The beloved and diverse Corydoras catfish have long been a staple in the aquarium hobby. But a recent study has shaken up the taxonomy of these "armored" catfish, revealing surprising genetic diversity and leading to a major reclassification. Say goodbye to the single genus Corydoras and hello to seven distinct genera, each with unique characteristics and evolutionary history. Dive in to discover the exciting changes in the world of Corydoras!
It is hard to find another group of fish as popular as the small Corydoras catfish. Nijssen & Isbrücker called them the elegance group (die Eleganz-Gruppe) in 1980, which says a lot about their appearance. For a long time, Corydoras catfish were uniformly referred to as Corydoras. However, Corydoras includes over 200 described species and is actually a much larger group. It is expected that there are between 450 and 500 species in total. Over the years, aquarists themselves have been able to notice obvious differences between some Corydoras species in terms of body structure, size, behaviour, and also different larval stages (from the moment the fish has consumed the yolk sac and begins to take in external nutrition until the moment the organism takes on the shape of a fish body and the final body organs are formed). As early as 2011, genetic tests confirmed that there are huge differences between the individual species. Now the time has come when this has been confirmed and described, and the species are newly classified into 7 genera based on published research.
Corydoras Catfish: A Deep Dive into the "Elegance Group"
It's hard to find another group of fish as popular as the small Corydoras catfish. Nijssen & Isbrücker called them the "Elegance Group" (die Eleganz-Gruppe) in 1980, which speaks volumes about their appearance. For a long time, all Corydoras catfish were simply referred to as Corydoras. However, Corydoras encompasses over 200 described species and is, in reality, a much larger group. It is estimated that there are between 450 and 500 species in total.
Over the years, aquarists have noticed clear differences between some Corydoras species in terms of body structure, size, behavior, and larval stages (from the moment the fish consumes the yolk sac and begins to take in external nutrition until it develops a fish-like body with fully formed organs). As early as 2011, genetic tests confirmed huge differences between individual species. Now, based on recent published research, these species have been reclassified into 7 genera.
Reclassification of Corydoras Based on New Research
A Brazilian research team, Dias et al., published their findings this year (June 11, 2024) in the Zoological Journal of the Linnean Society. The research confirms extraordinary genetic differences, providing clear evidence that these are distinct genera. The results identified different groups of species, each with an assigned genus name. These names are not new, however; they were already available, making it possible to build on previous studies and use existing designations based on similarities with previously described genera.
There are now 7 genera within the family Callichthyidae, subfamily Corydoradinae:
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Corydoras (lineage 1) now contains only 32 species (out of over 175 that belonged here before the publication of the new division). Described: 1803 in "A review of the genus Corydoras". Representatives of this genus can be found in almost every river system in the South American lowlands. From a phylogenetic (evolutionary) point of view, it is considered a sister group to the others. This is also evident in the diagram of the attached image for this article. Typical features are dark markings above the eyes (sometimes may not be fully visible).
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Aspidoras (lineage 2) 18 species (rather small and mostly have black and white spots, and all come from Brazil). Described: Ihering, R. von. 1907: "Diversas especies novas de peixes nemathognathas do Brazil". Representatives of this genus are 5 cm or just over 5 cm long and are very variably colored.
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Scleromystax (lineage 3) consists of seven species and is closely related to Aspidoras, but compared to Aspidoras it is slightly larger and has a longer and more massive head profile (only one species is kept in hobby aquaristics - Scleromystax). Described: Günther, Albert C. L. G. 1864. "Catalogue of the fishes in the British Museum". It occurs only in southeastern Brazil in the original untouched Amazon rainforest. Visually, it has a high-contrast black and white coloration, a cylindrical and elongated body, and males have skin teeth ("Bart") on the sides of their heads, which are a distinctive feature of this group. In addition, skin teeth run along the inside of the first rays of the pectoral fin and point towards the root and not towards the tip, as in other species.
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Gastrodermus (reinstated), (lineages 4 and 5) has the shortest head profile of all species and is probably the cutest, with a total of 14 species. Described: Cope, Edward D. in his work Synopsis of the fishes of the Peruvian Amazon from 1878 identified morphological differences between individual catfish already at that time. In later published works, the results of the research were not taken into account, and so this genus was not used for classification purposes from 1940 onwards. It has only now been reinstated as valid. This genus is characterized by a very short head and, in conjunction with a relatively oval (egg-shaped) body shape and a very high back, clearly distinguishes them from other groups. Furthermore, the rays of the first pectoral fin on the inner side are covered with tiny teeth, which are all aligned towards the tip.
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Osteogaster (reinstated), (lineage 7) is the second group, which is very similar in color, which is green, and there are only 9 of them so far. Osterogaster aeneus from this group is probably the best known and most widespread. Described: Cope, Edward D. in Synopsis of the fishes of the Peruvian Amazon from 1878. These representatives are strikingly greenish and their coloration is often accompanied by a black mask and dark back coloration. In addition to a relatively short head, they have moderately to strongly toothed first rays of the pectoral fin on the inner side, which point towards the tip. Osterogaster is most closely related to the genus Brochis, specifically 8a.
Other Species and their Designations
As already mentioned, it is assumed that there are more than 400 species of Corydoras catfish, so it can be expected that new information and newly described forms will be constantly discovered. An example is a new species from Peru, which has now been scientifically described, formerly designated C53, which it received from Hans-Georg Evers as early as 1996. Newly, thanks to Tencatt et al., it boasts the designation Corydoras iiap. This is a strongly territorial Corydoras catfish.
It will be interesting to see how other new species, all designated with C- and CW- numbers, such as Sp. CW153, which are still awaiting their scientific designation, will be classified.
Evers,H.G. (2024): Alles neu - mit alten Namen, Amazonas,
