Discus (fish)

''Symphysodon'', colloquially known as discus, is a genus of cichlids native to the Amazon river basin in South America. Due to their distinctive shape, behavior, and bright colors and patterns, discus are popular as freshwater aquarium fish, and their aquaculture in several countries in Asia is a major industry.Sands D (1994) ''A fishkeepers guide to Central American cichlids.'' Tetra Press. Belgium pg 59-60. They are sometimes referred to as pompadour fish. The discus fish has attracted a cult following of collectors and has created a multimillion dollar international industry complete with shows, competitions, and reputed online breeders.

Species

Following a review published in 2006, three species are recognized by FishBase:
* ''Symphysodon aequifasciatus'' Pellegrin, 1904 (blue discus or brown discus)
* '' Symphysodon discus'' Heckel, 1840 (red discus or Heckel discus)
* ''Symphysodon tarzoo'' E. Lyons, 1959 (green discus)

Taxonomy

Discus are fish from the genus ''Symphysodon'', which currently includes the species ''S. aequifasciatus'', ''S. discus'' and ''S. tarzoo'', based on a taxonomic review published in 2006. A review published in 2007 largely came to the same result, but differed in nomenclature, as the species called ''S. tarzoo'' in the 2006 study was called ''S. aequifasciatus'' in the 2007 study, and ''S. aequifasciatus'' in 2006 was ''S. haraldi'' in 2007. Further arguments have been made that ''S. tarzoo'' was not described in accordance with ICZN rules and thus should be considered invalid and replaced with ''S. haraldi'', currently considered a synonym of ''S. aequifasciatus'' by FishBase.

Other species and subspecies have been proposed, but morphometric data (unlike in ''Pterophyllum'', the freshwater angelfish) varies as much between individuals from one location as across the whole range of all discus fish species. ''S. tarzoo'' was described in 1959 and applies to the red-spotted western population. ''S. aequifasciatus'' and ''S. discus'', meanwhile, seem to hybridise frequently in the wild or have diverged recently, as they lack mitochondrial DNA lineage sorting but differ in color pattern and have dissimilar chromosomal translocation patterns. ''S. discus'' occurs mainly in the Rio Negro. Whether ''S. haraldi'' is indeed distinct from ''S. aequifasciatus'' remains to be determined; if valid it is widespread but it might just be a color morph.

A molecular study in 2011 found five main groups, which generally matched previously recognized phenotypes. They recognized them as evolutionarily significant units and species. Their assigning of scientific names to species differed to some extent from that used by earlier authors: Heckel (''S. discus''; Rio Negro, upper Uatumã, Nhamundá, Trombetas and Abacaxis), green (''S. tarzoo''; West Amazon drainages upriver from the Púrus Arch, blue (''S.'' sp. 1; central Amazon from Purus arch to the Meeting of Waters), brown (''S. aequifasciatus''; East Amazon downriver from Meeting of Waters), Xingu group (''S.'' sp. 2; Xingu and Tocantins). The Xingu group currently lacks a scientific name, but it is possible that the correct name for the blue is ''S. haraldi''. This taxonomy with four described valid species, ''S. discus'', ''S. tarzoo'', ''S. haraldi'' and ''S. aequifasciatus'', has been adopted by the Catalog of Fishes. Some hybridisation occurs (or has occurred) between the brown discus and neighbouring forms, but overall they maintain their separate evolutionary trajectories.

In addition to the wild discus, several captive variants achieved by selective breeding exist. Based on RAPD sequences, the captive variants popularly known as turquoise, pigeon, ghost, cobalt and solid red are derived from wild green, blue and brown discus (not Heckel discus).

Description

Like cichlids from the genus ''Pterophyllum'' (angelfish), all ''Symphysodon'' species have a laterally compressed body shape. In contrast to ''Pterophyllum'', however, extended finnage is absent giving ''Symphysodon'' a more rounded shape. It is this body shape from which their common name, "discus", is derived. The sides of the fish are frequently patterned in shades of green, red, brown, and blue. Some of the more brightly marked variants are the result of selective breeding by aquarists and do not exist in the wild. Discus typically reach up to in length, but captives have been claimed to reach . Adults generally weigh . No clear sexual dimorphism is seen for this fish, but males may reach a larger size than females. In breeding form varieties, solid red discus (red melon, red cover) females are generally redder than males.

Behavior

''Symphysodon'' spp. are highly social, typically occurring in groups that may number many dozens of individuals, which is unique among cichlids of the Americas. When breeding, the pair moves away from the group, possibly to reduce the risk of cannibalism of the young. As with most cichlids, brood care is highly developed with both the parents caring for the young. Additionally, adult discus produce a secretion through their skin, on which the larvae live during their first 4 weeks. During the first two weeks, the parents stay near their young allowing them to feed easily. In the last 2 weeks, they swim away, resulting in the young being gradually "weaned off" and starting to fend for themselves. Although rare in fish, more than 30 species of cichlids are known to feed their young with skin secretion to various extent, including ''Pseudetroplus'' and ''Uaru'' species. Sexual maturity is reached in a year.

Research has shown that, through this unique parental care behaviour (discus parents feeding their progeny with skin mucus), discus fish parents transmit key microorganisms to their fry. This parent-to-offspring transmission of important microorganisms might explain the high survival rate of discus fry raised with their parents, compared to the low survival rate of progeny raised artificially by fish breeders (e.g. on egg yolk, brine shrimp, or other replacement foods).

''Symphysodon'' spp. primarily feed on algae, other plant material, and detritus (periphyton), but also eat small invertebrates. Invertebrates can make up 38% of the stomach content in wild ''S. aequifasciatus'' during the high-water season, but this decreases during the low-water season, and year-round it is generally lower in the other species. Unlike more predatory cichlids, ''Symphysodon'' spp. have relatively long intestines typical of a herbivore or omnivore.

Distribution and habitat

''Symphysodon'' species inhabit the margins of floodplain lakes and rivers in the lowland Amazon basin, where they are part of the highly diverse Neotropical fish fauna. ''S. discus'' is restricted to blackwater habitats, but periodically these may experience brief floods of white water. ''S. tarzoo'' is found in both black and white water, and ''S. aequifasciatus'' also occurs in clearwater. Because of their preference for lentic habitats such as floodplains and flooded forests, white water inhabited by discus contains little suspended material (unlike main sections of whitewater rivers).

The three species of ''Symphysodon'' have different geographic distributions. ''S. aequifasciatus'' occurs in the East Amazon downriver from the Purus arch and ''S. tarzoo'' in the West Amazon upriver from the Purus arch. In contrast, the distribution of ''S. discus'' appears to be limited to the lower reaches of the Rio Negro, upper Uatumã, Nhamundá, Trombetas ,and Abacaxis Rivers.

The Nanay River in far western Amazonas is outside the native range; discus in this river were introduced from stock originating in the Tefé area by an aquarium exporter more than 30 years ago.

A significant number of discus now live in fish farms in Southeast Asia. These discus go to home aquaria around the world.

Home aquarium

Discus are kept by fishkeepers in the home aquarium, where they are valued for their striking appearance. They are considered difficult to keep, due to strict requirements for water quality and the need to be kept in groups. Breeders have selected for individuals with more adaptability to tap water conditions.

In home aquariums, discus live for an average of 10 years, but can live up to 15 years, and can grow up to 8 inches. Like many fish in the home aquarium, they will eat almost anything that fits within their mouth. Due to their size, they often require a minimum 55–75 gallon aquarium.

Each year the World Discus Competition takes place in Guangzhou, China. The North American Discus Association focuses on both supporting hobbyists and helping to support wild discus.

See also

* List of freshwater aquarium fish species
* Project ''Piaba''

References

External links

Discus Society MalaysiaFanatik-Discus-International informations, pictures, videos about discus fish

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Heroini
Taxa named by Johann Jakob Heckel