Discus genus revised

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Heiko Bleher and co-workers have published a revision of the Discus genus Symphysodon which provides evidence for the existence of three genetically distinct clades.

Bleher worked with Kai Stlting, Walter Salzburger and Axel Meyer, who revised the Symphysodon genus based on partial DNA sequences from portions of the mitochondrial control regions of 48 specimens collected from 20 locations across the lower and central Amazon.

The study, which has just been published in Bleher's journal Aqua, draws upon Bleher's extensive field work undertaken over the past 40 years, in which he has examined the fishes' distributions and documented their adaptation to specific types of water.

Three speciesThe authors recognise the three clades as S. aequifasciatus, S. haraldi and S. discus, but came across problems when trying to determine what was happening in the "discus" and "haraldi" clades:

"One of these genetic clusters is composed of specimens that morphologically are S. discus, but also of S. haraldi and natural hybrids of S. discus x S. haraldi.

"This indicates that either the "discus" clade is composed, at least partially, of hybrids or, alternatively, that a "haraldi" phenotype evolved (or was retained) independently in this clade.

"The other two clades consist of S. aequifasciatus and S. haraldi."

The study differs slightly to last year's paper by the leading cichlid taxonomist Sven Kullander and his co-workers Jonathan Ready and Efram Ferreira. (See New Discus named Symphysodon tarzoo).

Kullander's work suggested that Symphysodon aequifasciatus and S. discus were closely related or the same, and that a third species, S. tarzoo could be identified by its characteristic red spots.

Kullander says that the main differences are merely in the names chosen, as his paper refers to the Green discus as S. tarzoo, while Bleher refers to it as S. aequifasciatus. The Blue and Brown discus are considered to be S. aequifasciatus by Kullander et al., while Bleher names them S. haraldi.

ColourationBleher's study, like previous analyses of the Symphysodon genus, has also taken their differing colour patterns into consideration.

S. discus and S. aequifasciatus both have nine vertical bars on their flanks, but the first, fifth and ninth bars of S. discus are much more prominent than the same bars in S. aequifasciatus, in which the bars are normally of equal width.

Bleher's study claims that S. aequifasciatus can also be recognised by its rust-brown or red dots on the body, which range from a few spots to a dense pattern covering the fish, or more rarely, red spotted lines on the anal fin region.

The colour patterns of haraldi are much more variable, and then there is the problem of the hybrids, say the authors:

"Symphysodon haraldi displays a wide range of colours, colour patterns and a larger number of vertical bars (8 up to 16), which may differ substantially in shape.

"Except for hybrids of S. discus x S. haraldi, S. haraldi does not resemble the other two species.

"A study of geographic distribution patterns of the three species was carried out throughout the central and lower Amazon basin: in the western Amazon in almost every tributary of the Solimes and the Maraon Rivers to Iquitos, and in the eastern part in most tributaries of the Amazon River down to its mouth."

Blue, green or brown?Since the study has come from an author with a background in the aquarium trade, it makes reference to the common names for the fish, which the Kullander study did not.

The authors wrote: "The valid names of the three species are: S. discus - the Heckel discus; S. aequifasciatus - the green discus; and S. haraldi - the blue discus.

"The 'brown' or 'common' discus of the aquarium trade is the same as the 'blue' discus."

"In S. aequifasciatus a congruence of genetic and morphological (colour) characters has been found, whereas some specimens that would phenotypically be assigned to S. haraldi, genetically group also with the S. discus clade."

Future work using nuclear rather than mitochondrial DNA is required to analyse the genetics of Bleher's S. discus clade.

It remains to be seen which classification scientists will adopt for the placement of the species.

It's probable that most will wait until further detailed analysis of Bleher's haraldi and discus clades has been published before drawing any firm conclusions on the group's taxonomy.

For more information see the paper: Heiko Bleher, Kai N. Stlting, Walter Salzburger, and Axel Meyer (2007) - Revision of the Genus Symphysodon Heckel, 1840 (Teleostei: Perciformes: Cichlidae) based on molecular and morphological characters, pp. 133-174