stick. Not only has evolution taken away its eyeballs and a considerable chunk of its brain, it's turned it into an albino insomniac as well. On the upside, it's also given it a larger jaw, more teeth and enhanced taste buds. So how does this bizarre little fish find food in the dark? According to a new study by scientists from the University of Cincinnati, the answer could very well lie in its bones.

Mexican cavefish or tetras (Astyanax mexicanus) are surface-dwelling fish found in creeks and rivers. Sometime during the course of its evolutionary history, a bunch of them ended up getting trapped in the cisterns that dot the limestone caves of Mexico's Sierra del Abra Tanchipa rainforest. With no way of getting out, it was a case of adapt or die for the fish.

One of the interesting things about them is that these cave dwellers are not actually born blind. Rather, their eyes slowly regress and by the time they mature into adults, fat deposits have taken the place of their eyeballs in the sockets, which are covered with the same scales that run down the length of their bodies. Since they no longer have to see, the brain region responsible for processing visual inputs also shrinks, resulting in an overall decrease in the size of their brain compared to their surface-dwelling cousins. A previous study suggests this helps the fish save energy.

"They have been able to invade this really extreme environment," says lead author and biologist Amanda Powers. "They've evolved changes to their metabolism and skull structure. They've enhanced their sensory systems. And they can survive in an environment where not many animals could."

But how exactly do these blind fish navigate their way in the darkness to find food? Apart from their enhanced sensory systems, the researchers believe that the asymmetrical nature of their skulls also play a role.

Most animals, including fish, have bodies that are symmetrical. Scientists have come up with various hypotheses for this; one is that it helps make visual perception easier on the brain. Another is that it helps with mate selection as symmetry often indicates good health and dependable genes.

Skull of a surface-dwelling Mexican tetra on the left, with a skull of a cave-dwelling Mexican tetra on the right. Note the differences in the size of the eye orbit and jaw(Credit: Andrew Higley/UC Creative Services)

To survive in its habitat, the blind cavefish trade the symmetrical features they are born with for a skull that skews to the left as they mature. This transformation is enabled by the fusion and fragmentation of some of the bones below their eyes. Why exactly this happens remains a mystery but the researchers suggest it may be due to feeding behavior in the early stage of its life and that this evolutionary adaptation helps the cavefish find their way around by using sensory organs called neuromasts to swim in a counterclockwise pattern.

"Cavefish swim continuously at high speeds, maintaining a parallel body orientation to walls surrounding their environment," note the authors in the study. This is in contrast to surface fish, which vary their swimming speeds and direction, and are often found lying motionless in the darkest area of the tank.

This behavior is likely associated with differences in their lateral line systems (i.e. the system of sense organs used by fish to detect movement and changes in pressure in the water). As for why the blind cavefish tend to swim close to the edges of walls, the researchers suggest they might be doing this to get the greatest sensory input from the surrounding structures. When you're swimming blind and food is scarce, anything that gives you a leg up over the competition is worth doing. Unsurprisingly, it is not a picky eater. In the lab, the fish are fed a mix of flakes, brine shrimp and blackworms.

"The fact that they're all moving in the same evolutionary direction is not a coincidence," says co-author and biology professor Joshua Gross. "They're all living in total darkness with a limited food supply."

"You could see how asymmetry might be an advantage in navigation," adds Powers. "We think having asymmetry in their skull 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."

For scientists, the Mexican cavefish offers valuable insight into the evolutionary development process since it is one of the few species where both the ancestor (surface fish) and descendants (cave dwellers) are still available for study. Since they belong to the same species, they can breed with each other and this allows scientists to study interesting genetic traits and combinations, as well as understand why cave animals lose certain features.

"The traits they've lost are very conspicuous – their eyes, their pigmentation," says Gross. "The beauty of studying cave animals is it's a very robust model for understanding why features are lost, and it's a simple, stable set of environmental pressures that cause those features to go away."

The study was published in PLOS One and the researchers talk about the study in the video below.

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