In 1985, scientists discovered a walking fish living in a cave system in Thailand. The waterfall-climbing cavefish, Cryptotora thamicola, is completely blind and yet can make its way up an incline by using what closely resembles a tetrapod (four-limbed vertebrate) gait.
The fish’s movements draw a remarkable parallel to what scientists believe creatures did when they first left their watery homes behind for life on land. But until now, researchers had only a guess of how this fish makes its strides.
Through a collaboration with a museum in Thailand, because living members of the walking fish species are protected in the country, scientists at the New Jersey Institute of Technology and Maejo University in Thailand got their hands on one of the fish’s preserved specimens. This allowed them to image the dead fish using a CT scanner.
The images were then used to construct a three-dimensional model of the fish’s anatomy so that researchers could see if there were any special things in their makeup that enabled them to walk.
In typical fish, there is a pelvis, but it’s made of just two tiny bones that float in the body wall. These give fish a small amount of stability for their pelvic fins. But there was much more going on in the pelvic area for the cave-dwelling fish. Researchers saw that the pelvis in this fish was a complex of bones fused to the spine by a series of elongated ribs.
This construction is similar to the ones found in tetrapods, scientists say, which allow the creatures the ability to hold themselves up with their hind legs. It gives them the support they need by stiffening their spines.
By considering the pelvis with videos of the fish actually moving up an incline, researchers say that it makes sense that such a fusing of bones would be there. But the find is still surprising to see inside of a fish.
Videos of the fish walking helped confirm that they are using their pelvis to walk like four-limbed vertebrates do. The movements most closely appear to resemble those of salamanders. In its version of locomotion, the fish keeps its tail straight as it steps its fins forward, in stark contrast to the typical wriggling most fish do when they’re out of water.
The waterfall-climbing fish is incredibly rare and only inhabits eight caves on the border of Thailand and Myanmar. Its numbers are estimated to be just lower than 2,000. It is mostly pink, and is most closely related to goldfish in its biology. The fish has two sets of fins on its body.
Full results of the investigation uncovering the dynamics of the walking fish’s movements are published under open-access license in the journal Nature Scientific Reports.