bichir, (genus Polypterus), any of about 10 species of air-breathing tropical fishes of the genus Polypterus native to freshwater river and lake systems in western and central Africa. Bichirs are classified in the family Polypteridae, order Polypteriformes.
These fishes are elongated in form with fleshy lobes in the pectoral fins, rounded tails, thick ganoid (hard, rhombic, enameled) scales, and functional lungs for breathing air (in addition to rudimentary gills used to extract oxygen from the water). The dorsal fin is a series of sharp spines, each with a flaglike end cluster of fin rays. The bichir’s upper body is brown, grayish, or greenish, the lower side often white or yellowish. Most species grow to between 30 and 60 cm (11.8 and 23.6 inches) long. Some members of the largest two species, the saddled bichirs (P. endlicherii) and Congo bichirs (P. congicus), grow to lengths of 75 cm (29.5 inches) and 97 cm (38.2 inches) and weights of 3.3 kg (7.3 pounds) and 4.4 kg (9.7 pounds), respectively.
Air at the water’s surface is taken into the bichir’s paired lungs through two spiracles on the top of its head. The bichir’s lungs, as well as those of reedfishes and extinct members of Polypteridae, are the result of an ancient adaptation that may have arisen late in the Silurian Period (443.8 million to 419.2 million years ago). The lungs of these fishes differ from lunglike breathing organs in other fishes, such as the tarpon, which are derived from the swim bladder (or air bladder, an organ that is used for buoyancy in most bony fishes). Polypterid lungs arose from the development of a pouch on the ventral (lower) side of the endoderm. The swim bladder, in contrast, developed along the dorsal (upper) side of the endoderm. Studies suggest that lung development began with the sarcopterygians, a group of primitive bony fishes thought to have given rise to the tetrapods (amphibians, reptiles, birds, and mammals).
Bichirs live along the edges of streams and floodplains. They remain concealed by day and forage at night for worms, aquatic insects, small fishes and amphibians, and other animals. At least one species, the Senegal or gray bichir (P. senegalus), is capable of using its pectoral fins to propel itself across land for short distances—an adaptation that may have evolved to help the species move between temporary ponds that were drying up or hunt terrestrial insects.
Male bichirs leap from the water and twist and dart during courtship before they fertilize a female’s eggs. Bichirs apparently spawn in swamps and floodplains during the rainy season, and young fish have external branching gills and are newtlike in appearance.
Several species are harvested by humans for food and aquaria. All bichir species are considered species of least concern by the International Union for Conservation of Nature and Natural Resources.
Bichir classification is a matter of debate. Some researchers consider bichirs and the reedfish (Erpetoichthys calabaricus), its only living relative, to be chondrosteans (members of subclass Chondrostei) alongside sturgeons and paddlefishes. Others argue that bichirs may be more closely related to the crossopterygians (the group of lobe-finned fishes that includes living coelacanths). Most classifications, however, place them in their own clade, Cladistia, a sister group to subclass Chondrostei.