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A newly identified fish species has surprised scientists with an unusual organ never seen before in its group. Discovered in the Colombian Amazon, the tiny fish, named Priocharax rex, measures only 0.8 inches in length but displays a rare disc-like flap of skin between its pelvic fins, forming a curtain-shaped structure beneath its belly.
This previously unknown feature, found in such a small fish, challenges long-held assumptions about miniaturized species. Instead of simply losing complex features over time, Priocharax rex shows that even tiny bodies can evolve entirely new structures.
The finding was detailed in a 2025 study led by George Mendes Taliaferro-Mattox, a fish biologist at the Federal University of São Carlos in Brazil.
Mysterious organ raises questions about function and evolution
Taliaferro-Mattox, who specializes in miniature South American freshwater species, explained that while the purpose of the skin flap remains uncertain, it may play a role in stabilizing movement near the streambed or could aid in sensing the surrounding environment.
Since only preserved specimens have been examined, further research is needed to determine its function. He added that the structure might also be linked to mating behavior or communication, but testing these theories will require live observation and biomechanical studies.
Priocharax rex belongs to a little-known group of South American tetras found across the Amazon and Orinoco basins. Most species in this genus remain under 1.2 inches long and live in calm waters like creeks and floodplain lakes.
These fish are typically found near the bottom, feeding on insect larvae and becoming prey for young predators. They also serve a key ecological role by transferring energy up the food chain.
Unique adaptations set the tiny Amazon fish apart with an unusual organ
This species was collected from a blackwater forest stream near San Rafael in southern Colombia, where water tinted by decaying leaves creates dim lighting, even at midday. The fish was found among submerged roots and debris in shallow areas. When alive, it appears nearly transparent, with large eyes suited for low-light habitats.
Biologists describe Priocharax rex as strongly paedomorphic. Its adult form retains several juvenile features, such as a soft, cartilage-based pectoral fin and a reduced skeletal system, typical of many miniature fish. The belly flap adds to this pattern of developmental alteration.
Because of its extremely limited range in the Putumayo basin, the species may be at risk from habitat changes like deforestation and road construction. Researchers warn that such tiny Amazon fish are often overlooked in environmental assessments, despite their unique evolutionary traits.
By documenting species like Priocharax rex, scientists highlight the biological diversity hidden in remote river systems and the importance of protecting even the smallest habitats.
