Osteoglossiformes

(vertebrate zoology) An order of soft-rayed, actinopterygian fishes distinguished by paired, usually bony rods at the base of the second gill arch, a single dorsal fin, no adipose fin, and a usually abdominal pelvic fin.

(Bony tongues and relatives)

Class: Actinopterygii

Order: Osteoglossiformes

Number of families: 6

Evolution and systematics

The osteoglossiformes are an unusual group of teleost fishes comprising about 220 species of freshwater fishes, most of which are in one African family, the Mormyridae (19 genera; 182 species). The other species are scattered about the continents and are generally considered to be relicts of a once more abundant group. Fossil records of the family Osteoglossidae indicate these fishes to be between 38 and 55 million years old. However, the present distribution of members of the Osteoglossidae family suggests that the group was present on Gondwana prior to Gondwana's fragmentation. Biogeographic evidence thus suggests a considerably greater age than the 55 million years inferred from the fossil record. Most osteoglossiformes have most of their teeth located on the tongue and on the roof of the mouth. They also have a caudal fin with 16 or fewer branched rays (most bony fishes have more), lack intermuscular bones on the back, and have cycloid scales with ornate microsculpturing. The intestine curls around to the left side of the esophagus rather than to the right as in most other bony fishes.

Six living families are recognized. The monotypic family Gymnarchidae (Gymnarchus niloticus) together with the Mormyridae, comprises the superfamily Mormyroidea; this group is considered the sister group of the Notopteroidea (family Notopteridae; four genera and eight species). The position of the three remaining families is somewhat uncertain. The Osteoglossidae comprise seven species (four genera) and the Pantodontidae but one. The phylogenetic position of the Hiodontidae (two species) is not very clear. The two species of this family have a similar ear–swim bladder connection as do the clupeomorph fishes.

Recent data indicate the presence of a group of mormyrid fishes of the genus Brienomyrus in Gabon of uncertain taxonomic status. Morphological, physiological (electric discharge), and molecular genetic data indicate these fishes represent a species flock. A comparable situation is known from the East African lakes Victoria, Malawi, and Tanganyika. A very limited number of riverine cichlid species adjusted to lacustrine conditions and have evolved into a species flock now comprising more than 200 species in each lake and dominating the fish fauna of these lakes.

Physical characteristics

The Mormyridae, the elephantfishes, are odd-looking fishes ranging from 1.6 in to 5 ft (4 cm to 1.5 m) in length. The head morphology varies considerably related to feeding specializations: some species possess prolonged heads, others trunklike snouts or appendages on the lower jaw, hence the common name. The tail is often deeply forked and the caudal peduncle very narrow. The skin is thick and of high electrical resistance; all species indeed are weakly electric. The electric organ is located in the caudal peduncle. Larvae possess a larval electric organ in the lateral muscle. The electric field set up around the fish is used for electrolocation and electrocommunication. Related to this sensory modality is the enlarged cerebellum; thus brain volume, relative to body size, is roughly the same size as that of humans. Male elephantfishes in most species can be distinguished from females by the lobed, enlarged front part of the anal fin. The sperm of mormyrids lacks flagellum. In the remaining osteoglossiform fishes, sexual dimorphism is not very pronounced or lacking.

Gymnarchus niloticus, the only species of the family Gymnarchidae, can reach 5 ft (1.5 m) in length. It possesses a long snout and a long dorsal fin used for locomotion; the anal, caudal, and pelvic fins are absent. The fish produces sinusoidal weakly electric discharges.

The eight species of knifefishes of the family Notopteridae have long, strongly compressed bodies tapering to a point. The long anal fin extends from just behind the head to the tiny caudal fin, which it joins. The dorsal fin, which is absent from Xenomystus nigri, is small and featherlike, so these fishes are commonly called featherbacks. The swim bladder is connected to the gut and is used for air breathing. The species of the African subfamily Xenomystinae, genera Xenomystus and Papyrocranus, possess cutaneous electroreceptors. Knifefishes range from 7.9 in (20 cm) in length (in Xenomystus nigri) up to 5 ft (1.5 m) (in Chitala lopis, the giant featherback).

Species of the family Osteoglossidae, the bony tongues, have heavy, elongate bodies covered with large scales. The dorsal and anal fins are long and placed on the rear part of the body. All these fishes can apparently breathe air with their lunglike swim bladders. Arapaima gigas can reach lengths of about 14.7 ft (4.5 m); other species attain lengths of about 3.3 ft (1 m).

The African freshwater butterflyfish, the only species of the family Pantodontidae, reach 3.9 in (10 cm) in length. The fishes possess a large gape and a straight dorsal profile. The pelvic fins with the prolonged fin rays are located under the greatly enlarged, winglike pectorals. The swim bladder can act as an air-breathing organ.

The two species of the family Hiodontidae, the mooneye (Hiodon tergisus) and the goldeye (H. alosoides), superficially resemble clupeid fishes. Their most distinctive external features are their large eyes, which have bright gold irises (goldeye) or gold/silver irises (mooneye). Goldeyes have only rods in their retinas and are known to feed mostly at night.

Distribution

Elephantfishes occur all over tropical Africa. The highest diversity is found in the Congo River basin, where mormyrids comprise about 20% of the total number of about 600 fish species. Gymnarchus niloticus is found in all large rivers of the Sahelo/Sudanean region in Africa.

African knifefishes inhabit coastal streams in West Africa (Xenomystus nigri and Papyrocranus afer), or the Congo basin (P. congoensis). The Asian knifefishes are found in the Indus, Ganges-Brahmaputra, and Mahanadi River basins in India (Chitala chitala), in Indonesia, Malaysia, and Thailand (Chitala lopis), and in Laos, Thailand, Cambodia, and Vietnam (Chitala ornata). Notopterus notopterus is very widely distributed, inhabiting rivers in India, Indochina, Thailand, Malaysia, and Indonesia.

The three South American bony tongues are either restricted to the Rio Negro (Osteoglossum ferreira), or occur in the Amazon River system and French Guiana (Arapaima gigas and O. bicirrhosum). Heterotis niloticus occurs in Africa in all river basins of the Sahelo/Sudanian region. The Asian bony tongue (Scleropages formosus) is native to Indonesia, Malaysia, Thailand, Cambodia, and Vietnam. S. jardini is found in New Guinea and Northern Australia; S. leichardti is restricted to northeastern Australia.

Pantodon buchholzi, the only representative of the family Pantodontidae, occurs in various rivers of central and West Africa.

The two species of Hiodon are found in the central part of North America, with H. alosoides being more widely distributed than H. tergisus.

Habitat

African elephantfishes are mainly riverine species and rarely occur in lakes. They are pelagic, midwater, or bottom-oriented fishes. The knifefishes inhabit stagnant backwaters of the large rivers, and are sometimes found in lakes; the smaller species prefer habitats with dense vegetation. Large bony tongues are found in open, slow-moving, or stagnant water and are surface-oriented hunters. Pantodon buchholzi prefers surface water of habitats with stagnant water. Hiodon alosoides lives in turbid waters in large lakes and muddy rivers, occasionally in swift current. H. tergisus is usually found in the clear waters of large lakes and streams.

Behavior

Elephantfishes are nocturnal, often hiding during the day in dense vegetation or under other kinds of cover. Aquarists have long admired mormyrids for their learning abilities and the fact that many species engage in apparent "play" behavior consisting of batting around small objects, including air bubbles, with the head. They usually swim slowly with their body rigid, presumably to avoid distorting the electrical field they are generating. The electric field is used for electrolocation and electrocommunication. The frequency of the electric signals can be modified to communicate with other fish, and thus can be used in courtship, aggressive behavior, and other intraspecific encounters. Because each species has its own set of electrical patterns, recognition and avoidance of other species is also possible. Mormyrids possess a well-developed sense of hearing; a part of the small swim bladder is in contact with the inner ear. Mormyrids use acoustic signals during courtship behavior. Gymnarchus niloticus, the close relative of the mormyrids, produces sinusoidal electrical discharges (the elephantfishes produce pulse-type discharges); this slow moving fish also uses its discharge for electrolocation. Knifefishes generally remain quietly in cover during daytime, but come out to prey in the evening. Bony tongued fishes are active during the day, spending most of the time patrolling very close to the surface. From aquaculture and aquarium observations, it has been deduced that the Australian spotted barramundi (Scleropages leichardti) can withstand water temperatures of between 44.6 and 104°F (7 and 40°C). During the summer, when surface temperatures exceed 87.8°F (31°C) in their natural habitat, surface cruising ceases and the fish remain in deeper, cooler areas. Pantodon buchholzi is a slow-moving fish of surface waters; it can jump out of the water and has been observed gliding over 13.1–16.4 ft (4–5 m). The pronounced tapetum lucidum of the two Hiodon species enables these fishes to hunt effectively at night.

Feeding ecology and diet

The mormyrid fishes eat various kinds of zooplankton or feed on a variety of benthic organisms such as insect larvae, crustaceans, oligochaets, and snails. The species with the long snouts find their prey in holes and crevices. Large Mormyrops species are piscivorous. Elephantfishes themselves are eaten by the large predator Gymnarchus niloticus (who also feeds on other fishes) and large piscivorous catfishes. Smaller knifefishes feed on insect larvae, crustaceans, worms, and snails; the larger species are mainly piscivorous. The bony tongues are midwater and surface feeders. Species of the genus Osteoglossum and Scleropages are carnivorous, feeding in roughly equal measure upon smaller fishes and terrestrial insects. While large specimens of both are known to take small terrestrial vertebrates opportunistically, these items do not constitute a significant portion of their diet in nature. The large Arapaima gigas prefers fishes. Heterotis niloticus has its fourth gill arch modified into a spiral-shaped filtering apparatus. This organ secretes mucus in which phytoplankton and bits of organic matter are trapped and then swallowed. The surface-oriented Pantodon buchholzi lives on crustaceans, insects, and small fishes. The two species of the family Hiodontidae feed on a variety of prey, including aquatic insects, crustaceans, mollusks, small fishes, frogs, shrews, and mice. They are preyed upon by birds, some mammals, and other fishes.

Reproductive biology

Most osteoglossiform fishes breed during the rainy season. Experimental studies with elephantfishes have shown that gonad maturation is triggered by decreasing water conductivity. About nine species have been bred in captivity. Parental care in the male is found in Pollimyrus isidori; the eggs are transported after oviposition by the male into the nest of about 2 in (5 cm) in diameter, generally made from plant material. After hatching (three days after fertilization) the male guards the embryos until the beginning of exogenous feeding (on days 13–14) and also during the larval period. Courtship behavior is characterized by acoustic and electrical behavior. Species of the genus Stomatorhinus probably also show parental care. The remaining species bred so far (Petrocephalus

soudanensis, Brienomyrus brachyistius, Marcusenius sp., Mormyrus rume proboscirostris, Mormyrus sp., Hippopotamyrus pictus, Campylomormyrus cassaicus, and C. phantasticus) do not show parental care.

Egg size ranges between 0.07 in (1.8 mm) in P. soudanensis) and 0.12 in (3 mm) in H. pictus. Eggs number between a few hundred in P. isidori, and more than a thousand in C. cassaicus. Spawning intervals range between a few days in P. isidori and several weeks in most other species.

Gymnarchus niloticus breeds in swamps during the high-water season. Prior to spawning, these fishes construct a floating nest of plant fibers in which the thousand or so eggs, each about 0.39 in (10 mm) in diameter, are laid. The newly hatched young have long gill filaments and an elongate yolk sac. They come to the surface for air. Young fishes feed on insects and other invertebrates.

Reproduction in knifefishes is not well known. Xenomystus nigri females lay 150–200 eggs of 0.08 in (2 mm) diameter; in Notopterus notopterus, eggs (1,000–3,000) are deposited in small clumps on submerged vegetation. Chitala chitala lays eggs on a stake or stump of wood, the male fans them with his tail and guards them against predators. Arapaima gigas males build a nest about 6 in (15 cm) deep and 20 in (50 cm) wide in sandy bottoms at the end of the dry season; the large eggs and young are guarded by the male and occasionally by the female. Parental care lasts up to 14 weeks. The two Osteoglossum species are male mouth brooders. The large eggs (about 0.6 in/16 mm diameter) are incubated for 50–60 days. At release, the juveniles measure 3.1–3.9 in (8–10 cm).

The Scleropages species are female mouth brooders. S. leichardti incubates 70–200 eggs about 0.4 in (10 mm) in diameter. Spawning occurs in small ponds during spring, when water temperatures rise to 68–73.4°F (20–23°C). Hatching takes place between one and two weeks after spawning; the embryos with their large yolk sac are about 0.6 in (15 mm) long. After the total incubation period of five to six weeks, the juveniles are released at a total length of about 1.4 in (35 mm). During a three-day period, the female shows a "release-and-recall" behavior. When the young become independent of the female, they take up territories around the edge of the pond. Heterotis niloticus is a nest builder, breeding in still waters close to the river, and excavating a nest some 3.9 ft (1.2 m) in diameter with thick walls of vegetation and mud. Within this nest, eggs about 0.1 in (2.5 mm) in diameter are laid; protected by the parents; they hatch in two days. The newly hatched embryos have external gills. Pantodon buchholzi has a prolonged spawning season, spawning 80–200 small buoyant eggs every day; the small embryos hatch after 36 hours. Goldeye (Hiodon alosoides) spawn in late spring on gravelly shallows of tributary streams. Their eggs are about 0.16 in (4 mm) in diameter, and are semibuoyant even after hatching, as the oil globule in the yolk buoys up the newly hatched 0.3 in (7 mm) embryo.

Conservation status

Four species are listed by the IUCN: Arapaima gigas is listed as Data Deficient; Chitala blanci is listed as Lower Risk/Near Threatened; Scleropages formosus is listed as Endangered; and Scleropages leichardti is listed as Lower Risk/Near Threatened. Scleropages formosus is also included on Appendix I of CITES, as a result of which international trade is banned (CITES Appendix I). For Arapaima gigas, international trade is restricted (CITES Appendix II).

Significance to humans

Most osteoglossiform fishes, particularly the larger species, are economically important food fishes. Even many of the medium-sized African elephantfishes, which measure approximately 7.9–23.6 in (20 to 60 cm) in length, are regularly fished for food. Some of the larger osteoglossiforms are used in aquaculture, including Arapaima gigas; Scleropages leichardti and S. jardini; Heterotis niloticus; Chitala blanci and C. chitala; and Notopterus notopterus. The larger species are important as food fishes, as well as for exhibition in public aquaria. The various color breeds of Scleropages formosus are favored as ornamental fishes in Asia. The elephantfish, Gnathonemus petersii, is a well-known species in the international aquarium trade. Several species of the weakly electric mormyrids are intensively studied by scientists.

Species accounts

Resources

Bullock, T., and W. Heiligenberg. Electroreception. New York: John Wiley & Sons, 1986.

Daget, J., J. P. Gosse, and D. F. E. Thys van den Audenaerde, eds. Check-List of the Freshwater Fishes of Africa (CLOFFA), Vol. 1. Paris: ORSTOM, Tervuren: MRAC, 1984.

Merrick, J. R., and G. E. Schmida. Australian Freshwater Fishes, Biology and Management. North Ryde, Australia: School of Biological Sciences, 1984.

Moller, P., ed. Electric Fishes: History and Behavior. London: Chapman & Hall, 1995.

Kirschbaum, F. "Reproduction and Development of the Weakly Electric Fish Pollimyrus isidori (Mormyridae, Teleostei) in Captivity." Env. Biol. Fishes. 20 (1987): 11–31.

Kirschbaum, F., and C. Schugardt. "Reproductive Strategies and Developmental Aspects of Gymnotiform and Mormyrid Fishes." J. Physiol., in press.

Roberts, T. R., "Systematic Revision of the Old World Freshwater Fish Family Notopteridae." Ichthyological Explorations of Freshwaters 2, no. 4 (1992): 361–383.

[Article by: Frank Kirschbaum, PhD]

An order of soft-rayed, actinopterygian fishes that includes the mooneyes, featherbacks, mormyrids or elephantfishes, and bonytongues.

Osteoglossiforms have the primary bite between the well-toothed tongue and the roof of the mouth, usually the strongly toothed parasphenoid but occasionally the endopterygoids. The mouth is bordered by the premaxilla, which in some forms is fused to its mate from the opposite side, and the maxilla. A unique feature is the presence of paired, usually bony rods at the base of the second gill arch.

Osteoglossiforms represent one of the basal stocks among the teleosts. The Ichthyodectidae and two related families are well represented in Cretaceous marine deposits, and Allothrissops and Pachythrissops of the Jurassic and Cretaceous probably belong here. Recent members may be classified in 2 suborders, 6 families, 22 genera, and about 135 species. All inhabit fresh water, and all are tropical except for the two species of Hiodontidae or mooneyes of North America, where they have a history to the Eocene.

Numerically the Osteoglossiformes are dominated by the Mormyridae, African river and lake fishes of varied size and form. In some the snout is very blunt and rounded, in others it is elongated (see illustration), the source of the vernacular name elephantfish. The related family Gymnarchidae consists of a single species. Scientifically, mormyrids and gymnarchids are of especial interest in that all 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 fish is under stress. The mechanism operates like a radar device, since the fish is alrted whenever an electrical conductor enters the electromagnetic field surrounding it. See also Actinopterygii; Electric organ (biology).

African elephantnose (Mormyrus probosctrostrls). (After G. A. Boulenger, Catalogue of Fresh Water Fishes of Africa In the British Museum, Natural History, vol. 1, 1909)

Osteoglossiformes Fossil range: Late Jurassic–Recent PreЄ Є O S D C P T J K Pg N [1]
Osteoglossum bicirrhosum
Scientific classification
Kingdom: Animalia Phylum: Chordata Class: Actinopterygii Order: Osteoglossiformes
Families
Suborder Notopteroidei Gymnarchidae Mormyridae (elephantfishes) Notopteridae (featherfin knives) Ostariostomidae (extinct) Suborder Osteoglossoidei Heterotididae Osteoglossidae (arowana) Pantodontidae (butterflyfishes) Singidididae (extinct)

Osteoglossiformes (Gk. "bony tongues") is a relatively primitive order of ray-finned fish that contains two sub-orders, the Osteoglossoidei and the Notopteroidei. All of the living species inhabit freshwater. They are found in South America, Africa, Australia and southern Asia, having first evolved in Gondwana before that continent broke up.^[2]

The mooneyes (Hiodontidae) are often classified here, but may also be placed in a separate order Hiodontiformes.

Members of the order are notable for having toothed or bony tongues, and for having the forward part of the gastrointestinal tract pass to the left of the oesophagus and stomach (for all other fish it passes to the right). In other respects, osteoglossiform fishes vary considerably in size and form; the smallest is Pollimyrus castelnaui, at just 2 centimetres (0.79 in) long, while the largest, the arapaima (Arapaima gigas), reaches as much as 2.5 metres (8.2 ft).^[2]

References

1. ^ "Osteoglossiformes". FishBase. Ed. Ranier Froese and Daniel Pauly. January 2009 version. N.p.: FishBase, 2009.
2. ^ ^{a b} Greenwood, P.H. & Wilson, M.V. (1998). Paxton, J.R. & Eschmeyer, W.N.. ed. Encyclopedia of Fishes. San Diego: Academic Press. pp. 81–84. ISBN 0-12-547665-5. 

External links

• Mikko's Phylogeny for Osteoglossiformes
• Osteoglossiforms for aquaria
• Li, Guo-Qing and Wilson, Mark V. H. 1998. Osteoglossomorpha. Bonytongues. Version 6 October 1998. [1] in The Tree of Life Web Project

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