Source: https://commons.wikimedia.org/wiki/File:Pristella_maxillaris.jpg

The X-ray fish, X-ray tetra (Pristella maxillaris) is freshwater fish species with translucent body. It is a widely distributed and adaptable fish, found in the Amazon and Orinoco basins, as well as coastal rivers in the Guianas in both acidic and alkaline waters. Unlike most other characins, it is tolerant of (and sometimes found in) slightly brackish water. It is small (up to around 5 cm in in length) and lives in large groups, and males can be distinguished from females by being smaller and thinner than the females. Like most other tetras, it feeds primarily on small insects and planktonic animals.
Order: Characiformes
Family: Characidae
Genus: Pristella
Species: Pristella maxillaris (Ulrey, 1894)
Synonyms:
Aphyocarax maxillaris Ulrey, 1894
Holopristis riddlei Meek, 1907
Pristella riddlei (Meek, 1907)