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 naturally resides, A. ocellatus specimens are often found for sale as a food fish in the local markets.^[2][3] The fish can also be found in other areas, including China, Australia, and the United States. Although its slow growth limits its potential for aquaculture, it is considered a popular aquarium fish.^[4][5][6]

Taxonomy

The species was originally described by Louis Agassiz in 1831 as Lobotes ocellatus, as he mistakenly believed the species was marine; later work assigned the species to the genus Astronotus.^[7] The species also has a number of junior synonyms: Acara compressus, Acara hyposticta, Astronotus ocellatus zebra, and Astronotus orbiculatus.^[8]

Description

Ocelli on dorsal fin and caudal peduncle

A. ocellatus examples have been reported to grow to about 45 cm (18 in) in length and 1.6 kilograms (3.5 lb) in weight.^[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] These ocelli have been suggested to function to limit fin-nipping by piranha (Serrasalmus spp.), which co-occur with A. ocellatus in its natural environment.^[7][9] The species is also able to rapidly alter its colouration, a trait which facilitates ritualised territorial and combat behaviours amongst conspecifics.^[10] Juvenile oscars have a different colouration from adults, and are striped with white and orange wavy bands and have spotted heads.^[7]

Distribution and habitat

Two tiger oscars

A. ocellatus is native to Peru, Ecuador, 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,^[11] northern Australia,^[12] and Florida, USA^[13] 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).^[14]

Sexual dimorphism

Although the species is widely regarded as sexually monomorphic,^[5] males have been suggested to grow more quickly, and in some naturally occurring strains, males are noted to possess dark blotches on the base of the dorsal fin.^[6][7] The species reaches sexual maturity at approximately one year of age, and continues to reproduce for nine to ten years. Frequency and timing of spawning may be related to the occurrence of rain.^[15] A. ocellatus fish are biparental substrate spawners, though detailed information regarding their reproduction in the wild is scarce.

Young oscar, about 2 inches

The closely related Astronotus crassipinnis has been observed, in times of danger, to store food in its mouth, possibly for protection, 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 1,000 to 3,000 eggs. Like most cichlids, A. ocellatus practices brood care, although the duration of brood care in the wild remains unknown.^[6]

Feeding

Captive oscars may be fed prepared fish food designed for large carnivorous fish, crayfish, worms, and insects (such as flies or grasshoppers). They even eat small mice, and crickets are also good live food. Its feeding habits mostly depends on live food so feeding with live food may increase the rate of growth but also may cause endoparasites. Since these fish eat fruit in the wild, it can also be used as a type of food. Just about anything that fell into the water would be eaten by oscars. Live feeder fish can be given, but fish such as goldfish and rosy red feeder minnows should not be fed. They also contain an enzyme within their flesh which binds vitamin B₁, leading to deficiency. Most fish eaten by A. ocellatus in the wild are relatively sedentary catfish, including Bunocephalus, Rineloricaria, and Ochmacanthus species.^[9] The species uses a suction mechanism to capture prey,^[16] and has been reported to exhibit "laying-on-side" death mimicry in a similar fashion to Parachromis friedrichsthalii and Nimbochromis livingstonii.^[17][18] The species also has an absolute requirement for vitamin C, and develops health problems in its absence.^[19]

Selective breeding

A leucistic long-finned oscar

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 individuals; 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]

References