The X-Ray Fish, Pristella maxillaris, represents one of the most intriguing species in the freshwater aquarium trade. This small, peaceful fish is a member of the family Characidae, which includes many popular tetra species. Native to South America, the X-Ray Fish is recognized globally for its unique physical characteristic that sets it apart from its relatives.
Defining the X-Ray Fish
The species Pristella maxillaris is commonly known as the X-Ray Tetra, Water Goldfinch, or Golden Pristella Tetra. It is a relatively small fish, typically reaching a maximum total length of about 4.5 centimeters. The body shape is fusiform, slender and tapered at both ends, which is typical for many active swimming species.
Its general coloration features a subtle yellowish-silver or gold tint across the body, which is faint enough to contribute to its translucent appearance. The fins provide the most distinct markings, with a pattern of black, yellow, and white bands visible on the dorsal and anal fins. A small, dark spot near the gill area, known as a humeral spot, is also present on its side.
The Biological Mechanism of Transparency
The remarkable transparency that gives the X-Ray Fish its common name is a result of specific biological adaptations in its integumentary system. This translucency is achieved due to the near absence of certain pigment cells, iridophores and melanophores, in the skin and peritoneal tissues. Iridophores are responsible for the light-reflecting, silvery-white appearance seen in many fish, while melanophores contain black pigment.
The lack of these light-absorbing and light-reflecting cells allows light to pass directly through the soft tissues of the body. This optical clarity makes the underlying skeletal structure and internal organs, particularly the spine and swim bladder, clearly visible. This anatomical visibility creates the signature “X-ray” effect, a form of cryptic camouflage that offers an evolutionary advantage in its natural environment. By being nearly invisible in clear water, the fish can better evade visual predators.
Natural Habitat and Social Behavior
The X-Ray Fish originates from the vast river systems of South America, primarily inhabiting the Amazon and Orinoco river basins, as well as coastal rivers of the Guianas. This species demonstrates a high degree of environmental adaptability, thriving in both clearwater and blackwater tributaries. They are found in slow-moving or still waters, often where there is dense aquatic vegetation or submerged roots.
The X-Ray Fish is an omnivorous micropredator, and its diet in the wild consists mainly of small invertebrates, insect larvae, worms, and tiny crustaceans. As a social species, it exhibits strong schooling behavior, moving in coordinated groups that offer protection and enhance foraging efficiency.
Keeping X-Ray Fish in Aquariums
The X-Ray Fish is a popular and hardy choice for freshwater aquarists due to its peaceful nature and adaptability to different water chemistries. A minimum tank size of 20 gallons is generally recommended to provide adequate swimming space for a small school. They exhibit their best behavior and appearance when kept in groups of six or more individuals.
The aquarium setup benefits from dense planting and a darker substrate, which mimics their natural blackwater or heavily vegetated environments and encourages them to display their natural colors. Their omnivorous diet is easily satisfied with a combination of high-quality flake food, micro-pellets, and supplemental frozen or live foods like brine shrimp.