Cardinal Tetra (Paracheirodon axelrodi)
Cardinal tetras are a species of freshwater fish native to the upper Orinoco and Negro Rivers of South America. Similarly to the popular neon tetra, they have an iridescent blue line running across the length of its body.
Cardinal tetras are shoaling fish. This serves as a protective measure, as it is harder for predators to single out an individual. The flashing neon blues may act to further confuse predators.
Mike Kamermans on Flickr, Kristof Borkowski on Flickr