At Least 11 Species Of Fish Could Walk On Land If They Wanted To

The strong pelvic girdle connecting the backbone of the waterfall climbing cave fish to its fins allows it to walk on land like many four-legged animals. This bone structure is unusual for fish, but not, as it turns out, unique. Zachary Randall/Florida Museum

Somewhere around 430 million years ago a fish decided to try walking on land and the world was never the same. The question of how this happened fascinates biologists, so it may prove useful to have discovered quite a few fish have bodies suited to land locomotion, although most are apparently too lazy to bother.

In 1985 scientists were shocked to discover a species they named the waterfall climbing cavefish (Cryptotora thamicola) living in deep caves beneath Thailand's Mae Hong Son Province. Like other cave dwellers, C. thamicola has lost its sight, but makes up for this by being able to climb rocky outcrops to get between bodies of water within the cave environment.

Other fish have developed a capacity to leap from the water and take a few bounces on land using powerful tails, but something akin to actual walking is a different matter. “Mexican Walking Fish”, for example, aren't actually fish, and musical claims of training fish to walk regarded skeptically.

In 2016 a study of the way C. thamicola gets around found it has a large pelvic girdle similar to land animals and uses the most common four-legged gait – right front and back left together, followed by the reverse. However, in this case, its fins rather than legs are being moved.

The cave angel fish’s pelvis more closely resembles that of some amphibians, enabling its salamander-like gait. Video By Zachary Randall/Florida Museum

Zachary Randall of the Florida Museum wondered if C. thamicola had developed the anatomy to do this all on its own, or it's built on a pre-existing structure found in related species. The waterfall climbing cavefish is the only one of its genus known, but it belongs to the larger family of hillstream loaches, so Randall and others studied the bone structures of other loaches.

In the Journal of Morphology, they report 10 other loaches with pelvic girdles strong enough they could support the fishes' weight walking out of water.

"Fishes don't usually have any connection between their spine and pelvic fin," Randall said in a statement. "But before, the idea was that the cave angel fish was totally unique. 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."

Although Homaloptera bilineata doesn’t have the cave angel fish’s robust pelvis, Randall observed it using its fins to propel itself forward. The extent to which it can walk is still unknown. Zachary Randall/Florida Musuem

More than 100 hillstream loaches are known, so this is a rare feature, even on this branch of the ichthyic family tree. The study helped place C. thamicola relative to other hillstream loaches. Lead author Callie Crawford, a New Jersey Institute of Technology PhD student, noted that while the physical characteristics of the loaches vary by species, they share a genetic capacity to adapt to fast-flowing river environments.

Many fish, such as Tasmania's endangered handfish species, “walk” along the bottom of the ocean or lake. However, with the buoyancy of water holding most of their weight this requires much less adjustment of the conventional fish bone structure.

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