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The giant of freshwater fishes, the arapaima or pirarucu (Arapaima gigas) is a legend among fish. Weighing up to 440 lb (200 kg), this species, which has only been recorded in the rivers of Brazil and the Guianas, may reach a length of some 16.5 ft (5 m), although most specimens today are less than 10 ft (3 m) long. The origins of the arapaima, which belongs to the bony-tongued fishes (Osteoglossidae), date back to the Cretaceous period (some 65–135 million years ago); it is one of just five remaining species of this ancient group.

In appearance the arapaima exhibits many archaic characteristics, such as an asymmetrical tail fin and a swim bladder that also functions as an air-breathing organ. Only very young arapaima have functional gills; adult fish always come to the surface to breathe in oxygen and expel carbon dioxide, usually at intervals of 10–15 minutes.

Arapaima are active predators and often seek out fish in pools that are drying out, or backwaters where the fish are slowly being starved of oxygen. Captured prey are held in the jaws and the toothed tongue is then used to press and grind the hapless prey against the roof of the mouth. When prey is abundant, arapaima gorge themselves on fish, laying down rich fat deposits that will see them through the breeding season. Reproduction is marked by vivid changes in color and the pairing of adult fish. Both male and female participate in excavating a small hole in the substrate, usually in shallow water and well concealed by vegetation. When the nest site is completed, the female proceeds to lay her eggs, which are then fertilized by the attendant male. Both parents remain in the vicinity of the nest to ward off potential predators. Upon emerging from the eggs, the young fish remain close to one or both of the adults until such time as they are able to fend for themselves. Despite such lavish parental attention, the predation rate on young arapaima is thought to be considerable.

Prior to the nineteenth century, this species was seldom captured as neither the techniques nor the means of preserving such a large amount of food were available. In recent decades, however, the introduction of steel-tipped harpoons and gill nets have resulted in large catches of this giant fish. Smoking and salting techniques have also developed, enabling people to store larger quantities of fish for longer periods. As a result, this species has developed as one of the most important fish caught for food throughout the Amazon basin. No estimates of the population size of this species exist, but scientists have expressed concern for its future as a result of increasingly heavy hunting pressures in some regions.

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