Black pacu are tall, narrow fish shaped like a disk or plate. This body shape makes them well-adapted for movement between aquatic plants. They have small eyes and an arched back. The body is dark below and lighter above. This is unusual in aquatic animals which usually exhibit the opposite as it helps with camouflage. Pacu have an adipose fin (a small, fleshy dorsal fin) located behind the dorsal fin. Despite its large size, black pacu are a peaceful, non-aggressive species.

Pacu can reach 3 feet in length and weigh over 65 pounds.

Red hook pacu live in the upper Amazon River basin in South America.

Pacu live about 15 years.

Pacu eat fruit. They are generalist feeders but seem to always include plants in their diet. At the Smithsonian's National Zoo, they are fed grapes, banana, papaya, sweet potato, omnivore gel diet, herbivore gel diet, earthworms, shrimp, smelt, squid and herring. They prefer to eat fruits, decaying plant matter, snails and grains as they would in the wild. However, at the Zoo they seem to accept a more varied diet.

Black pacu are spawning fish, with the young living within floating vegetation in the black waters of the Amazon River basin.

Pacu are caught for food in South America and are bred in fish farms for food and the pet trade. Many species of pacu are imported for the aquarium trade as piranha.

This confusion often leads to release in nonnative waters as they quickly outgrow their aquariums. Pacu have been introduced to United States in rivers from Washington to Florida.