Pristella maxillaris, the only species in the genus Pristella, is commonly known as the X-Ray fish or X-Ray Tetra because of its translucent body. The X-Ray Tetra is a small species of schooling fish that is naturally found in the Amazon River coastal waters in South America.

They were first described by Ulrey in 1894 and have since become one of the most popular freshwater fish kept in artificial aquariums today. Although the X-Ray Tetra is the only known species in its genus, it is closely related to other small and colorful South American fish, including the nearly 100 other tetra species.

The most distinctive feature of the X-Ray Tetra is the translucent layer of skin that covers its small body, allowing the fish backbone to be clearly seen. The scales of the X-Ray Tetra are a silvery-yellowish color that is very faint, looking almost golden in some lights.