Danionin

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, Devario and Rasbora. They are primarily native to the fresh waters of South and Southeast Asia, with fewer species in Africa. Many species are brightly coloured and are available as aquarium fish worldwide. Danio species tend to have horizontal stripes, rows of spots, or vertical bars, and often have long barbels. Devario species tend to have vertical or horizontal 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. 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 1941, H.M. Smith attempted to unite all the Brachydanios and Danios species 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 subgenus of Allodanio with one member, Allodanio ponticulus, but Myers later pointed out that A. ponticulus was actually a member of the genus Barilius. 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 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 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.