Esomus caudiocellatus

Esomus: from the Latin preposition e-, meaning 'out of', and Ancient Greek σῶμα (sôma), meaning 'body', presumably in reference to the extremely long maxillary barbels.

caudiocellatus: from the Latin cauda, meaning 'tail', and ocellatus, meaning 'having small eyes', in reference to the dark spot on the caudal peduncle of this species.

Unclear, and no type locality was provided in the Ahl's description. It has been collected from the Salween river system in eastern Myanmar and northwestern Thailand but may also occur elsewhere.

An aquarium with base dimensions of 120 ∗ 30 cm48 ∗ 12 in should be the smallest considered.

There currently exist a dozen described Esomus specie,s although few are seen in the ornamental trade and none are especially popular. Most are commonplace in their native countries but are generally overlooked by collectors due to their relatively plain colouration. Their most common use in some areas is actually as a feeder fish in the aquaculture of larger species. They are characterised by greatly-enlarged pectoral fins and two pairs of barbels, of which the maxillary pair are extremely long and usually reach the pectoral fins.

E. caudiocellatus can be told apart from congeners by lacking a dark lateral stripe and possessing a prominent dark marking on the caudal peduncle.

In recent years a number of phylogenetic studies involving Esomus and its near relatives have been conducted and conflicting results published. For example a 2003 study by Fang et al. concluded that the genus is the sister group, i.e., most closely-related to, Danio whereas Mayden et al. (2007) placed the genera Chela, Microrasbora, Devario and Inlecypris as sisters to Danio with Esomus as a basal sister group to that larger clade.

A further analysis by Fang et al. published in 2009 recovered Esomus as sister to Danio but noted that its exact placement is uncertain due to conflicting results depending on the type of analysis performed. The authors go on to state that more detailed studies into the molecular and morphological characters of the genus are needed in order to identify its exact relationships with its closest relatives.

Some aquarium hobbyists have recorded a type of wasting disease that can appear in Esomus spp. characterised by a rapid reduction in body mass of affected specimens. The problem manifests itself as an initial thinning of the ventral part of the body just behind the gills and usually results in death within a couple of weeks. As yet no treatment is known.