|Silver arowana, Osteoglossum bicirrhosum|
Arowanas are freshwater bony fish of the family Osteoglossidae, also known as bonytongues. In this family of fish, the head is bony and the elongated body is covered by large, heavy scales, with a mosaic pattern of canals. The dorsal and anal fins have soft rays and are long based, while the pectoral and ventral fins are small. The name "bonytongues" is derived from a toothed bone on the floor of the mouth, the "tongue", equipped with teeth that bite against teeth on the roof of the mouth. The arowana is a facultative air breather and can obtain oxygen from air by sucking it into its swim bladder, which is lined with capillaries like lung tissue.
Osteoglossids are basal (primitive) fish from the lower Tertiary and are placed in the actinopterygiid order Osteoglossiformes. Of the 10 described living species, three are from South America, one from Africa, four from Asia, and the remaining two are from Australia.
The Osteoglossidae are the only exclusively freshwater fish family found on both sides of the Wallace Line. This may be explained by the theory that Asian arowanas (S. formosus) diverged from the Australian Scleropages, S. jardinii and S. leichardti, about 140 million years ago (Mya), making it likely that Asian arowanas were carried to Asia on the Indian subcontinent.
A genetic study showed the lineage leading to the arapaima and African arowana diverged about 220 Mya, during the Late Triassic; the lineage leading to the silver and blackish blue arowanas of South America diverged about 170 Mya, during the Middle Jurassic. The lineage leading to the Australian arowanas diverged from that leading to the Asian arowanas about 140 Mya, during the Early Cretaceous.
At least five extinct genera, known only from fossils, are classified as osteoglossids; these date back at least as far as the Late Cretaceous. Other fossils from as far back as the Late Jurassic or Early Cretaceous are widely considered to belong to the arowana superorder Osteoglossomorpha. Osteoglossomorph fossils have been found on all continents except Antarctica. These fossil genera include Brychaetus, Joffrichthys, and Phareodus.
Osteoglossids are carnivorous, often being specialized surface feeders. They are excellent jumpers; Osteoglossum species have been seen leaping more than 6 ft (almost 2 m) from the water surface to pick off insects and birds from overhanging branches in South America, hence the nickname "water monkeys". Arowana species typically grow to around 2 to 3 ft in captivity.
Several species of osteoglossids exhibit parental care. They build nests and protect their young after they hatch. All species are mouthbrooders, the parents holding sometimes hundreds of eggs in their mouths. The young may make several tentative trips outside the parent's mouth to investigate the surroundings before leaving permanently.
In the aquarium
Arowanas are solitary fish and only allow company while young; adults may show dominance and aggression. Some compatible species often partnered with this fish are clown knifefish, pacu, oscars, plecostomus, jaguar cichlids, green terrors, gar, tinfoil barb, Siamese tigerfish, and any other somewhat aggressive fish that cannot fit in the arowana's mouth. These fish are best kept with live or frozen feed and they easily outgrow the tank within 8 to 10 months. An aquarium of at least 150 gallons is preferable. Australian species are best kept alone in aquaria.
The name comes from the Indonesian arwana or nirwana, meaning fish of paradise.
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