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 genera
Danio and
Devario were originally placed in the genus
Danio upon discovery. However, in the first part of the 20th century,
George S. Myers split them into three genera,
Danio,
Brachydanio, and
Daniops. The sole species within Myers'
Daniops,
D. myersi, has long ago been found to be a synonym of
Devario laoensis, but his genus
Brachydanio 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
Brachydanio (with the exception that they were much larger than
Brachydanio species) were placed within
Danios. (Due to this and other misplacing, both
Danio and
Brachydanio were found to be paraphyletic by Fang Fang in 2003. In the late 1980s and 1990s, doubts grew about the validity of
Brachydanio, with species being referred to their original naming of
Danio, and Fang Fang determined that the genus
Danio, recognized up to that point, was
paraphyletic. Fang 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
Danio species to have been consistently called
Danio were
D. dangila and
D. feegradei. As
D. dangila was the first discovered
Danio (or type) the name
Danio had to remain with
D. dangila, which is why the vast majority of species were moved to
Devario. Also, the sister group to
Devario was deemed to be a
clade formed by
Inlecypris and
Chela, and more controversially,
Esomus was found to be the
sister group of
Danio. The relationships of
Sundadanio,
Danionella, and
Microrasbora remained unresolved. The danionin notch was found to not supported to be a danionin
synapomorphy. In another paper,
Celestichthys margaritatus was described as a new species of the 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 fish of Rasborinae almost invariably have anal fins with three spines and five rays.
Celestichthys has three anal fin spines and 8–10 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 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 Roberts as being likely to include
Erythromicron, but with further research needed. In 2009 and 2010, detailed mitochondrial and nuclear DNA analyses of the phylogenetic interrelationships of the danionins were published by Fang et al (Zoologica Scripta, 38, 3, 2009) and Tang et al (Molecular Phylogenetics and Evolution 57, 2010). These two significant studies confirmed or established several taxonomic standings: The Danionin Tribe (of the Cyprinid Sub-family, Danioninae) were reduced to 3 clades -
danio (
Danio and
Danioella genera),
devario (
Devario, Chela, Laubuka, Microdevario genera), and a combined
esomus/paedocypris/sundadanio genera clade.
Rasbora and related genera were excluded. The aquarium popular Celestial Pearl Danio / Galaxy Rasbora was confirmed as
Danio margaritatus, being most closely related to
D. erythromicron, and next to
D.choprae. Within the
devario clade,
Microdevario was erected for all but one of the former
Microrasboras. There is still uncertainty over the exact relationships for the three miniature taxa: Danionella, Paedocypris, and Sundadanio as all three have shown variability in phylogenetic position among different studies, both molecular and morphological.
Tanichthys is often regarded as a danionin by aquarists and grouped as such in some older aquatic publications, but no scientific basis exists for this, a fact stated on numerous occasions by Brittan and others. However, in Nelson, 2006, Danioninae was listed as a
synonym of Rasborinae. However, neither inter- nor intrarelationships among the "rasborins" has 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. ==References==