Oscar (fish)

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]

1831 watercolor of Astronotus ocellatus by Jacques Burkhardt.

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 yellow-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]

Two tiger oscars

A. ocellatus is native to Brazil, Colombia, Ecuador, Guyana, French Guiana, Suriname, Peru, and Venezuela, and occurs in the Amazon River basin, along the Amazon, Içá, Negro, Solimões, and Ucayali River systems, and also in the Approuague and Oyapock River 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 byproduct 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 (55.22 °F).[14]

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 their dorsal fins.[6][7] The species reaches sexual maturity around one year of age,[15] and continues to reproduce for 9–10 years.[15] Frequency and timing of spawning may be related to the occurrence of rain.[16] A. ocellatus fish are biparental substrate spawners, though detailed information regarding their reproduction in the wild is scarce.

Young Oscar, about 2 in

In captivity, pairs are known to select and clean generally flattened horizontal or vertical surfaces on which to lay their eggs.. Smaller females lay around 300-500 eggs, while larger female oscars can lay about 2,500-3,000 eggs.[15] Like most cichlids, A. ocellatus practices brood care, although the duration of brood care in the wild remains unknown.[6]

The oscar at the Särkänniemi Aquarium in Tampere, Finland

The oscar is one of the most popular cichlids in the aquarium hobby.

Territorial behavior

An albino oscar

Oscars will often lay claim to an area of the aquarium and will be very aggressive towards other fish encroaching on their newly established territory inside the aquarium or lake. The size of the territory varies depending on the size and aggressiveness of the fish, and its surroundings. Once the oscar establishes a territory, it will vigorously defend it by chasing away other fish.[23]

Varieties

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 the mainly red colouration of the flanks are frequently sold under the trade name of red oscars.[24] The patterning of red pigment differs between individuals. In recent years long-finned varieties have also been developed. The species is also occasionally artificially coloured by a process known as painting.[25]