A fingernail-sized fish living in a shaded Amazon stream has astonished scientists with an organ that has never been recorded before. Researchers working in the Colombian Amazon have described a new species called Priocharax rex, only about 0.8 inches long, but with a strange skin “wing” on its belly that no other known fish possesses.
The discovery, published in the journal Zootaxa in 2025, suggests that even miniature species continue to evolve new and complex structures rather than simply losing parts of their anatomy over time.
The research team was led by George Mendes Taliaferro-Mattox, a fish biologist at the Federal University of São Carlos in Brazil. His work focuses on miniature freshwater fishes in South America and how their bodies change when they stop growing earlier than most species.
The new fish was found in a forest stream feeding into the middle reaches of the Río Putumayo in southern Colombia. It lives in shallow blackwater, close to submerged roots, fallen leaves and dark stream beds where light remains dim even during daylight hours. In this habitat, Priocharax rex shows pale, almost transparent skin and unusually large eyes, which help it survive near the bottom of the forest water channel.
A tiny fish with an unexpected feature
The central mystery of the study is a rounded, curtain-like flap of skin stretching between the bases of the fish’s two pelvic fins. It hangs beneath the body like a wing or disc, and cannot be explained by any known structure in other related species.
Scientists do not yet know its role. Since only preserved specimens have been examined so far, researchers cannot confirm whether the structure helps the fish move, attract mates, or detect changes in water flow. Biologists believe it may alter water currents under the belly, provide stability near the stream bed, or possibly function in courtship communication.
This unexpected organ is especially remarkable because Priocharax rex belongs to a group known for body simplification. Members of its genus Priocharax are extremely small tetras that typically retain juvenile characteristics into adulthood. They often have simplified skulls, fewer fin rays, reduced bones, and missing sensory canals. The new species fits this pattern, with a soft, larval-type pectoral fin supported by a flexible cartilage plate rather than stiff skeletal rays.
Scientists say this combination of reduced structures and a completely new feature makes Priocharax rex a unique example of evolutionary experimentation at small body size.
Hidden diversity in Amazon waters
The new species lives in a blackwater forest stream stained brown by decaying leaves. Its habitat undergoes seasonal flooding and recession, creating pools, channels and dense networks of submerged roots where insect larvae thrive. Priocharax rex feeds on tiny invertebrates and serves as prey for larger fish, helping maintain energy flow in the ecosystem.
So far, it has only been recorded at a few sites in the middle Putumayo basin in Colombia. Biologists warn this narrow range makes the species vulnerable to pollution, deforestation along stream banks and road development. Because such miniature fishes often go undetected during surveys, researchers believe this discovery hints at many more undiscovered tiny species in the region.
Evolution at small scale
Miniature fish are often the result of shortened early development. Adults resemble the larvae of larger relatives, and while this process can lead to the loss of structures, it can also create radically new anatomical ideas. The unusual belly wing in Priocharax rex shows that innovation does not vanish at small sizes; instead, evolution continues testing new solutions that allow species to adapt to dark, crowded water channels.
Researchers say documenting such tiny, easily overlooked species is essential to understanding river ecosystems and protecting unique evolutionary lineages. Conserving even small, obscure forest streams can protect habitats that shelter remarkable species found nowhere else on Earth.
The study underscores a basic lesson: the smallest creatures often hold the biggest scientific surprises.