Pacu fish are large freshwater species native to South America, often recognized for their deep, plate-shaped body and unique dental structure. They belong to the family Serrasalmidae, which also includes the much smaller and more infamous piranhas, making them close relatives in the aquatic world. The common name “pacu” is applied to several different genera, highlighting a group of fish that share a similar appearance and general ecological niche in their native rivers.
Defining Physical Traits and Origin
Pacu species are classified under the family Serrasalmidae, a group of fish that are predominantly found in the tropical and subtropical waters of South America. The most commonly recognized pacu species fall under genera such as Colossoma and Piaractus, which are found primarily across the vast Amazon, Orinoco, and Paraguay-Paraná river basins. A defining feature of the pacu is their potential for massive growth, which far surpasses that of most other aquarium fish. Large species, such as the black pacu (Colossoma macropomum), can reach lengths exceeding 3 feet (1 meter) and can weigh up to 88 pounds (40 kilograms). Their body shape is deep and laterally compressed, often described as round or plate-shaped, with mature specimens typically displaying a dark or black coloration.
Diet and Distinctive Dental Structure
The most remarkable feature of the pacu is their dentition, which is perfectly adapted for their largely herbivorous diet. Unlike the triangular, razor-sharp teeth of their cousins, pacu possess square, blunt teeth that bear a striking resemblance to human molars. This unique dental plate is designed not for tearing flesh but for crushing and grinding tough materials, earning them their reputation as powerful crushers. This specialized dental structure is used to process the tough fruits, seeds, and nuts that fall from the rainforest canopy into the water. Pacu play an important role in their ecosystem by consuming these items and then dispersing the seeds through their waste, though they are technically omnivorous and will opportunistically consume smaller invertebrates or fish.
Differentiation from Piranhas
Pacu are frequently mistaken for piranhas, but a closer examination reveals clear biological differences that define their distinct ecological roles. The most obvious contrast lies in their teeth: piranhas have a single row of interlocking, pointed, and razor-sharp teeth designed for a shearing bite, whereas pacu have two rows of flat, molar-like teeth built for crushing. This difference in dental morphology reflects their primary feeding strategies. In terms of diet, piranhas are primarily carnivorous, scavenging or actively hunting fish and other small animals. Pacu, by contrast, are mainly frugivorous and herbivorous, consuming plant material as their main food source.
Pacu in the Aquarium Trade
The pacu’s journey into the aquarium trade begins with their small, attractive juvenile size, making them a popular choice for inexperienced fish keepers. However, the black pacu and red-bellied pacu grow exceptionally fast, quickly outgrowing all but the most massive home aquariums. An adult pacu requires a tank that can hold hundreds of gallons of water, a commitment few hobbyists can maintain. This rapid growth often leads to the fish being released into non-native waterways by overwhelmed owners, posing a serious ecological threat as they compete with native species for food resources. Reports of pacu caught in temperate climates across the United States and Europe underscore the problem of improper disposal. While tropical fish cannot typically survive cold winters in temperate zones, their introduction carries the risk of establishing permanent populations in warmer regions.