Danionin

Danionins are small, slender, pelagic freshwater fish belonging to the family Danionidae within the suborder Cyprinoidei. Based on phylogenetic research from 2009, the group includes species from the genera *Danio* and *Devario*, as well as the genera *Chela*, *Laubuka*, *Microdevario*, and *Microrasbora*. They are mostly found in shallow freshwater environments such as ponds, streams, floodplains, marshes, and rice paddies across South and Southeast Asia, with a smaller number of species in Africa. Many danionins are brightly colored and are commonly kept in aquariums worldwide. Fish in the *Danio* clade typically have horizontal stripes, rows of spots, or vertical bars, and often possess long barbels. Those in the *Devario* clade tend to have vertical or horizontal bars and short, rudimentary barbels, if they have them at all.

All danionins are egg scatterers, timing their reproduction to the monsoon season in the wild. They are opportunistic feeders, consuming a variety of tiny insects, crustaceans, plankton, and other small organisms and invertebrates.

The earliest known fossil records of danionins come from the Eocene of Sumatra, Indonesia, where two species tentatively assigned to *Rasbora*—*'Rasbora' antiqua* and *'Rasbora' mohri*—have been found.

Since 2004, many new danionins have been discovered that lack scientific names, and numerous species previously known only to scientists have entered the aquarium trade. This has led to confusion in naming, with some species having up to five different common names and some common names applied to as many as four different species. Scientific names have also been unstable, as many species have been moved between genera repeatedly over recent decades, and some have been synonymized and later unsynonymized.

The genera within the danionin group are organized into several subfamilies: Chedrinae (including *Barilius*, *Bengala*, *Cabdio*, and others), Rasborinae (including *Amblypharyngodon*, *Boraras*, *Rasbora*, and others), Danioninae (including *Betadevario*, *Chela*, *Danio*, *Danionella*, *Devario*, and others), and Esominae (including *Esomus*).

In an aquarium, danionins are active swimmers that occupy the top half of the tank and will eat almost any type of aquarium food, though they generally avoid plants and algae. They are boisterous and may chase each other or other fish, but they are good community fish and do not typically attack others. They may occasionally nip fins and, like most fish, will eat eggs and any fish small enough to fit in their mouths. They are easily stressed by flowing water and bright light, and in the wild they occur in stagnant water with a low pH of 3 to 5, caused by peat accumulation under a dense canopy. This often makes them subtropical, with temperatures of 20 to 22 °C (68 to 72 °F) being suitable. They are good jumpers, so a tight-fitting lid is recommended.

The taxonomy of danionins has been the subject of ongoing research and debate throughout the 20th century. Nearly all fish now placed in *Danio* and *Devario* were originally assigned to *Danio* upon discovery. In the early 20th century, George S. Myers split them into three genera: *Danio*, *Brachydanio*, and *Daniops*. The sole species in *Daniops*, *D. myersi*, was later found to be a synonym of *Devario laoensis*, but *Brachydanio* persisted for much longer, containing most of the fish now classified as *Danio*, while *Danio* held most of those now in *Devario*. However, *Danio dangila* and *Danio feegradei*, which shared most characteristics of *Brachydanio* except for their larger size, were placed in *Danio*. This and other misplacements led Fang Fang in 2003 to find both *Danio* and *Brachydanio* paraphyletic. In 1941, H.M. Smith attempted to unite all *Brachydanio* and *Danio* species into one genus based on a fish from Thailand thought to bridge the gap, downgrading both to subgenera and erecting a new subgenus *Allodanio* with one member, *Allodanio ponticulus*. Myers later pointed out that *A. ponticulus* was actually a member of the genus *Barilius*. The danionin group was once thought to include *Parabarilius*, *Danio*, *Brachydanio*, and *Danionella*, distinguished from other cyprinids by the shared "danionin notch" character.