The Difference Between a Piranha & a Pacu

Notorious for feeding frenzies that can strip animals to the bone in minutes, piranhas evoke fear in all who encounter them. A school of piranhas might have 1,000 specimens, all competing for the same food sources. Their main diet isn't large animals, though. They stick mostly to smaller animals that live in or near the water, as well as snails, plants and fruit. They engage in feeding frenzies only under starvation conditions, in most cases. When food is scarce, they've been known to cannibalize each other. Pacus, on the other hand, are considered herbivores, relying mostly on aquatic vegetation as well as fruit and seeds that fall into the water. They occasionally feast on snails or other insect, but vegetative matter is their preferred food source.

While piranhas tend to gather in groups, pacus aren't big on being social. They can become increasing independent and territorial as they grow. After mating, the parents go their separate ways, allowing the eggs to hatch on their own and the young to fend for themselves. Piranha parents, meanwhile, stick close to their eggs, which helps 90 percent of the eggs survive to hatching. Piranha young cluster together under aquatic plants for protection; when they're old enough to survive outside the plant shelter, they join larger schools of piranhas.

The easiest way to tell a pacu from a piranha is size. A pacu can top 30 inches long, usually sticking to about 28 inches, and can weigh up to 50 pounds. Piranhas are much smaller, usually less than 17 inches long and topping out at about 7½ pounds. If you decide to swim in tropical rivers, giant piranha-looking fish aren't as much of danger – they're likely pacus.

When looking at an adolescent pacu and an adult piranha -- which can be about the same size and shape -- an easy way to tell them apart is to check out their teeth. Piranha teeth are small, triangular and razor-sharp. They're strong enough to bite through silver fish hooks. Pacu teeth are short and blunt, resembling human teeth. These are designed to crush seeds and nuts, and are not powerful enough to inflict damage if they bite an animal. The difference is they aren't trying to feed on the animal like a piranha might; they're likely to bite only if provoked.

While the U.S. government allows import of fish such as pacus and piranhas, many states outlaw importing or owning such fish. People who have brought them in as pets can cause environmental problems if they later release them into the wild. Piranhas have been found in U.S. rivers, but these fish don't pose much of a problem; they tend to die off when the winter cools the water. Pacus, however, seem to survive in American rivers better than piranhas. These fish are considered invasive in many states, needing so much food to sustain their size that they squeeze out indigenous species.