A surprising number of hillstream loaches—a family of Asian fish—are capable of walking on land using all four limbs, according to a new study. It’s a discovery that could explain how some of the earliest animals managed to stroll on solid ground.
South Asian hillstream loaches are a family of small fish that can often be found clinging to rocks in fast-moving waters. New research published in the Journal of Morphology suggests at least 11 species of hillstream loaches can also walk on land, as evidenced by their peculiar anatomies. At least one species, a blind cavefish known as Cryptotora thamicola, has actually been caught in the act, but the new research suggests other hillstream loaches can do it as well.
Brooke Flammang, a biologist at the New Jersey Institute of Technology and the study’s lead principal investigator, along with her colleagues, analyzed 29 hillstream loach specimens. Using micro-CT scans, the team studied and compared the various specimens, looking at their distinctive shapes, muscle groups, and skeletal structures.
This international team of researchers, which included scientists from the Florida Museum of Natural History, Louisiana State University, and Thailand’s Maejo University, also conducted some genetic work, sampling the DNA of 72 loaches in order to reconstruct their evolutionary family tree.
Together, the physical and genetic analysis revealed the fishes’ unusual land-walking capabilities.
“In most fishes, there is no bony connection between the backbone and the pelvic fins. These fish are different because they have hips,” explained Flammang in an email. “The hip bone is a sacral rib, and within the fishes we studied, we found three morphological variants ranging from very thin and not well-connected to robust and having a sturdy connection. We expect that those with the largest, most robust ‘hip’-bones have the best walking ability.”
Of the fish studied, 11 were found to have these robust hips, or pelvic girdles. Interestingly, the resulting gait is reminiscent of the way salamanders walk on land. As noted, the only documented example of a walking hillstream loach is Cryptotora thamicola, also known as the cave angel fish. These blind fish, in addition to walking on land, have been seen climbing up waterfalls, which they do using all four limbs.
Flammang said these fish don’t represent an intermediate species, that is, some kind of missing link between fully aquatic animals and those capable of living on land.
“But we know that throughout evolution, organisms have repeatedly converged on similar morphologies as a result of facing similar pressures of natural selection,” she said. “And we also know that physics does not change with time. Therefore, we can learn from the mechanics of how this fish walks and use it to better understand how extinct early animals may have walked.”