Evolution is weird. Sometimes, it feels like nature just gets bored and decides to break its own rules. You’ve probably seen fish jump. You might have even seen a mudskipper shimmy across a tide pool. But the waterfall climbing cave fish—specifically Cryptotora thamicola—is doing something entirely different. It’s not jumping. It’s not flopping. It is quite literally hiking up vertical rock faces through rushing water.
Found deep within the karst cave systems of Northern Thailand, this tiny, blind creature has forced biologists to rethink what a fish’s body is actually capable of.
If you were to look at one, you’d see a pale, pinkish-white sliver about two inches long. It lives in total darkness. It has no eyes because, honestly, what’s the point? But it has developed a pelvis that looks suspiciously like yours. That's the part that trips people up. Fish aren't supposed to have a pelvic girdle fused to their spine. It defies the standard "fishy" blueprint we've studied for centuries.
The anatomy of a hiker
Most fish have pelvic fins that sort of just float in their body wall. They’re held there by muscle, not bone-to-bone contact. If you try to make a goldfish walk, its fins will just collapse because there’s no structural support to hold its weight against gravity. The waterfall climbing cave fish changed the game.
Back in 2016, a team from the New Jersey Institute of Technology, led by Brooke Flammang, published a study in Scientific Reports that blew the lid off this thing. They used high-resolution CT scans and saw something wild. The fish has a massive, robust pelvic girdle. It’s actually fused to the vertebral column via an enlarged rib. This is a tetrapod feature. It's the kind of thing you see in salamanders or dogs, not something swimming in a subterranean stream in Mae Hong Son Province.
Because of this rigid connection, the fish can support its body weight. It moves with a "diagonal-pair lateral sequence" gait. Basically, it moves its front left fin with its back right fin, then switches. It looks like a lizard walking. Except it’s a fish. And it’s doing it up a slippery, moss-covered waterfall in the dark.
Why climb at all?
You might wonder why a fish would bother working that hard. Climbing a waterfall is a massive energy drain. In the harsh environment of a cave, every calorie counts.
The prevailing theory is about survival and niche stability. The cave systems in Thailand are prone to flash flooding. If a massive surge of water comes through, most fish get washed downstream, out of the cave, and into open waters where predators are waiting and the environment is totally different. By being able to grip the rock and climb up into faster-flowing or more elevated shallow seeps, Cryptotora thamicola stays exactly where it wants to be.
It’s about staying put by moving up.
A rare glimpse into the past
Biologists get really excited about the waterfall climbing cave fish because it offers a living laboratory for the Devonian period. This was the era, roughly 390 million years ago, when our ancestors first crawled out of the water.
We used to think that the transition from water to land required a very specific set of environmental pressures and a very long time. While that’s still true, this fish shows us that the "walking" blueprint can emerge in specialized environments even today. It’s not a "missing link"—it’s a modern fish that happened to land on a similar evolutionary solution.
It’s a specialist. It’s not trying to become a land animal. It’s perfectly happy being a fish that just happens to have the hips of a land-dweller.
The Mae Lana and Tham Susa connection
These fish aren't everywhere. They are endemic to a very specific set of caves within the Salween River basin. Specifically, the Tham Susa and Mae Lana caves.
If you ever found yourself in these caves—which is unlikely, as they are strictly protected—you’d notice the water is incredibly low in nutrients. There isn't much to eat. These fish likely scrape biofilms and microbes off the rocks. That’s another reason for the climbing; the best "grazing" spots might be on the vertical faces where water thin-films provide oxygen and fresh nutrients without the competition found in deeper pools.
The Thai government classifies them as protected. You can’t just go in there with a net. In fact, the locations are kept somewhat low-profile to prevent poaching for the exotic pet trade, though keeping one alive outside its cave environment would be nearly impossible for a hobbyist.
Misconceptions about "walking" fish
People often confuse this fish with the walking catfish (Clarias batrachus). They aren't the same. Not even close.
👉 See also: Why Pleated Wide Leg Pants Womens Styles Are Actually Taking Over Your Wardrobe
- Walking catfish "crutch" themselves along. They use their pectoral fins to pivot and wiggle their tails to push forward. It’s clumsy. It’s a desperate move to find a new pond.
- The waterfall climbing cave fish is graceful. It has a steady, rhythmic gait. It can move against a vertical flow of water that would sweep other fish away.
- The climbing fish doesn't breathe air in the same way. While it can survive in the damp "splash zone" of a waterfall, it still needs its gills kept wet.
It’s the difference between a person dragging themselves across the floor and a rock climber tackling a Grade 5 pitch.
The threat of the outside world
Even though they live deep underground, these fish aren't safe. The karst systems they inhabit are incredibly sensitive.
Agricultural runoff from farms above the caves can seep through the limestone. If pesticides or heavy fertilizers hit that groundwater, it can wipe out the delicate biofilms the fish eat. Because their population is so small and localized, a single pollution event could lead to extinction.
Climate change plays a role too. The monsoons in Thailand are becoming more unpredictable. Since the waterfall climbing cave fish relies on specific water flow rates to feed and move, extreme shifts in the water table can disrupt their entire life cycle.
Practical insights for the curious
If you’re interested in the intersection of biomechanics and evolution, there are a few things you can do to dive deeper into this specific miracle of nature.
Read the source material
Don’t just take a summary for granted. Look up the work of Dr. Daphne Soares or Dr. Brooke Flammang. Their research into the functional morphology of Cryptotora thamicola is the gold standard. They used 3D modeling to show how the muscles actually attach to that unique pelvis.
Understand the Karst landscape
The fish exists because of the geology. Learning about how limestone caves form in Southeast Asia provides context for why these "micro-habitats" produce such strange species. The isolation of a cave is basically an island in reverse.
Support cave conservation
The best way to ensure these fish keep climbing is to support organizations like the Cave Conservationist Society or international groups focused on subterranean biodiversity. Protecting the surface protect the depths.
The waterfall climbing cave fish is a reminder that the tree of life is messy. It doesn't always move in a straight line from "simple" to "complex." Sometimes, it takes a fish, strips away its eyes, and gives it the hips of a lizard just so it can survive a Tuesday afternoon in a flooded cave.
If you want to see the mechanics in action, look for the peer-reviewed footage from the NJIT labs. Seeing the skeletal overlay on the moving fish is the only way to truly appreciate how bizarre this movement really is. It’s not a flop; it’s a march.
To truly appreciate these creatures, focus on the study of functional morphology. It’s the field that explains why things are shaped the way they are. By studying how this fish moves, researchers are gaining insights that could eventually help in the development of soft robotics or better understanding the fossil record of the first land animals. Stay updated on the research coming out of the Florida Museum of Natural History, as they often collaborate on Southeast Asian subterranean biodiversity projects.
Protecting these environments starts with awareness of the groundwater systems. Avoid supporting unregulated tourism in sensitive karst areas, as foot traffic and light pollution can devastate cave ecosystems. Instead, look for virtual tours or documentaries that respect the "no-entry" status of these critical habitats.
The story of the climbing cave fish isn't just about a weird animal; it's about the resilience of life in the most extreme corners of our planet.