The title for the smallest fish in the Amazon is often a contest between several "miniaturized" species, but the Leptophilypnion fittkaui is a top contender. This tiny goby, discovered in the Amazon basin, rarely exceeds 9 to 10 millimeters (0.3–0.4 inches) in length as a fully grown adult. However, a more recent and sensational discovery in 2025/2026 is the Priocharax rex, a miniature tetra found in the Colombian Amazon that measures just 20 millimeters (0.8 inches) but is notable for being almost entirely transparent with "wing-like" skin flaps. These "nanofish" have evolved to live in the "leaf litter" at the bottom of shallow forest streams where larger predators cannot reach them. Because they are so small and transparent, many of these species are only discovered when scientists specifically sieve the mud and submerged leaves of blackwater streams. They are so small that their entire skeleton is often reduced, and they retain many larval characteristics even when they are sexually mature adults.