Mexico’s Blind Cave Fish: Freshwater Species of the Week

A blind cave fish and two surface fish from the same species (Astyanax mexicanus) searching for food at the bottom of an aquarium
A blind cave fish and two surface fish from the same species (Astyanax mexicanus) search for food at the bottom of an aquarium. (Photograph by Richard Borowsky)

If you don’t use it, you lose it—at least this appears to be the case for a blind cave fish found in Mexico and the southern U.S. known as Astyanax mexicanus. Descended from an eyed surface fish, over long periods of time the subterranean form of the same species has adapted to darkness, accumulating new mutations that make them blind.

As a result, Astyanax mexicanus is a freshwater fish that is currently found in two forms: cave and surface fish. Both forms are omnivores and approximately the same size, around 3 inches long (7.5 centimeters). But unlike the surface fish, the cave fish are blind, albino, insomniacs, and have taste buds on the outside of the lower jaw. freshwater species of the week

The cave and surface fish can interbreed, which makes them an ideal species to investigate the genetic basis of their differences.

“Evolution is harder to study than laboratory science because frequently important adaptations arise only once,” says scientist Richard Borowsky, a professor of biology at New York University. “For example, the ability for birds to fly was an evolutionary adaptation that happened only once in history.”

However, in the case of these fish, different cave populations have repeatedly and independently adapted to cave life. Basically, if a surface fish colonizes in a cave environment, eventually they will lose their eyesight and pigmentation.

Because different genes are changed in independent colonizations it is possible to hybridize cave fish from different caves and restore vision in their offspring. (Read: “Cross-breeding restores sight to blind cavefish.”)

According to Borowsky, this recurring adaptation comes about through genetic mutation. Over the past few months, biologists have been searching for the genetic mutatutions that cause blindness and skin transparency using a genetic website called Ensembl.com to crowdsource the fish’s entire genome.

Borowsky estimates that between 15 and 20 different genes have been altered by mutation in each cave population leading to the loss of vision, although the total number of genes needed to develop and maintain normal eyes is in the hundreds.

“We’re really identifying the larger set of genes that are needed to properly develop and maintain eyesight,” said Borowsky. Through the use of genetic research, biologists can locate many of the genes responsible for normal eye development. (Read: “Blind fish see shadows.”)

“There are tons of questions that people are interested in knowing, and we have people that are investigating,” Borowsky adds. “Now we’re able to look at the genome and see the standout pieces that really makes this fish different.”

Angie McPherson is the Digital News Intern at National Geographic.

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