Hillstream loach anatomy
High-resolution computed tomography reconstruction of Cryptotora thamicola.
Image: Zach Randall, Florida Museum of Natural History

A recent study has found that some Asian Hillstream loaches have the ability to walk on their fins. There are several well-known “walking fish” including the aptly named Walking Catfish and Climbing Perch, although those species use much more of a swimming motion when crossing land, only gaining grip with their pectoral fins in the case of the catfish or serrated gill covers in the case of the perch. 

The difference with the Hillstream’s form of locomotion is that they can actually raise themselves up on all four fins, lifting their body up off the substrate and walk in a more conventional manner, with a Salamander-like gait. The discovery could explain how some of the earliest animals managed to leave the water and evolved to walk on solid ground. 

The study was carried out by the New Jersey Institute of Technology, where 29 hillstream loach specimens were micro-CT scanned for their shapes, muscle groups and skeletal structures and then analysed by the team. The DNA of 72 loaches was then added by an international team from the Florida Museum of Natural History, Louisiana State University, and Thailand’s Maejo University to construct a family tree. 

Hillstream loach anatomy
Cryptotora thamicola as seen in multiple perspectives.
Image: Zach Randall, Florida Museum of Natural History, and BE Flammang, NJIT

Hips don’t lie

Brooke Flammang, a biologist at the New Jersey Institute of Technology and the study’s lead principal investigator explained: “In most fishes, there is no bony connection between the backbone and the pelvic fins. These fish are different because they have hips,” “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.”

South Asian hillstream loaches are a family of small fish that can often be found clinging to rocks in fast-moving waters and are available in the tropical freshwater aquarium hobby. The 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, walking on land and up waterfalls using all four limbs but the new research suggests other hillstream loaches can do it as well. Of the fish studied, 11 were found to have these robust hips, or pelvic girdles. 

“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 angelfish, Cryptotora thamicola, this increased mobility could allow for enhanced access to oxygenated waters, which is an important trait among this family group.

Zach Randall, a biologist at the Florida Museum of Natural history and a co-author of the study, explained the traits are “likely key to helping these fishes avoid being washed away in the fast-flowing environment that they live in. 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.”

Cave angelfish, Cryptotora thamicola in the wild.
The Cave Angelfish, Cryptotora thamicola in the wild.
Image: Florida Museum

The missing link

Flammang said these fish don’t represent an intermediate species, however, or 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.”

The team is now turning their attention to the walking mechanics employed by hillstream loaches 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 Angelfish and the resulting scientific insights.”