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.
These pelvic adaptations allow the fish to push their fins against the ground, pushing their bodies up and forward with each step, said Flammang. These features likely evolved as adaptations to fast-flowing waters, such as rivers and streams. In the cave angel fish, this increased mobility could allow for enhanced access to oxygenated waters, which is an important trait among this family group.
And as Zach Randall, a biologist at the Florida Museum of Natural history and a co-author of the study, explained in a NJIT press release, these traits are “likely key to helping these fishes avoid being washed away in the fast-flowing environment that they live in.” To which he added: “What’s really cool about this paper is that it shows with high detail that robust pelvic girdles are more common than we thought in the hillstream loach family.”
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.”
Indeed, the team is now turning their attention to exactly that—the walking mechanics employed by these remarkable fish. To that end, the team is studying hillstream loach locomotion in the lab using high-speed video and a technique that records their muscle activity.
“We can then use the information of live fish walking to program the amphibious robotic fish we are building,” said Flammang. “The fish robot can then be adapted to represent fossil forms for us to study their functional morphology and locomotor biomechanics.”
This sort of thing has been done before, particularly with extinct species, including an early land walker known as Orobates. We’re now looking forward to the team’s robotic cave angel fish and the resulting scientific insights.