Researchers Find Fish That Walks the Way Land Vertebrates Do

It’s one of the most famous chapters in evolution, so familiar that it regularly inspires New Yorker cartoons: Some 375 million years ago, our ancestors emerged from the sea, evolving from swimming fish to vertebrates that walked on land.

Scientists still puzzle over exactly how the transition from sea to land took place. For the most part, they’ve had to rely on information gleaned from fossils of some of the intermediate species.

But now a team of researchers has found a remarkable parallel to one of evolution’s signature events. In a cave in Thailand, they’ve discovered that a blind fish walks the way land vertebrates do.

The waterfall-climbing cave fish, Cryptotora thamicola, has even evolved many of the skeletal features that our ancestors did for walking, including a full-blown pelvis.

“It’s really weird,” said John R. Hutchinson, a biologist at the Royal Veterinary College at the University of London who was not involved in the new study. “It’s a good example of how much fish diversity there’s left to be discovered.”

Drop an ordinary fish on the ground, and it will flop around helplessly: Its fins are adapted for pushing against water, not fighting gravity.

The early land vertebrates, known as tetrapods, evolved adaptations that enabled them to move efficiently over solid ground. A pelvis joined their hind limbs to their spines, for example. Their vertebrae grew flanges so that they interlocked, helping the spine hold itself stiff and straight even when being pulled down by gravity.

These adaptations led tetrapods to walk in a distinctive fashion, moving their forelegs and hind legs together in a cycle. Early tetrapods probably walked much the way salamanders do today, bending their trunk from side to side as they traveled.

All tetrapods descend from a single ancestor — a single lineage of fish that managed to spread on land. Some other fishes evolved vaguely similar ways of moving around.

ABOVE: An excerpt from a video of the cave fish walking up an incline of approximately 90 degrees on an aquarium wall in the direction of a trickle of water. Credit Apinun Suvarnaraksha

On coral reefs, for example, frogfish can push off surfaces with their fins. They have a gait that looks something like a slow-motion walk. But they can manage this movement only underwater.

Other fish can move on land, although none of them use a tetrapod gait to do so. Some simply squirm, while others, like mudskippers, rely on their front fins as crutches. In Hawaii, the Nopili rock-climbing goby climbs up rock faces by using its mouth as a suction cup.

The waterfall-climbing cave fish is leaps ahead of them, it turns out. Pale and blind, the two-inch-long fish feeds on microbes and organic matter growing on the cave walls. It was discovered in 1985, deep inside a system of caves in northern Thailand, and has been found nowhere else.

While other fish in the caves enjoy a life in quiet pools, the waterfall-climbing cave fish clambers up slick rocks as water crashes over it.

On a recent expedition to the caves, Apinun Suvarnaraksha, a biologist at Maejo University in Thailand, and Daphne Soares, of the New Jersey Institute of Technology, came across the climber and took some grainy videos of it.

Back in New Jersey, Dr. Soares showed the videos to her colleague, Brooke E. Flammang, an expert on biomechanics.

“I was completely blown away,” Dr. Flammang said. Instead of flopping or crutching, the cave fish were using what looked like a full-blown tetrapod gait.

“These guys seemed to be very leisurely walking up the rock face,” Dr. Flammang said.

She wanted to study the fish more closely, but the species is rare and protected, and she could not bring any of them into her lab.

Dr. Suvarnaraksha did the next best thing. In a Thai museum collection, he found one of the few preserved specimens of the fish. He took it to a dental school and used a high-resolution CT scanner to make images of the fish.

After Dr. Suvarnaraksha emailed the images to Dr. Flammang, she was able to line up the images together to reconstruct the fish’s three-dimensional anatomy. – NY Times