In a remote corner of Colombia’s Amazon rainforest, researchers have identified a tiny fish species with a body structure never documented before. The fish, named Priocharax rex, is just under an inch long, but its most striking feature is a thin, disc-like flap of skin between its pelvic fins, forming a curtain-like shape beneath its body.
This discovery challenges the idea that very small animals tend to lose body complexity over time. Instead, the fish appears to have developed an entirely new structure — something researchers didn’t expect to find in such a miniature creature.
The details of the find were published in 2025 by George Mendes Taliaferro-Mattox, a fish expert at the Federal University of São Carlos in Brazil. He has spent years studying tiny freshwater species across South America.
Strange skin flap sparks curiosity over its purpose
While the exact function of this belly flap remains a mystery, early theories suggest it could help the fish move steadily along the streambed or improve its awareness of its surroundings. Some scientists also believe the flap might be used during mating or as a form of communication, but more testing is needed.
So far, researchers have only studied preserved specimens. To better understand how the structure works, future studies will require observing live fish in their natural habitat or under lab conditions.
Priocharax rex belongs to a little-known group of freshwater tetras that live in calm, shallow waters across the Amazon and Orinoco river systems. These fish are often found near the bottom, feeding on small insect larvae and serving as prey for larger species. Though small in size, they play a key role in keeping the food chain balanced.
New discovery highlights importance of small species
This species was collected in a blackwater stream near San Rafael, southern Colombia. The area is known for its dark, leaf-stained waters and dim lighting. Priocharax rex was found in shallow spots, hidden among tangled roots and leaf litter. When alive, it appears almost see-through, with large eyes adapted to low light.
Researchers describe the fish as “paedomorphic,” meaning it keeps some features from its juvenile stage into adulthood. These include a soft, flexible fin structure and a simple skeletal system, traits often found in other tiny fish species. The strange belly flap adds to this list of unusual characteristics.
With such a small geographic range in Colombia’s Putumayo basin, the fish could be vulnerable to changes in its environment. Deforestation and infrastructure projects pose a risk, yet species like Priocharax rex are often ignored during environmental reviews.
By studying and documenting these hidden species, scientists aim to shine a light on the lesser-known animals that contribute to the Amazon’s complex ecosystem — and why protecting even the smallest habitats matters.