OSTEOGLOSSOMORPH

any member of the superorder Osteoglossomorpha, a group of morphologically and biologically diverse primitive fishes primarily found in freshwaters, although a few species enter slightly brackish water. Their relationship with other teleosts (i.e., advanced bony fishes) is obscure; they probably were an early offshoot from the basal teleost stock. Osteoglossomorpha comprises six extant families and about 150 species. Although the group is of little importance to man, in parts of Africa, Asia, and South America certain osteoglossomorph species are sometimes sought commercially as food fishes. Except for one North American family (Hiodontidae), the Osteoglossomorpha are tropical fishes. The families Mormyridae (elephant-snout fishes, mormyrs), Gymnarchidae, and Pantodontidae (butterfly fishes) are confined to Africa; the Notopteridae (featherbacks) occur in Africa, Southeast Asia, and India. The distribution of the Osteoglossidae (e.g., pirarucu, arawana) in Africa, South America, and Australasia (believed by many authorities to have once been joined as a single landmass called Gondwana) is of particular zoogeographical interest. The pirarucu (Arapaima) of the Amazon, one of the world's largest freshwater fishes, attains a length of three metres (about 10 feet); other osteoglossomorphsfor example, certain mormyridsare only a few centimetres long. any member of the superorder Osteoglossomorpha, a group of primarily primitive freshwater fishes, including the elephantsnout fish, mormyrid, pirarucu, and awawana. Except for the family Hiodontidae (e.g., goldeye and mooneye), the Osteoglossomorpha are tropical fishes. Their distinguishing features are mostly skeletal; they have strongly toothed jaws, a wide variety of head and mouth forms, and a characteristic caudal (tail) fin. Osteoglossomorph fishes have well-developed teeth on the tongue, skull base, and bones of the mouth cavity. They have existed since the Cretaceous Period (65,000,000 to 136,000,000 years ago), and it is believed that they descended from an early teleost (bony fishes) stock. The breeding habits of some Osteoglossomorphs are quite interesting. Gymnarchus niloticus, one of the electrical species, prepares a large, floating nest from swamp grasses, biting off the stems and working them into a type of trough. The fish spawn in the nest and take turns guarding the developing young for about 18 days. Other species carry the eggs and young in their mouths. Arapaima gigas, the huge pirarucu of South America, digs a spawning pit; after about seven days, the young hatch and leave the pit, guarded by the male. The dark colour of the male's head is perhaps recognized by the young, which congregate around it, but there are probably other stimuli, such as taste and smell attractions, to keep the young nearby. Members of the Mormyridae and Gymnarchidae families have electrical organs that allow the fish to distinguish prey, predators, its own species, and obstacles in the water. The electrical organs also probably account for the peculiar swimming habits of some mormyriforms. Gymnarchus swims with its long body held rigid, propelled entirely by undulations of its dorsal fin. Other rigid swimmers are the featherbacks (family Notopteridae), which use their long anal fin to propel them; as they have no electrical organs, this behaviour is probably associated with the long, gas-filled swim bladder that extends into the tail. Additional reading P.H. Greenwood, On the Genus Lycoptera and Its Relationship with the Family Hiodontidae (Pisces, Osteoglossomorpha), Bull. Br. Mus. Nat. Hist. (Zool.), 19:259285 (1970); and et al., Phyletic Studies of Teleostean Fishes, with a Provisional Classification of Living Forms, Bull. Am. Mus. Nat. Hist., 131:339455 (1966), includes a classification of the Osteoglossomorpha and a discussion of the reasons for the arrangement; E.S. Herald, Living Fishes of the World (1961, reprinted 1972), a popular, well-illustrated account of the various families and of some species, particularly their biology; K.H. Luling, Arapaima, Giant Fish of Amazonas, Animals, 11:222225 (1968), a popular account; G.J. Nelson, Infraorbital Bones and Their Bearing on the Phylogeny and Geography of Osteoglossomorph Fishes, Am. Mus. Novit., no. 2394 (1969); Gill Arches of Teleostean Fishes of the Division Osteoglossomorpha, J. Linn. Soc. (Zool.), 47:261277 (1968); J.R. Norman, A History of Fishes, 3rd rev. ed. by P.H. Greenwood (1977), includes a general account of osteoglossomorph biology, distribution, and anatomy (high school and college level); G. Sterba, Freshwater Fishes of the World, rev. ed., 2 vol. (1989; originally published in German, 1959), particularly for the aquarist.