A blind, walking cavefish in Thailand can climb up steep, slippery rocks in rapidly flowing water thanks to a pelvis bone that’s remarkably similar to that of four-legged landlubbers. The findings, published in Scientific Reports this week, could help us understand the transition from finned to limbed appendages that took place some 420 million years ago.

From flying fish to mudskippers to gravity-defying gobies in Hawaii, fishes have adapted a number of different behaviors to move out of the water. But until now, none of them have been described as being able to walk on land with a tetrapod-like gait. Tetrapods are all four-legged, land-living vertebrates – from frogs to eagles to humans. The group also includes animals that have since lost their limbs (like snakes) and those that returned to a life at sea (like whales). 

Called Cryptotora thamicola, the blind cavefish from Tham Maelana and Tham Susa in northern Thailand "possesses morphological features that have previously only been attributed to tetrapods," Brooke Flammang from the New Jersey Institute of Technology says in a statement. Based on anecdotal evidence, researchers already knew that these cavefish can walk, but they’re rare, and because of their protected status, studies into their functional morphology have been limited. 

Flammang’s team scanned a 47-millimeter-long Cryptotora thamicola specimen using a computed microtomography scanner. They also scanned a common goldfish (Carassius auratus) and a long-tailed salamander (Eurycea longicauda), and then compared the 3D reconstructions of their micro-CT images. Additionally, the team conducted a motion (or kinematic) analysis by observing and filming wild cavefish at Mae Lana cave; they carefully scooped two of them into a glass tank for 15 minutes of kinematic sequence recording. 

Turns out, the cavefish climbs fast-flowing waterfalls with what the team describes as a diagonal-couplets lateral sequence gait: The semi-synchronous movement of the right forefin and left hindfin is followed by the semi-synchronous movement of the left forefin and right hindfin. The rotation of the pectoral (or chest) and pelvic girdles – the paired bones where limbs attach – creates a standing wave of the body midline. It seems the cavefish convergently evolved a salamander-like way of walking. 

"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 explains. In all other fishes, the pelvic bones are suspended in a muscular sling or loosely attached to the pectoral girdle. In contrast, the pelvic girdle of this walking cavefish is a large, broad plate, and it’s fused with other bones in a similar way as terrestrial animals. 

Interestingly, the fin shape and footfalls of this cavefish are similar to footprints found in a subaqueous trackway in New South Wales dating back between 382 and 358 million years.