Trigonostigma

The lambchop rasbora, Trigonostigma espei, is a small cyprinid from southwestern and southeastern Thailand, reaching into southwestern Cambodia. It can be confused with T. heteromorpha and T. hengeli but is generally bright copper to reddish, with a thinner 'lambchop'-shaped body marking. Very peaceful, it inhabits gently flowing forest streams where submerged plants grow thickly.

Trigonostigma hengeli is a small rasbora from the Greater Sunda Islands, collected in Jambi province, Sumatra and apparently West Kalimantan, Borneo. It is sometimes sold as the 'glowlight rasbora', a name also applied to Trigonopoma pauciperforatum. Slightly smaller than T. espei and T. heteromorpha, it has a greyish body with bright orange along the upper and anterior edges of the 'lambchop' marking.

The harlequin, Trigonostigma heteromorpha, is a small cyprinid from southern Thailand, Peninsular Malaysia, Singapore and Sumatra. First exported for aquaria in the early 1900s, it is now mass-produced, and farmed fish lack much of the colour of wild ones. The dark triangular body marking sets it apart from T. espei and T. hengeli, and two divergent lineages found in aquarium stock suggest the species as currently recognised may contain more than one taxon.

Trigonostigma somphongsi is a small rasbora, probably endemic to western Thailand and reckoned to occur somewhere in the Mae Klong basin. Exceptionally rare, it was considered potentially extinct for around a dozen years before a trio arrived in Germany among Boraras urophthalmoides in 2006. Its dark flank marking is reduced to a thickish black stripe, and captive stock is being bred to establish a sustainable population.