Arowanas are freshwater bony fish of the family
Osteoglossidae, also known as bonytongues (the latter name is now often
reserved for Arapaimidae).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.
Evolution
Osteoglossids are basal (primitive) fish from the lower
Tertiary and are placed in the actinopterygiid order Osteoglossiformes. As
traditionally defined, the family includes several extant species from South
America, one from Africa, several from Asia, and two from Australia. Today
Arapaimidae is often regarded as a separate family, which includes the arapaimas
and the African arowana.Consequently, the South American genus Osteoglossum,
and the Asian and Australian genus Scleropages are the only extant genera that
remain in the osteoglossid family.Arapaimidae and Osteoglossidae split about
220 million years ago (Mya), during the Late Triassic.
Within Osteoglossidae, the South America Osteoglossum
arowanas diverged from the Asian and Australian Scleropages arowanas about 170
Mya, during the Middle Jurassic.
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 Mya, making it likely
that Asian arowanas were carried to Asia on the Indian subcontinent.
Fossil record
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.
Behavior
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, 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.
