Here's a fish that sounds like something out of a fairytale. The walking cavefish (Cryptotora thamicola) lives its entire life swimming through subterranean rivers and streams, with a remarkable ability to climb underground waterfalls. It's blind due to living in the pitch black of its cave habitats, but has evolved fins unlike any other fish ever discovered — fins that have an underlying structure comparable to a salamander's legs.
Researchers believe that by studying the cavefish's remarkable climbing ability, they can better understand the evolutionary transition from fins to limbed appendages that happened some 420 million years ago, which led to the first land-walking tetrapods, reports Phys.org.
Other fish have been discovered that can scurry across land. For example, there are walking catfishes, climbing perch, and mudskippers, to name a few. But the waterfall-climbing cavefish is unlike any of these.
"It possesses morphological features that have previously only been attributed to tetrapods," said Brooke E. Flammang, one of the researchers that studied the cavefish. "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."
Tetrapods are the first land-living creatures that eventually made life possible for all of us vertebrates that live on terra firma: amphibians, reptiles, birds and mammals. To find a fish that may have evolved some of these same traits independently gives scientists a living example of how legs may have first developed.
The cavefish climbs fast-flowing waterfalls by latching to rock and wiggling its body using what is called a "diagonal-couplets lateral sequence gait." This involves the semi-synchronous movement of the right forefin and left hindfin, followed by the semi-synchronous movement of the left forefin and right hindfin. It's how a salamander walks, basically.
Out of the around 30,000 species of fish known to exist, this basic body plan is unlike any other. A video showcasing these remarkable discoveries, which also shows some of these cavefish in their natural habitat, can be viewed here: