X-Ray Fish

The Basics

X-Ray Fish are small, schooling aquatic fish native to the Amazon basin in South America. In addition to being known as the Golden Pristella Tetra, X-Ray Tetra, and Water Goldfinch, they also have translucent skin that allows their organs and skeleton to be observed, much like an X-ray.

The description

Small, the X-Ray Fish grows to a maximum of 1.9 inches, with females slightly larger than males. It has striking black, white, and yellow stripes on its dorsal and anal fins. As a result of its translucent skin, its bony internal structure can be observed.

Despite being the only member of its genus, the X-ray fish is closely related to more than 100 other Tetra species native to South America. Aside from that, it is one of the most popular freshwater aquarium species in the world.

Distribution and Habitat

Brazilian, Guyana, and Venezuelan coastal Amazon regions are home to the X-Ray fish. The brackish waters in this region make them different from most other tetras. The species can also thrive in more typical freshwater environments, such as streams and tributaries during the dry season, and flooded marshlands during the wet season.

Diet and Predators

X-Ray fish are omnivorous, eating a wide variety of animals and plants. Small worms, aquatic insects, their larvae, and shrimp-like animals are commonly hunted by tetras. It is also possible for them to feed on some plant and algal material as a supplement to their diet, but they are mainly predators.

They are also vulnerable to predation by a variety of other species due to their small size. All tetra species, including the X-ray tetra, are hunted by larger predatory fish. Various species of birds and snakes, as well as amphibians like frogs, also pose threats. 

Therefore, X-Ray fish and other similar species are often found in bottles or at the bottom of water columns to avoid predation by fish that rely on swimming close to the surface for detection.


In the wet season, flooded environments allow the X-Ray fish to access environments that would otherwise be inaccessible to them. As a result, breeding grounds become more accessible and food becomes more readily available. Breeding of the X-Ray fish occurs during this season.

Courtship and Copulation

In contrast to related species that give birth to live young, X-ray fish lay eggs. As the grasslands and marshes are flooded, the X-ray fish scatter 300-400 eggs among the blades of grass and other vegetation. 

As soon as 24 hours after hatching, fry are free-swimming. By this point, they are able to access more food sources, and by about 5-8 months of age are able to develop their characteristic adult markings. In the wild, individuals typically live for three to four years. Some fish in aquariums can live even longer, up to 7-8 years.

One of the reasons they are so popular in aquariums is their high reproduction rate.

Protecting the environment

While the X-ray fish is still considered common, it and its entire ecosystem remain threatened by unprecedented habitat loss. This is mainly due to illegal deforestation practices, often for grazing cattle, that destroy the rainforests. 

In addition to pollution, this is a very real threat to the entire Amazon rainforest, which includes the ecosystems where X-ray fish live.

Fun Facts about the X-Ray Fish!

The fast breeding rate and unique appearance of the X-Ray fish make it one of the most popular aquarium species around the world. However, there are more fun facts and interesting biological concepts to explore with this species besides just its famously translucent skin.

The Weberian Apparatus

One of the most fascinating features of this small fish is a bony structure inside its body known as the Weberian apparatus. This unique structure is seen in other related species and allows for sound waves to be conducted through their vertebrae and thus detected by their inner ear. 

Depending on the species, the Weberian apparatus generally consists of two parts: a movable portion, known as the pars auditum or Weberian ossicles, and a supporting structure, known as the pars sustentaculum, several uniquely modified anterior vertebrae. 

In combination with the air in their swim bladder, this unique structure allows the X-Ray fish excellent hearing which can aid in the avoidance of predators as well as the detection of prey.

Invisibility Cloak

The X-ray fish is known for its translucent skin, hence its common name. But like many morphological features, it has evolved to serve a specific purpose. In this case, it likely acts as a form of predator avoidance as it makes them more difficult to spot by most of their predators. This is due not only to the translucency of their skin but also to the way it reflects light, shimmering much like the aquatic environment in which it is found. 

This, combined with their distinct yellow, black, and white markings on some of their fins, makes them difficult to spot, particularly amongst the grasses and other vegetation at the bottom of the bodies of water they normally inhabit.

The World Traveller

Tetras are a very common species in aquaria. Some have unique features such as iridescent ‘neon’ stripes on their flanks. Others such as the X-Ray fish are known for their translucent skin. In addition, they have a very high productivity rate. They are also relatively long-lived and durable. 

As long as they are provided with a good-sized fish tank with a lack of predatory species and a few others to school with, they can live for up to 8 years long with minimal care and attention. In this way, this small fish native to streams and marshes in coastal South America has been able to find its way around the world.

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