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This blind, cave-dwelling fish can climb walls

New Jersey Institute of Technology

Scientists have long been trying to figure out how exactly our ancestors evolved from fish to land vertebrates some 375 million years ago. Now a tiny, eyeless fish that walks and climbs up waterfalls could offer some clues, The New York Times reports.

tiny fish, big questions

The fish, Cryptotora thamicola, was first spotted in 1985. It's about two inches long, blind as a bat, and a bit of a recluse —€” Cryptotora have never been spotted anywhere but the caves of northern Thailand. That's where Dr. Apinun Suvarnaraksha of Maejo University in Thailand and Dr. Daphne Soares of the New Jersey Institute of Technology recently came across the little fish and decided to look deeper into it.

Because the cave fish is a protected species, researchers acquired a preserved specimen from a Thai museum collection and ran it through a high-resolution CT scanner. The images they received from the CT revealed some pretty zany things about Cryptotora's skeletal structure. The standout? Its impressive pelvis.

Most fish have pelvic bones, but they're usually just suspended in muscle or loosely attached to the skeletal framework of the front fins. That keeps fish flexible for swimming. In the case of Cryptotora, however, the pelvis is a broad plate joined to a sacral rib, which in turn binds to the vertebrae. Its bone structure resembles those of tetrapods — four-legged, land-roaming vertebrates. And it allows the cave fish to climb up waterfalls.

Other fish can walk and climb: eels, killifishes, and pricklebacks can temporarily wriggle onto land to avoid predators or move between bodies of water. The frogfish can use its fins as legs, but only underwater, where its weight is already mostly supported. Polypterus, mudskippers, and some catfish can crutch along for short distances by pushing off of their tails. But Cryptotora is likely the only one that has grown such sophisticated, tetrapodal bone structures.

This fish's pelvis is out of this world

It’s also the first fish species that’s been found to walk in a "diagonal-couplets lateral sequence," the researchers wrote. In other words, the front right leg moves at the same time as the back left leg, just like a salamander. Until now, that move was considered unique to tetrapods. Researchers caution that, however amazing, the cave fish isn't exactly the missing link between fish and land vertebrates. But it's one piece of the puzzle that could eventually shed light on how the transition from sea to land took place.