The X-ray fish, scientifically known as Pristella maxillaris, is a small species of freshwater fish popular in the aquarium trade due to its hardiness and peaceful nature. This fish derives its common name from a translucent body that permits a view of its skeletal structure and internal organs. While commonly seen in home aquariums, the wild origins of the X-ray fish are rooted in diverse aquatic environments.
Geographical Distribution
The X-ray fish is endemic to a large area of Northern South America, establishing a notably wide distribution for a single species of tetra. Its native range encompasses the major river systems of the continent’s northern region. This includes the massive Amazon River Basin, extending into the lower Brazilian Amazon, and the extensive Orinoco River Basin in Venezuela. The species is also found throughout the coastal drainages of the Guianas, including Guyana, Suriname, and French Guiana.
Native Habitat Characteristics
In the wild, Pristella maxillaris favors environments characterized by slow-moving or still water, which provide refuge from strong currents. These habitats include the quiet tributaries of major rivers, densely vegetated swamps, and the flooded forests of the savannah. During the annual rainy seasons, the fish often migrates into temporary, flooded areas to spawn among the submerged vegetation.
The water conditions are variable, as the fish is known to tolerate both acidic and alkaline waters, adapting to a pH range between 6.0 and 8.0. Its preferred natural state involves soft, acidic water stained brown by decaying organic matter. This natural “blackwater” effect, caused by tannins and humic acids from leaf litter and wood, helps camouflage the fish.
The water is typically tropical, remaining warm with temperatures ranging from 24°C to 28°C. Uniquely among many tetras, the X-ray fish can occasionally be found in slightly brackish water near coastal areas. The presence of dense aquatic or riparian vegetation is a consistent feature, offering cover from predators and serving as a foraging ground for small insects and planktonic animals.
Distinctive Physical Traits
The most defining feature of Pristella maxillaris is its semi-transparent body, which gives the fish its popular name. The body itself is small and streamlined, reaching a maximum length of about 4.5 centimeters. This transparency functions as a form of camouflage, making the small fish difficult for predators to spot in the dark, vegetated waters of its natural habitat.
While the body is mostly clear, it possesses a faint yellowish or silvery-golden tint. The dorsal, anal, and caudal fins display a striking combination of black, white, and yellow markings. Specifically, the dorsal fin features a prominent black spot lined with white and yellow, and the pelvic and anal fins have symmetrical bands of the same colors.
The fish also has a small, round black spot, called a humeral spot, located just behind the gill cover.