You can literally see right through this fish, and that’s part of its genius. The X-ray tetra, Pristella maxillaris, has a translucent body that reveals its skeleton and swim bladder, a natural sleight of hand that helps it vanish into the dappled light of its home waters. That transparency gave rise to its common name and a starring role in countless children’s science books as the “X-ray fish,” but there’s more to this tiny characin than a cool party trick.

Pristella hails from the floodplains and lowland rivers of northern South America, especially the Guianas and parts of the Amazon–Orinoco region. It’s a fish built for change: during the rainy season, vast forests flood and become temporary nurseries; in the dry season, channels shrink back to meandering creeks and lagoons. X-ray tetras form tight schools that sweep the midwater in these shifting habitats, using numbers and near-invisibility to confuse predators. Unusually for a tetra, they also tolerate slightly brackish water near coastal deltas, a testament to their flexibility in a landscape where freshwater and tides blend.

The species’ signature look is a set of high-contrast “flags” on otherwise ghostly fins: bold black and yellow bands tipped in white on the dorsal and anal fins, often echoed in the caudal fin. In tannin-stained or silty water, those flashes act like visual beacons that help schoolmates keep formation and may double as a warning or courtship signal. The rest of the fish stays understated—minimal pigment, delicate silver sheen—so the silhouette melts away while the signals stay crisp where they matter.

In the wild, X-ray tetras spend their days picking at tiny drifting fare—microcrustaceans, insect larvae, and other planktonic morsels—feeding strategy aligned with constant movement in the water column. When the rains come, they scatter eggs among submerged grasses and leaf litter, offering no parental care but banking on numbers and timing. The floods create a nutrient pulse and a maze of cover for fry, and the species’ quick maturation dovetails with habitats that appear and disappear on a seasonal clock.

This little characin has been known to science for over a century, and it has become a quiet ambassador for South American floodplain ecology. Most individuals seen in aquaria today are captive-bred, and selective breeding has produced “gold” or paler strains alongside the classic transparent form. Researchers and educators alike have leaned on Pristella as a living example of transparency as camouflage, schooling dynamics, and how animals adapt to highly variable environments.

If you’re looking for a fish with a story, the X-ray tetra wears its biology on its sleeve—or rather, beneath it. From brackish estuaries to tea-colored forest creeks, it’s a master of subtlety whose bold fin pennants and see-through body tell a tale of communication, concealment, and survival in waters that never sit still.