The earliest vertebrate animals to walk on land were ancient four-limbed tetrapods that waggled their way across the ground like salamanders. Yet researchers still haven’t found many intermediate species showing just how swimming fish evolved to walk on land. Now, an endangered species only found in a handful of caverns in Thailand might finally help straighten things out.
The species in question is a type of blind cave fish called Cryptotora thamicola, or the waterfall-climbing cave fish. Documented in a study recently published in the journal Scientific Reports, the creature uses its four fins to crawl over rocks and up slick walls. The fish even has a full pelvis fused to its spine—a skeletal feature absent from any of the other 30,000 fish species in the world. This particular feature, however, is found in terrestrial vertebrates and fossils of the earliest tetrapods, making the waterfall cave fish a unique window into evolution.
“It’s really weird,” John R. Hutchinson, a biologist at the Royal Veterinary College at the University of London tells Carl Zimmer at The New York Times. “It’s a good example of how much fish diversity there’s left to be discovered.”
The species was first found in Northern Thailand in 1985 in eight caves near the Myanmar border. The Thai government is now extremely protective of those caves, allowing only a handful of researchers to examine them and their strange fish.
Last year, Apinun Suvarnaraksha, an ichthyologist from Maejo University in Thailand and Daphne Soares, a biologist at the New Jersey Institute of Technology observed the fish on an expedition to those caves and took a video . When Soares shared the images with her NJIT colleague Brooke Flammang, a biomechanics researcher, she was stunned. “I was like, ‘Fish can’t do that,’” Flammang tells Diane Kelly at Wired. “That’s ridiculous.”
Flammang hoped to get specimens of the rare fish to study, but that was not possible. So she began working with Suvarnaraksha, who returned to the caves and began briefly capturing the fish and putting them in an aquarium for filming before releasing them. He was also able to perform a CT scan of a preserved museum specimen of Cryptotora thamicola at a local dental school.
Armed with that data, Flammang began to unravel the secrets of the cave fish. It didn’t take long. “When they sent me the files, I thought someone was playing a trick on me,” she tells Kelly. “There was this gigantic pelvis [on the CT scan] that looks nothing like any fish pelvis.”
While it’s highly unlikely that the waterfall cave fish is an ancestor of ancient tetrapods, its evolution sheds some light on how other fish could have evolved to move on land. It also calls into question some of the 400-million-year-old tetrapod “footprints” scientists have found in recent years.
Researchers may now need to evaluate those prints—the next likely candidate is the giant waddling fish. “The physics are the same,” Flammang tells Zimmer.