Discus found in the western Amazonian region are a distinct species which has been given an old scientific name.
Scientists from the Swedish Museum of Natural History and Instituto Nacional de Pesquisas da Amazonia (INPA) have named the species Symphysodon tarzoo.
Symphysodon aequifasciatus and Symphysodon discus, the other two Discus species, were found to have no significant genetic differences, suggesting they are very closely related or a single variable species.
"Surprisingly," wrote Ready, Kullander and Ferreira in the Journal of Fish Biology, "mitochondrial DNA indicates no difference between the two historically described species, Symphysodon discus and Symphysodon aequifasciatus, but shows that non-clinical variation exists with a lineage found in the western Amazon."This form is recognised as a distinct species, Symphysodon tarzoo...""This lineage is consistent with a colour form that is distinct from other Symphysodon lineages. This form has a parapatric distribution and is recognised as a distinct species, Symphysodon tarzoo."
The discoveries were made following a study of the colour pattern, morphology and mitochondrial DNA of large numbers of Symphysodon collected from the length of the Amazon river.
Previous work on the Symphysodon genus has been hampered by the small number of specimens used and the uncertainty surrounding their origin.
The current study is the most comprehensive work on the genus published to date.
Symphysodon tarzooSymphysodon tarzoo differs in colouration from other Discus species by the presence of red spots on the anal fin and the body and stems from a different mitochondrial lineage to the forms of Discus from the eastern Amazon.
The species was first identified by Lyons in 1959, who mentioned the red spots on its anal fin and body. But the description was based on aquarium specimens from Leticia in Colombia, to the far west of the Rio Negro, and the type specimens weren't preserved, so the species name didn't get accepted and the fish was placed in synonymy."The red spots on the anal fin and in the body distinguishes S. tarzoo from all other Symphysodon species..."Kullander believes that the name tarzoo is available under the International Code of Zoological Nomenclature, but outlined some nomenclatural problems:
"Because of the nomenclatural and taxonomic problems in Symphysodon, a neotype for S. tarzoo was selected. INPA 25960 from Brazil, Est. Amazonas, Rio Jutai. 1998, SO Kullander and EFJG Ferreira. It is an adult male of 132.4mm SL and has the following meristic data: Dorsal fin, X.30; anal fin, XIII.31; vertebrae, 14 abdominal, 17 caudal; scales, 58 on row E1, 20 in anterior lateral line, 14 in posterior lateral line.
"The red spots on the anal fin and in the body distinguishes S. tarzoo from all other Symphysodon species that have reticulations."
How many species?The findings raise questions about forthcoming changes to the names of Discus.
Previous studies by Kullander suggested just two species - aequifasciatus and discus - were valid.
Now, new evidence suggests that Symphysodon aequifasciatus and S. discus are very closely related or a single species which shares the genus with the new S. tarzoo.
The authors do not yet know why the genetic differences between Symphysodon aequifasciatus and discus are so minimal.
One theory is that the two upper and lower Amazon species diverged so recently that mitochondrial DNA differences could not be found.
Alternatively, the authors suggest that the genomes of all three species have diverged significantly but the similarity between the mitochondrial DNA of S. aequifasciatus and S. discus is due to a process called introgressive hybridisation.
"Under this scenario", the authors wrote, "the observed mtDNA sequence probably originally belonged to S. aequifasciatus because of the smaller number of sequences obtained for S. discus".
Jonathan Ready told Practical Fishkeeping magazine that he had no intention of synonymising Symphysodon aequifasciatus and S. discus.
"With the number of samples, observed variation and sequence data we had in our analysis it was possible to show the difference between S. tarzoo and the others both in terms of genetics and colour pattern, and the difference in melanic colour pattern and scale count difference between S. aequifasciatus and S. discus clearly remains.
"I am aware of numerous non-peer-reviewed articles online or in print that suggest that the melanic colour difference may not be a valid diagnostic character for the species, but for the time being I have not seen sufficient evidence to contradict the idea that it is (it would require quite a considerable data set and effort to do so).
"The reliability of images from many sources is something which unfortunately is rather questionable at times, and as such only the people involved in sampling can really know the confidence of the link between the collection location and the image (showing colour pattern of fish)."
BleherOthers have also been working towards a similar goal.
The fish collector Heiko Bleher told Practical Fishkeeping in January 2004 that he had undertaken a field study across wide areas of the Amazon basin.
Bleher said that the taxonomic organisation of Symphysodon at that time was incorrect.
He said that he was working in conjunction with Dr Axel Meyer from the University of Konstanz and would propose splitting the genus into three species, Symphysodon aequifasciatus, S. discus and S. haraldi.
Jonathan Ready told Practical Fishkeeping: "Efram Ferreira and Sven Kullander had in fact collected the samples we analysed quite a while ago (1998) with the very intention of trying to clear up the knowledge situation for the taxonomy of these fishes. However, such a number of samples require a reasonable amount of time to work through."
Ready said that he was aware that Bleher and Meyer were also studying the same thing. "We had the material and had already begun to do the work, so it seemed rather pointless to stop," Ready said.
"Not knowing whether our dataset/analysis was really identical to that others might be doing or not meant that there was clearly potential for different results, which is always interesting. We just kept doing what we had planned to do from the start and put out the results as we should do."
BiogeographySymphysodon tarzoo is found in Rio Ica, Rio Madeira, Rio Jurua, Rio Bauana and Lago Tefe areas in the Amazon flood basin upstream or west of Manaus, the capital of Amazonas.
The species has been sold in the aquarium trade for many years, most obviously under the name Tefe Discus.
AAccording to this study, those species found downstream, or east, of Manaus, include Symphysodon aequifasciatus and S. discus.
For more information on the new Discus species see the paper: Ready JS, Ferreira EJG and SO Kullander (2006) - Discus fishes: mitochondrial evidence for a phylogeographic barrier in the Amazonian genus Symphysodon (Teleostei: Cichlidae). Journal of Fish Biology (2006) 69 (Supplement B), 200-211.