Arapaima have broad, bony heads, upturned mouths and streamlined bodies with a dorsal fin stretching along their backs toward their tails, which are massive, yet stumpy in appearance. The head of the arapaima is copperish-green in color, their bodies are black with a white center and their tails are red. In Brazil, they are known as "pirarucu," a word from the Tupi language, which translates roughly as "red fish." In Peru, they are known as "paiche." When the rivers overflow, fish can be dispersed into the surrounding forest for miles, sometimes to remote areas where floodwater contains so much decaying vegetation that oxygen levels are too low to support most fish. This predicament does not affect the arapaima, as it has the ability to breathe air. Its gills are so small that it takes in air using a modified swim bladder that opens into the fish's mouth and acts as a lung. It can survive as long as 24 hours outside the water.

The arapaima is possibly the largest freshwater fish in the world, reaching up to 440 pounds (200 kilograms) and 10 feet (3 meters) in length. However, sizes of 200 pounds (90.7 kilograms) and 7 to 8 feet (2.2 to 2.4 meters) in length are more common.

The arapaima is found in Brazil, Peru and Guyana. They live in the slow-moving and typically oxygen-deficient rivers of the Amazon River basin floodplain.

The arapaima can live for up to 20 years.

In the wild, the arapaima eats mostly fish but is also known to eat fruits, seeds, insects, birds and mammals found on the surface of the water. In order to eat, they use a "gulper" feeding strategy: by opening their large mouths they create a vacuum that pulls in nearby food objects. Their tongues and sharp, bony teeth, combined with the teeth on the roof of their palates, allow them to debilitate and shred their prey. The arapaima can easily feed in low-oxygen areas where gill-breathing fish are forced to slow down. They can also use short bursts of speed to attack potential prey hitting the surface of the water. At the Smithsonian's National Zoo, their diet consists of herring, squid, smelt, shrimp, fruit and gels.

During the dry season between February and March, arapaima lay thousands of eggs in nests about 28 inches (70 centimeters) long and 10 inches (25 centimeters) deep hollowed out in the sand by their fins. The eggs hatch at the beginning of the wet season (between October and November), which allows for the fry (baby fish) to gather food. It is also thought that mouth incubation takes place. The male protects the fry by drawing them into his flat, long mouth and moving them to another location if predators get too close. Sexual maturity is reached after about 4 or 5 years of age.

In the 1970s, the arapaima population began to decline due to overfishing until it went commercially extinct near major Amazonian cities. Pesticides and the amount of other animals dependent on aquatic habitats have also contributed to their decline.

A favorite food amongst the Amazonian natives, the arapaima is harpooned not only to be eaten but also for its scales, which can reach 4 inches (10 centimeters) in length and are used for jewelry. Their bony tongues can also be used as a scraper. In Brazil, commercial fishing of wild arapaima is banned, but they are being bred with some success in indoor ponds in Germany and artificial lakes in Peru. In 2011, arapaima (known as paiche in Peru, where it has been farm-raised and certified sustainable), appeared on the menu of several prominent U.S. restaurants.