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Oscar cichlid (fish)

Astronotus ocellatus is a species of fish from the cichlid family known under a variety of common names including oscar, tiger oscar, velvet cichlid or marble cichlid.[1] In South America, where the species occurs, A. ocellatus are often found for sale as a food fish in the local markets.[2][3] However, its slow growth limits its potential for aquaculture.[4] The species is also a popular aquarium fish.[5][6]
Appearance, size and colouration
A. ocellatus have been reported to grow to a length of 45
cm (approximately 18 in) and a mass of 1.6 kg (3.5 lb).[1] The wild caught forms of the species are typically darkly coloured with orange ringed-spots or ocelli on the caudal peduncle and on the dorsal fin.[5] It has been suggested that these ocelli function to limit fin-nipping by piranha (Serrasalmus spp.) which co-occur with A. ocellatus in its natural environment.[7] Further studies have suggested the ocelli may also be important for intraspecific communication.[8] The species is also able to rapidly alter its colouration, a trait which facilitates ritualised territorial and combat behaviours amongst conspecifics.[9] Juvenile A. ocellatus have a different colouration to adults and are striped with white and orange wavy bands and have spotted heads.[8]
Distribution and habitat
A. ocellatus is native to
Peru, Colombia, Brazil and French Guiana and occurs in the Amazon river basin, along the Amazonas, Içá, Negro, Solimões and Ucayali river systems, and also in the Approuague and Oyapock drainages.[1][2] In its natural environment the species typically occurs in slow moving white-water habitats, and has been observed sheltering under submerged branches.[5] Feral populations also occur in China,[10] northern Australia,[11] and Florida, USA[12] as a by-product of the ornamental fish trade. The species is limited in its distribution by its intolerance of cooler water temperatures, the lower lethal limit for the species is 12.9 °C (55.2 °F).[13]
Sexual dimorphism and reproduction
Although the species is widely regarded as
sexually monomorphic,[5] it has also been suggested that males grow more quickly, and in some naturally occurring strains, males are noted to possess dark blotches on the base of the dorsal fin.[8][6] The species reaches sexual maturity at approximately 1 year of age and continues to reproduce for 9-10 years. Frequency and timing of spawning may be related to the occurrence of rain.[14] A. ocellatus are biparental substrate spawners though detailed information regarding their reproduction in the wild are scarce. It has been observed that the closely related Astronotus crassipinnis may, in times of danger, protect its fry in its mouth in a manner reminiscent of mouthbrooding geophagine cichlids. This behaviour, however, has not yet been observed in A. ocellatus.[6] In captivity pairs are known to select and clean generally flattened horizontal or vertical surfaces on which to lay their 1000 to 3000 eggs. Like most cichlids, A. ocellatus practice brood care, although the duration of brood care in the wild remains unknown.[6]
Feeding and prey
Examination of the stomach contents of A. ocellatus by Winemiller (1990) demonstrated that the natural diet of this fish consists primarily of aquatic and terrestrial
insects (which comprise up to 60% of the diet), although small fish, and to a lesser extent crustaceans, are also consumed. Most fish eaten by A. ocellatus in the wild were relatively sedentary catfish, and included Bunocephalus, Rineloricaria and Ochmacanthus species.[7] The species uses a suction mechanism, generated by jaw extension, to capture prey,[15] and has been reported to exhibit "laying-on-side" death mimicry in a similar fashion to Parachromis friedrichsthalii and Nimbochromis livingstonii.[16][17] The species also has an absolute requirement for vitamin C and develops health problems in its absence.[18]

History, taxonomy and synoma
The species was originally described by
Louis Agassiz in 1831 as Lobotes ocellatus, as he mistakenly believed the species was marine, latter work assigned the species to the genus Astronotus.[8] The species also has a number of junior synonyms: Acara compressus, Acara hyposticta, Astronotus ocellatus zebra and Astronotus orbiculatus.[19]

Selective breeding
A number of ornamental varieties of A. ocellatus have been developed for the aquarium industry. These include forms with greater intensity and quantities of red marbling across the body, albino, leucistic and xanthistic forms. A. ocellatus with marbled patches of red pigmentation are sold as red tiger oscars, while those strains with mainly red colouration of the flanks are frequently sold under the trade name of red oscars.[20] The patterning of red pigment differs between inidividuals, in the United Kingdom one A. ocellatus reportedly had markings that resembled the Arabic word for "Allah".[21] In recent years long-finned varieties have also been developed. The species is also occasionally artificially coloured by a process known as painting.[22]

In the aquarium
A. ocellatus are popular as
pets, and are regarded as intelligent by aquarists. This is in part as they learn to associate their owners and food[6] and are purported to be able to distinguish their owner from strangers.[20]
Despite their large size, and predatory nature A. ocellatus are relatively placid aquarium residents best housed with other fishes too large to be considered food items.
[5][6][20] They also get along with aggressive fish such as a convict cichlid.
A. ocellatus are known to uproot
plants, and move other objects in aquariums[23] and are best maintained in volumes of 200-600 litre (75 or more gallons).[20][23] A. ocellatus is relatively tolerant of a range of typical aquarium water chemistries,[5] though its large size and messy feeding habits necessitates that efficient filtration be installed on the aquarium.[20] A. ocellatus is undemanding to feed in captivity and will accept a range of foods that include pieces of fish and prepared cichlid foods.[5][6][20] Oscars may also tend to jump out of an aquarium if the lid isn't supported sufficiently.

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