I have a new favorite animal, and it’s a fish. Not just any old fish, this one lives deep underground in caves in Mexico, and not only is it blind, it has lost its eyes altogether.
The Mexican tetra is a small and boring-looking animal, but appearances are deceptive. This fish is famous among evolutionary biologists, physiologists and sleep scientists for its hidden talents.
The Mexican tetra comes in two forms, a “normal” form that lives in rivers and streams, and a blind form that lives in caves. Charles Darwin wrote about blind cave fish in “Origin of Species”: “By the time that an animal had reached, after numberless generations, the deepest recesses, disuse will on this view have more or less perfectly obliterated its eyes, and natural selection will often have affected other changes, such as an increase in the length of antennae or palpi, as compensation for blindness.”
In other words, after many years of living in darkness, the fish starts to lose the ability to see. There is no point going to the bother of growing eyes if they are never needed, so the cave fish use the energy they would have put into making eyes into growing and reproducing.
Darwin guessed that animals losing their vision in this way would compensate in other ways, and this is what has happened with the blind cave fish. The animals are able to sense tiny vibrations in water and fluctuations in water pressure, and use this information to get around.
Blind cave fish are also of great interest to scientists trying to figure out how humans can better cope with diabetes. It turns out the fish carry a gene that induces an insatiable appetite. It makes evolutionary sense because the fish live in caves and often have to endure times of food scarcity.
When there is food present it means the animals can gorge and stock up on supplies. It’s one of the reasons why the fish don’t just go blind but entirely lose their eyes. Food resources are in such short supply that there is a big advantage in not having to grow eyes.
They eat pretty much anything — algae, and dead and rotten plants and animals. In people, when this kind of voracious appetite takes hold, we can get high blood sugar levels and develop diabetes.
Blind cave fish, however, are resistant to insulin, the hormone that turns blood sugar into energy. They do develop high levels of blood sugar, but not the problems of diabetes that usually go with it. If we can figure out how they do it, it might be able to help the tens of millions of people — 30 million in the United States alone — that suffer from diabetes.
Now Yoshiyuki Yamamoto, a developmental biologist at University College London, U.K., and colleagues have studied the Mexican tetra and discovered another remarkable feature. The fish living at the surface — the ones that have not lost their eyes — are able to regenerate their hearts after an injury.
Humans have only limited capacity to regenerate damaged tissue. Our livers are able to regenerate, for example, but if anything else gets worn out or damaged, we just have to live with it. It’s one of the reasons we get slower and less fit as we get older. Naturally, discovering the secrets of regeneration is a hot area of research.
It turns out that the surface fish have the ability to regenerate damaged hearts but the cave fish do not. Some other species of cave fish in different regions have retained the ability to regenerate, so it means ecological and geological differences among the caves may influence the evolutionary loss of regeneration. Yamamoto and colleagues cross-bred animals from the surface with those from caves to find out what happens with intermediate forms. The hybrid fish showed different levels of regeneration, indicating that the ability to regenerate heart tissue is heritable, and they narrowed the ability down to a gene called lrrc10.
“We have identified three regions in the DNA that contain genes that make the difference between regeneration or scarring after heart injury,” says Yamamoto.
The next step is to find out which genes are key to heart regeneration. If we can understand how to trigger regeneration, and especially in vital organs such as the heart or brain, it could open the door to a whole new kind of medicine.
“If the genetic mechanisms are the same between the cave fish and humans, it might be possible to switch it back on again for helping patients with wounded hearts,” says Yamamoto.
You can imagine — eventually — being able to replace and repair damaged tissue in the body, and so extend human lives for many years.
Rowan Hooper is managing editor of New Scientist magazine. He tweets at @rowhoop and his new book, “Superhuman: Life at the Extremes of Mental and Physical Ability,” is out now, published by Simon & Schuster.