Fish that walk seem to have turned evolution on its head, but while most of them just flap around on fins or proto-legs that never really evolved to take actual steps, there is a fish that walks. Literally.
Cryptotora thamicola dwells deep in the caves of Thailand and both looks and moves as if it came from some alien body of water. This blind, colorless species of balitorid is the fish whose movements are the closest to the walk of any tetrapod on Earth, and not because it either devolved or failed to evolve. This fish was not struggling to walk on land and frozen in transition millions of years ago. The force of rushing water from underground waterfalls threatened to sweep it off the cave walls it climbed, which made it develop a unique skeletal structure that can go against the flow.
Balitorids, or river loaches, live fast—at least in fast-flowing waters. Previous studies have suggested that the unique morphology of Balitoridae developed in the face of strong currents. Enlarged skeletal structures not only support their bodies but provide more surface area for muscle attachment that keeps them from getting washed away. It also explains how they are capable of movement that is just about passable as walking.
“The adaptations we see in these fish are not the same as the evolutionary trajectory of the fin-to-limb transition. Instead, these fish are converging on structural adaptations which support similar movements. This lineage was not trying to make it to land, but instead, the morphological requirements to support life in their fast moving environment happen to facilitate terrestrial locomotion," NJIT Ph.D. candidate Callie Crawford, who co-authored a study conducted by researchers from NJIT, Louisiana State University, and the Florida Museum of Natural History, and recently published in Journal of Morphology, told SYFY WIRE.
How Cryptotora thamicola ended up lurking in caves is still uncertain; it may be similar to other fishes that ended up living in places where flooding could wash ancestral populations into the eldritch depths. Balitoridae all have modified skeletal structures related to survival in fast waters. C. thamicola stands out because the species diverged even further from balitorids closer to the surface over millions of years. Over time, it lost its sight and pigmentation, developing an eerie and ethereal appearance that makes it look like a ghost in the water. It is the only known member of Balitoridae that adapted to cave survival and the only species in its genus.
“The specialized skeletal structures of Balitoridae appear across the family, and the degree to which the modifications are found may be related to the flow rates. The water movement in the caves is fast, similar to the rivers and streams in which the closely related surface fish are found,” Crawford explained.
Genetics factor into the skeletal structures of Balitoridae, but they have really been shaped by the rivers that generations them have spawned in.
When Crawford and her team studied various species of balitorids, they found skeletal advantages in the pelvic area that help these fish get around. They have a tough pelvic girdle and elongated sacral rib (fused to the bottom of the spinal column), built for contending with fast-flowing water.
“A consequence of these enlarged structures is that they allow for the forces from pressing against the ground to be transferred to the body, leading to the fish being able to lift its body and produce forward movement with each step,” explianed Dr. Brooke Flammang, lead Principal Investigator on the grant funding this research. "What is really exciting about this project is that physics don't change over time. So understanding how this fish supports and stabilizes walking forces informs our understanding of the constraints and challenges faced by early tetrapods 400 million yeas ago."
Crawford next plans to study how walking performance could change between balitorids with slightly differing pelvic morphologies. The strength of their fins, sacral ribs, and pelvic girdles is expected to impact how well these fish walk. She and her team are looking into how much distance is covered in each step and over time, as well as fin and tail movement. Fish with stronger sacral ribs and pelvic girdles are expected to use those their advantage and show more movement in those areas. This is what separates the walkers from the flappers.
“We expect to see movements more similar to those seen in terrestrial tetrapods, like salamanders, in the fish with the greater modifications,” Crawford said, with Flammang adding that "The walking motion data will be used in designing an amphibious fish robot; the scope of this project includes using that robot to investigate the walking performance of early tetrapods by incorporating replicas of their fossil morphology."
Until then, maybe a mutated altverse version of these fish could walk onto your screen right where Sharknado left off.