Back in 2016, researchers exploring a cave in Thailand discovered that a rare species of blind cave fish was capable of walking on land like a salamander. The strange fish in question, known as the cave angel fish or Cryptotora thamicola, doesn’t just walk the walk, it’s also got the skeleton to back it up.
A computerized tomography (CT) scan revealed the cave angel fish has a pelvis and spine similar to those of land dwellers, offering a tantalizing glimpse into how our own aquatic ancestors might have first adapted to life on terra firma some 375 million years ago, reported Carl Zimmer for the New York Times in 2016.
Now, researchers have discovered that at least ten other species of the cave angel fish’s relatives, an Asian family of fish called hillstream loaches, have the same strange anatomy and may themselves be able to walk on dry land, reports George Dvorsky for Gizmodo.
Other species of fish, such as the mudskipper, can get around out of the water using just their front fins as walking sticks or with a less than elegant full body wiggle, but the cave angel fish, and perhaps nearly a dozen of its brethren, are the only ones to use a four-finned gait, per the Times.
“In most fishes, there is no bony connection between the backbone and the pelvic fins. These fish are different because they have hips,” Brooke Flammang, a biologist at the New Jersey Institute of Technology (NJIT) and senior author of the new study, tells Gizmodo.
The new study, published last month in the Journal of Morphology, used CT scans and DNA analysis of 30 species of hillstream loaches to assess their evolutionary relationships and to see if any of them possessed the unique locomotive abilities of the cave angel fish.
CT scans of the hillstream loaches revealed that ten of the species had the same robust connection between the pelvic structures and the backbone that are present in the cave angel fish. The other 20 species studied had less sturdy hips, some of which are very thin and not well-connected to the spine.
Flammang tells Gizmodo that the researchers expect the ten species “with the largest, most robust ‘hip’-bones have the best walking ability.”
In a statement, Callie Crawford, a graduate student at NJIT and first author of the study, says, “these loaches have converged on a structural requirement to support terrestrial walking not seen in other fishes.”
The catch is that the salamander-like strut the beefy hip girdle theoretically makes possible has only been conclusively documented in the cave angel fish. The two-inch long, pale species uses its special abilities to climb up waterfalls deep inside its dank abode, which may help it access different parts of the cave stream ecosystem, according to a statement.
By combining their anatomical study of the loaches with DNA analysis, the team determined that the strong pelvis likely evolved multiple times within the family, rather than having been passed down to the 11 species which possess it from a single ancestor.
“We know that throughout evolution, organisms have repeatedly converged on similar morphologies as a result of facing similar pressures of natural selection,” Flammang tells Gizmodo. “And we also know that physics does not change with time. Therefore, we can learn from the mechanics of how this fish walks and use it to better understand how extinct early animals may have walked.”
According to the statement, the team collected additional specimens in Thailand earlier this year and is now using high-speed video to study the unique locomotion of these fishes.