It's a fish that swims, but one that can also walk like a land animal and is built like no other fish on Earth.
The blind, waterfall climbing cavefish Cryptotora thamicola is a distant relative of pet goldfish. It lives in fast-flowing streams and waterfalls in caves in Thailand. And its claim to fame is the unusual way it uses two front fins and two back fins to pull itself up steep rocks and waterfalls as the water rushes over them.
"They have fin structures unlike any fish that I have ever seen anywhere," said Brooke Flammang, a biologist at the New Jersey Institute of Technology who led the new discovery, in a YouTube video showing how the fish moves.
"They are able to climb up rock faces in very much the same way that salamanders do. From an evolutionary perspective, this is a huge finding."
The fish doesn't push itself with its tail using the side-to-side movements that other fish like mudskippers and climbing perch use when "walking" on land.
Instead, it plants and moves its fins the way four-legged vertebrates such as amphibians, reptiles and mammals — known together as tetrapods — plant their feet.
When Flammang and her colleagues looked at the cavefish's anatomy to see how it was able to do this, they were surprised — the fish's skeleton had adaptations for walking that have never been seen in a fish.
"The pelvis and vertebral column of this fish allow it to support its body weight against gravity and provide large sites for muscle attachment for walking," Flammang said in a news release.
Among its unique features was a bony connection between its pelvis and backbone. That's something other "walking" fish don't have, but tetrapods do.
Researchers know that the cavefish isn't related to the fishy common ancestor of all tetrapods because that ancestor had multiple digits or fingers on each of its limbs, and the cavefish doesn't.
But they say their discovery shows that fish have the innate ability to evolve a pelvis. That may also explain why some fossil track ways from a four-legged, walking animal have been found that are older than the oldest animal with multiple digits on its limbs.
The researchers published their discovery in the journal Scientific Reports.