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Osteoglossiformes

Article by: Bailey, Reeve M. Formerly, Division of , Museum of Zoology, University of Michigan, Ann Arbor, Michigan. Boschung, Herbert Department of Biological Sciences, University of Alabama, Tuscaloosa, Alabama. Publication year: 2012 DOI: http://dx.doi.org/10.1036/1097-8542.478600 (http://dx.doi.org/10.1036/1097-8542.478600)

Content

• Anatomy and development • Evolution and adaptations • Bibliography

An order of fishes consisting of two monophyletic clades, suborders and . The Osteoglossoidei consists of one family, the Osteoglossidae (bonytongues and butterfly- fishes), which occur in freshwaters of tropical South America, Africa, and Southeast Asia to northern Australia. The Notopteroidei consist of three extant families, (Old World knifefishes and featherfin knifefishes, or featherbacks), (elephantfishes), and Gymnarchidae (gymnarchid ). The featherbacks are native to central tropical Africa, India, and the Malay Archipelago. The elephantfish family, having a long proboscislike snout in many , is the most species-rich osteoglossiform taxon but is limited to tropical Africa and the Nile River. The family Gymnarchidae is represented by a single species, niloticus, of tropical Africa including the upper Nile.

These fishes, formerly assigned to the orders (Isospondyli) and Mormyriformes, are accorded a revised classification as a result of modern phylogenetic research on lower .

Anatomy and development

Members of the Osteoglossiformes have the primary bite between the well-toothed tongue and the strongly toothed parasphenoid and certain pterygoid bones in the roof of the mouth. The mouth is bordered by a small premaxilla, which is fixed to the skull, and the maxilla. The anterior part of the intestine passes posteriorly to the left of the esophagus and stomach, whereas in all but a very few fishes the intestine pass to the right. A unique feature is the presence of paired bony rods at the base of the second gill arch. Additional characters include the absence of epineural intermuscular bones; a single of soft rays (or absent in some featherbacks) [Figs. 1 and 2]; lack of adipose fin; caudal fin well developed, reduced, or absent, its supporting skeleton usually compact and consolidated; pelvic fin usually abdominal, not attached to cleithrum; pelvic rays usually 6 or 7, with the fin sometimes rudimentary or absent; supraoccipital

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separated from frontals by parietals; orbitosphenoid usually present; a mesopterygoid arch; no Weberian apparatus; branchiostegals 3–17; and cycloid scales that are commonly large and often have a complex reticulum formed by the radii. Unlike the Clupeiformes, there is no extension of the cephalic canal system onto the opercle. In contrast to the , development is without a leptocephalous (elongate and flattened side to side) larva. See also: Clupeiformes (/content/clupeiformes/142500); Elopiformes (/content/elopiformes/229100); Teleostei (/content/teleostei/680400)

Fig. 1 African bonytongue (Heterotis niloticus). (After G. A. Boulenger, Catalogue of Fresh Water Fishes of Africa in the British Museum, Natural History, vol. 1, 1909)

Fig. 2 African elephantnose ( proboscirostris). (After G. A. Boulenger, Catalogue of Fresh Water Fishes of Africa in the British Museum, Natural History, vol. 1, 1909)

Evolution and adaptations

Osteoglossiforms represent one of the basal stocks among the teleosts. The Ichthyodectidae, including such giants as Xiphactinus (or Portheus), and two related families are well represented in marine deposits, and Allothrissops and Pachythrissops of the and Cretaceous probably belong here.

Recent members may be classified in 2 suborders, 6 families, 29 genera, and about 217 species. All inhabit freshwater, and all are tropical.

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Bonytongues are well known because of their economic importance, large size, and frequent exhibition in public aquariums. They are represented by five Recent species: the microphagous, African Heterotis (Fig. 1), the giant gigas (over 10 ft or 3 m long), and the arawana ( bicirrhosum) of the Amazon, and two species of of southeastern Asia, New Guinea, and Australia. The North American is a typical osteoglossid. A related group consists of a single small species known as African butterflyfish (Pantodon bucholzi). Butterflyfish leap a meter or more out of the water and glide with the help of their expansive pectoral fins. The featherbacks, Notopteridae, with five species from Africa and southeastern Asia, are compressed fishes with tapering bodies and long anal fins that are continuous with the caudal fin; the dorsal fin, if present, is a tiny tuft of a few rays.

Numerically Osteoglossiformes is dominated by the Mormyridae. Of the roughly 198 species, many are valued as food. The elongate snout of some species accounts for the vernacular name elephantfishes (Fig. 1). The caudal peduncle is slim in all species; in most the dorsal and anal fins are approximately equal, but in some the anal is much longer and in others the dorsal greatly exceeds the anal (Fig. 1). In the related family Gymnarchidae, consisting of a single species that attains a length of 1.5 m, the pelvic, anal, and caudal fins are absent. Mormyrids and gymnarchids are of especial interest in that they are electrogenic. Modified muscles in the caudal peduncle generate and emit a continuous electric pulse. The discharge frequency varies, being low at rest and high when the is under stress. The mechanism operates like a radar device, since the fish is alerted whenever an electrical conductor enters the electromagnetic field surrounding it. The receptor mechanism is not fully understood, but the brain of mormyrids (especially the cerebellum) is the largest of all fishes. Mormyrids have small eyes and often live in turbid water, so this electric mechanism is an effective evolutionary solution to a need for keen awareness of the environment and is comparable, for example, to acute hearing or vision. Electric discharge has been developed in several groups of fishes; in the African mormyrids, the discharge is convergent with a similar but wholly unrelated acquisition in the gymnotids or knifefishes of South America. See also: (/content/actinopterygii/009100); (/content/cypriniformes/177800); Electric organ (biology) (/content/electric-organ-biology/216500)

Osteoglossiforms vary greatly in size, shape, and biology. Their trophic biology is greatly diverse, with the various species being piscivores, insectivores, filter feeders, or omnivores. The well-vascularized can act as a lung, which is especially helpful in oxygen-deficient habitats. Gymnarchus niloticus builds a floating nest; and the osteoglossid genera Arapaima and Heterotis are substrate nest builders, whereas Scleropages and Osteoglossus are buccal incubators. of South America is one of the largest freshwater fishes of the world, at least 3 m, perhaps 4.5 m, in length. Notopterids reach a meter in length; they are egg layers and the male guards the eggs. Most mormyrids are small, up to 50 cm in length; most are much less, but 1.5-m specimens have been reported. Species vary in having a blunt snout with a terminal or inferior mouth, to a long snout and small terminal mouth; hence, they have variable feeding strategies.

Reeve M. Bailey Herbert Boschung

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Bibliography

T. M. Berra, An Atlas of Distribution of the Freshwater Fishes of the World, University of Nebraska Press, 1986

P. H. Greenwood, Interrelationships of osteoglossomorphs, pp. 307–332, in P. H. Greenwood, R. S. Miles, and C. Patterson (eds.), Interrelationships of Fishes, Academic Press, 1973

V. G. Lauder and K. F. Leim, The evolution and interrelationships of the actinopterygian fishes, Bull. Mus. Comp. Zool., 150:95–197, 1983

J. S. Nelson, Fishes of the World, 4th ed., Wiley, 2006

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