The danionins are a group of small minnow-type fish belonging to the family Cyprinidae. Members of this group are mostly in the genera Danio and Devario. They are native to the fresh waters of southeast Asia. Many species are brightly coloured and are available as aquarium fish worldwide. Danio tend to have horizontal stripes and long barbels, Devario tend to have vertical bars and short rudimentary barbels, if barbels are present at all. All danionins are egg scatterers and breed in the rainy season in the wild. They are carnivores living on insects and small crustaceans in the wild.
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 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 of 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. Danio was restricted to the species known as the "D. dangila species group", which at the time comprised nine species including D. dangila, D. rerio, D. nigrofasciatus, and D. albolineatus, and the remaining Danio species were moved to Devario. Devario at this time included among many others 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.
Although boisterous and liable to chase each other and other fish, they are good community fish and will not generally attack each other or other fish, although they occasionally nip fins, more by accident than design and will, like most fish, eat eggs and any fish small enough to fit into their mouths.
They are best kept in a tank long enough for their active swimming, preferably with a current from a power filter (or at least airstone) as they often live in fast flowing streams in the wild. Generally this also results in them being sub tropical with temperatures of 20 to 22 degrees Celsius (low seventies degrees Fahrenheit) often being fine, however they are good jumpers and a tight fitting lid is recommended.
Individual Danionin species are listed within the relevant pages for each Genera, however many Danionin species have been changed into different genera over the last decades in some cases repeatedly, similarly some species have been synomymised with other species and in some cases later unsynonymised, all of which has caused a lot of confusion. As a result a separate page has been created which lists all danionin species and also lists defunct species which have since been synonymised or renamed.