Head to head
Red-Bellied Piranha vs Pacu
Piranhas and pacus belong to the same family of South American fish and share the same warm river basins, and a young pacu is regularly sold in aquarium shops as a piranha, or bought by people who want a piranha and do not know the difference. But their teeth tell you they make a living in opposite ways, and a pacu grows into a very different animal.
| Red-Bellied Piranha | Pacu | |
|---|---|---|
| Family | Serrasalmidae (with pacus and silver dollars) | Serrasalmidae (with piranhas and silver dollars) |
| Teeth | One row of interlocking triangular razor teeth | Squared, flat, human-like molars for crushing |
| Diet | Fish, insects, fruit, and carrion; a scavenger and omnivore | Seeds, nuts, and fruit; a seed disperser |
| Adult size | 15 to 30 cm | Up to about 1 m and 25 kg |
| Jaw | Prominent underbite | Jaws roughly even; less of an underbite |
| Behaviour | Shoals for defence; feeding frenzies are rare and stress-driven | Usually solitary or in small groups; not aggressive |
| Relationship with people | Occasionally bites; reputation far exceeds the reality | Farmed worldwide for food; now invasive in many countries |
| In the aquarium trade | Sold as a novelty; illegal to keep in some places | Often sold small as a "vegetarian piranha"; quickly outgrows tanks |
Piranha vs Pacu: common questions
How do you tell a piranha from a pacu?
The mouth is the giveaway. A piranha has a distinct underbite, with the lower jaw jutting forward, and its teeth are pointed, triangular, and interlock like the blades of scissors. A pacu has a less pronounced underbite, more even jaws, and broad, flat, squared-off teeth that genuinely look like human molars, sometimes unnervingly so. Size and body shape help too: a piranha stays fairly small and keeps its deep, chunky body, while a pacu keeps growing, often to the size of a dinner plate or much larger, and becomes more oval and heavy. In the aquarium trade, a "piranha" that is placid, growing fast, and has odd flat teeth is a pacu.
Are piranhas actually dangerous to swim with?
Far less than films suggest. Piranhas are opportunistic feeders and scavengers, and healthy people swim, bathe, and wash clothes in piranha-inhabited rivers throughout the Amazon basin with little trouble. Bites do happen, most often as single nips to the feet or hands, and they are concentrated in the dry season, when falling water levels trap large numbers of hungry fish in small pools, and around nests, which piranhas defend. The idea of a shoal stripping a large animal to the bone in minutes describes a rare, extreme situation involving an already dead or dying animal and dense, food-starved fish, not normal piranha behaviour.
Why is the pacu found in rivers all over the world?
Because it is a good aquaculture fish and a common aquarium purchase, and both routes leak fish into the wild. Pacu grow fast on a cheap plant-based diet, tolerate a range of conditions, and taste good, so they are farmed for food across South America, Asia, and Africa, and farmed fish escape. At the same time, pet shops sell young pacu, often mislabelled, and owners who did not expect a fish that reaches a metre release them into local lakes and rivers. As a result pacu now turn up in waterways in the United States, Europe, and Asia, where in cool climates they usually cannot survive the winter, but in warm regions they can establish and compete with native fish.