There is some debate as to whether the Pygocentrus genus actually does contain only three species, as there are several other contenders for membership. The first of these, Pygocentrus palometa, was first described by the ichthyologist A. Valenciennes in 1849. It is listed as occurring in the lower Rio Orinoco, Venezuela but little further information is available. Although the scientific name is still listed as valid, it is highly likely that this P. palometa is actually a synonym of Myleus schomburgkii, a member of the silver dollar family. The second species, held by many to be a valid Pygocentrus, is P. ternetzi. This issue is yet to be settled conclusively but current scientific thinking classes this fish as a yellow color form of P. nattereri. Finally, some ichthyologists believe that species most commonly sold as the red belly is in fact a species known as P. altus and that the true P. nattereri species is never exported from what is believed to be its relatively small geographic range. This theory has not found a great deal of acceptance amongst piranha enthusiasts, with most believing that P. altus is simply a junior synonym of P. nattereri. In any event, these misunderstandings highlight the need for further study of this group of fish.

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There are five genera of fish currently recognized under the broad definition of piranha species. These are all fish which fall within the taxonomic subfamily of Serrasalminae. This article is concerned with the two genera most widely recognized and sold as piranha: Serrasalmus and Pygocentrus.
The two genera themselves are relatively easy to tell apart. Pygocentrus species are the classic round-shaped, bulldog-jawed species. The Pygocentrus genus is generally held to be made up of only three recognized species; P. piraya, P. cariba (often referred to incorrectly as P. notatus) and P. nattereri. The last of these is, of course, the famous red bellied piranha and is the species most commonly seen in nature documentaries and horror films. The glaring inaccuracies and sensationalism of some documentaries can make it difficult to tell these two types of film apart, highlighting the degree to which these fascinating fish are misunderstood.

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