Piranhas have a reputation as vicious killers, but you're unlikely to encounter one swimming in your local river. These creatures are generally isolated to one part of the world, and it isn't the United States. Still, the varying legality of keeping piranhas as pets means that they sometimes turn up outside of their typical habitat, so they do occasionally lurk in North American waters.
Typically, piranhas in the wild live only in South America -- this is their natural habitat. They inhabit rivers and basins connected to the ocean, particularly the Amazon, the Guyana, the Essequibo and other coastal rivers. They're common in northeastern Brazil. They prefer loamy whitewater rivers to blackwater rivers.
One of the primary reasons piranhas thrive in these coastal rivers of South America is their feeding habits. Piranhas don't only kill and eat living animals, they scavenge for carcasses and feed on whatever meat they can find. Because of this, they benefit from pororoca, a natural phenomenon in which oceanic tides course into coastal rivers. When the waters rise like this, it drags animals living and dead into the river, where piranhas can feast on them.
Strength in Numbers
Piranhas are also notably found in groups -- they don't like to swim solo. Instead, they'll form schools of about 20 to 30 fish. They work as a team to scavenge for food or lay in wait for prey to pass through the water. When prey, especially wounded prey, passes by, the piranhas chase and swarm it, attacking all at once and devouring it quickly.
Legality as Pets
While a piranha's natural habitat is limited to coastal rivers of South America, they can have been found in the United States. In some states, piranhas are legal to keep as pets, and are occasionally -- illegally -- released into the wild. Because piranhas require relatively warm water temperatures to survive, though, they won't typically live long in cool parts of the country. If the water is colder than 54 degrees Fahrenheit, they won't eat and will eventually die. For that reason, states like Pennsylvania allow the purchase of piranhas as pets because, even if they're released into the wild, they won't survive long.
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