A-z - Animals

X-ray Tetra

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The X-Ray Tetra is a small school of fish that occurs naturally in the waters along the Amazon River in South America. X-ray Tetra fish are also known as Golden Pristella Tetra and Water Goldfinch because of their translucent skin which is a pale golden color.


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Scientific name and classification

First described by Albert Ulrey in 1894, the X-Ray tetra has since become one of the most popular freshwater fish in aquariums today. Although the X-Ray tetra ( Pristella maxillaris) is the only known species in the genus, it is closely related to other small, colorful South American fishes, including nearly 150 other tetrafish species. Among these other types of tetra are:

  • Neon Tetra (Paracheirodon innesi)
  • Rainbow Tetra (Nematobrycon lacortei)
  • Bloodfin Tetra (Aphyocharax anisitsi)
  • Nematobrycon palmeri
  • Buenos Aires Tetra (Hyphessobrycon anisitsi)
  • Lemon Tetra (Hyphessobrycon pulchripinnis)
  • Ember Tetra (Hyphessobrycon amandae)
  • Congo Tetra (Phenacogrammus interruptus)
  • Neon black (Hyphessobrycon herbertaxelrodi)
  • Ruby Tetra (Axelrodia riesei)

For descriptions and pictures of these ten species, click here.

anatomy and appearance

The X-ray tetra, Pristella maxillaris , is named for its almost transparent skin.

© Debivort at English Wikipedia/Creative Commons

The most notable feature of the X-Ray Tetra fish is the layer of translucent skin covering its small body, which allows the fish's spine to be clearly seen. The scales of X-Ray Tetra are a very faint silvery yellow that can appear almost golden in certain lights. The X-Ray Tetra also has a red tail and strikingly striped dorsal and anal fins in yellow, black and white.

This is a relatively small fish (1.5 inches) that actually has a skeletal internal structure called a Weber apparatus that picks up sound waves and aids x-ray fish's keen sense of hearing (this skeletal structure is are also found in many of their relatives).

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Females are generally slightly larger and rounder than the more slender males, although the two are very similar in appearance.

Distribution and Habitat

X-Ray Tetra occurs in the Amazon coastal waters of Brazil, Guyana, and Venezuela in South America. They are quite different from other Tetra species in that they are able to tolerate the harder salt water closer to the coast as well as their usual freshwater environment. During the dry season, they inhabit clear streams and tributaries, and as the rainy season arrives, X-ray fish move to flooded swamps, where the water is softer and more acidic. X-Ray Tetra breed during the rainy season because their water conditions are better and food is more plentiful.

Behavior and Lifestyle

The X-ray tetra is one of the most adaptable species of Tetra as it can inhabit both fresh and brackish water.

© Richard Bartz / Creative Commons

Like many other small, colorful fish, the X-Ray Tetra is a schooling fish that lives in the area between the bottom and the water. They are very peaceful and tolerant of other species in their habitat. X-Ray Tetra is one of the most adaptable species of Tetra as it can inhabit fresh and brackish water under both acidic and alkaline conditions. It has been widely observed that those x-ray fish kept in tanks quickly change from calm to timid when encountering larger predatory fish, and exhibit the same behavior if the school size is not large enough.

Interesting Facts and Features

The Weber device (bone structure) inside the X-Ray Tetra works by transmitting the sound waves received by the swim bladder, sending the sound waves through the vertebrae, and then carrying the sound waves to the inner ear. The end result is X-Ray Tetra with excellent hearing.

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The transparency of the skin is thought to be a form of protection, as predators find them (along with their buff markings) harder to spot in dense vegetation and sparkling water. Not only is the yellowish X-ray Tetra popular in tanks, but albino versions of this species are now common in captive communities around the world as well.

Reproduction and Life Cycle

During the rainy season, X-Ray Tetra returns to floodplains to spawn. Unlike many similar fish that lay live offspring, female X-Ray Tetra lay 300-400 eggs by scattering them in vegetation. When it's ready to lay eggs, the transparent skin means the eggs can be easily seen too. X-ray Tetra fry start hatching as early as 24 hours and are free swimming within a few days. Once able to swim, the small, sluggish white pups find better food sources and quickly develop their characteristic adult markings. X-ray fish typically live three to four years in the wild, but can live much older in captivity.

Neon tetras are shallow water fish that do best when kept in groups of 6 or more.
X-ray Tetras can be kept in the same aquarium with other small fish such as these neon tetras.


diet and prey

Like many other small fish that live in the Amazon, the X-Ray Tetra is an omnivore, with a diet consisting of both animals and plants. X-Ray Tetra preys primarily on worms, insects and small crustaceans that live near stream beds. Its fry feed on insect larvae. The chickadee is primarily a microscopic predator that feeds on small invertebrates, although it is also known to supplement its diet with aquatic plants. In captive communities, X-Ray Tetra requires a variety of food sources, including brine shrimp and bloodworms as well as standard flakes and pellets, to ensure it has an adequate nutritional diet.

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Predators and Threats

The small size of the X-Ray Tetra means it should not be kept in the same aquarium as larger predatory fish, but it can coexist peacefully with other small fish that do not pose a threat. In the wild, the X-Ray Tetra is preyed upon by many aquatic predators, including larger fish and frogs, and is threatened by birds and snakes if it is near the surface. Its populations are thought to be most threatened by increased pollution and habitat loss.

relationship with humans

Since it was first described in 1894, this unique animal has attracted attention for its nearly transparent body. X-Ray Tetras are now one of the most popular tropical fish in home tanks, primarily because they are hardy, adaptable and undemanding, making them easy to care for. The rapid reproduction rate is also believed to be one of the reasons why this fish is so popular. Today, aquarium X-Ray Tetra are not sourced from the wild, but are commercially farmed for trade, mainly in Eastern Europe and the Far East.

Protect the status quo and life today

X-Ray Tetra is not yet listed on the IUCN Red List as Threatened in its environment. However, increased industry in the Amazon Basin means increased levels of water pollution and therefore lower water quality, leading to slight declines in population in some areas.

See all 13 animals that start with X

X-Ray Tetras are omnivores, which means they eat both plants and other animals.

X-Ray Tetras belong to Kingdom Animalia.

X-Ray Tetras belong to the phylum Chordata.

X-ray Tetras belong to the Family Characidae.

X-Ray Tetras belong to the order Characiformes.

X-Ray Tetra is also known as golden pristella tetra or water goldfinch.