Taxonomy
The grouping of fish now deemed danionins has been the subject of constant research and speculation throughout the 20th century. Nearly all the fish classed within the Danio and Devario genera were originally placed in the Danio genus upon discovery. However, in the first part of the 20th century Dr George S. Myers split them into three genera, Danio, Brachydanio and Daniops. The sole species within Myers' Daniops, Daniops myersi has long ago been found to be a synonym of Devario laoensis, however his Brachydanio genus lasted for much longer, as it included most of the fish now classed as Danio, whereas Danio included most of the fish now classed as Devario.
However Danio dangila and Danio feegradei both of which had most of the characteristics of the Brachydanios (with the exception that they were much larger than the Brachydanios) were placed within the Danios. (Due to this and other mis-placing, both Danio and Brachydanio were found to be paraphyletic by Fang Fang in 2003.). In 1941 H.M. Smith attempted to unite all the Brachydanios and Danios into one genus on the basis of a fish from Thailand which was supposed to bridge the gap. He downgraded both Danio and Brachydanio into subgenera and erected a new subgenera of Allodanio with one member Allodanio ponticulus, however Myers later pointed out that ponticulus was actually a member of the Barilius genus.
In a 1986 paper, Tyson Roberts noted that the danionin group was thought to include Parabarilius, Danio, Brachydanio and Danionella. In this scheme, danionins were distinguished from other cyprinids by the uniquely shared character of the danionin notch, a large and peculiarly shaped indentation in the medial margin of the mandibles; this feature is not noted in rasborins, esomins, bariliins, or chelins. However, all of these categories at that time were informal. Microrasbora was not considered to be a part of the danionins, nor even closely related to Danionella, a part of the danionins as understood at that time.
In the late 1980s and 1990s doubts grew about the validity of Brachydanio, with species being referred to their original naming of Danio, and in a 2003 paper Fang Fang determined that the genus Danio, recognized up to that point, was paraphyletic. Fang restricted Danio to the species in the "D. dangila species group", which at the time comprised nine species including D. dangila, D. rerio, D. nigrofasciatus, and D. albolineatus; the remaining Danio species were moved to Devario, which at this time included D. malabaricus, D. kakhienensis, D. devario, D. chrysotaeniatus, D. maetaengensis, D. interruptus, and D. apogon.
The only Danios to have been consistently called Danio were D. dangila & D. feegradei. As D. dangila was the first discovered Danio (or type) the name Danio had to remain with dangila, which is why the vast majority of species were moved to Devario.
Also, in Fang's 2003 paper, the sister group to Devario was deemed to be a clade formed by Inlecypris and Chela and, more controverisally, Esomus was found to be the sister group of Danio. The relationships of Sundadanio, Danionella, Microrasbora remained unresolved. The danionin notch was found to not supported to be a danionin synapomorphy.
In 2007 in another paper by Tyson Roberts, Celestichthys margaritatus was described as a new species of Danioninae. Apparently, it is most closely related to Microrasbora erythromicron; the other Microrasbora species differ significantly from Celestichthys. The genus is identified as a danionin due specializations of its lower jaw and its numerous anal fin rays. Though it lacks a danionin notch, Celestichthys exhibits the "danionin mandibular knob", a bony process on the side of the mandible behind the danionin notch or where the notch should be; it is perhaps diagnostic of danionins. This knob is better developed in males than females. The author notes that fish of Rasborinae almost invariably have anal fins with three spines and five rays. Celestichthys has 3 anal fin spines and 8½-10½ anal fin rays. Also, rasborins have the generalized cyprinid principal caudal fin ray count of 10/9, while all Asian cyprinids with fewer than 10/9 principal caudal fin rays are all diminutive species of Danioninae, including Celestichthys, M. erythromicron, Danionella, and Paedocypris.
In 2007, an analysis of the phylogenetic relationships of the recently described genus Paedocypris was published, placing it as the sister taxon to Sundadanio. The clade formed by these two genera was found to be sister to a clade including many danionin genera as well as some rasborin genera such as Rasbora, Trigonostigma, and Boraras, making the danionin group paraphyletic without these rasborin genera based on these results. This paper considered the danionin genera to be within a larger Rasborinae.
Also in 2007, another study by various authors analyzed the relationships of Danio. These authors considered Rasborinae to have priority over Danioninae, suggesting that they have the same meaning. Also, Danio was found to be the sister group of a clade including Chela, Microrasbora, Devario, and Inlecypris, rather than in a clade exclusively with either Devario or Esomus as in previous studies. This paper supported the close relationship of "Microrasbora" erythromicron to Danio species; however, this study did not include Celestichthys, which was noted by Tyson Roberts in his 2007 paper as being likely to include erythromicron but with further research needed.
Tanichthys is often regarded as a danionin by aquarists and grouped as such in some older aquatic publications, however there is no scientific basis for this, a fact stated on numerous occasions by Brittan and others. It is more closely related to the Rasboras. The danionins can be classed as a subfamily Danioninae which is increasingly gaining credibility as a distinct subfamily from Rasboriniae within the Cyprinidae family. However, in Nelson, 2006, Danioninae was listed as a synonym of Rasborinae. On the other hand, neither inter- nor intrarelationships among the "rasborins" have as yet been thoroughly analyzed.
A number of the species have only been recently discovered, in remote inland areas of Laos and Myanmar and do not yet have scientific names. They are listed as Danio or Devario sp "xxxx" within the relevant genera and disambiguation pages, where details of the fish can be found by clicking on the hyperlink.
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