The harlequin rasbora (Trigonostigma heteromorpha) is a small fish in the family Cyprinidae. The species became an instant favorite among aquarists after its introduction in the early 1900s and is the best known and most widely kept species among the rasboras. In 1935, an image of a trio of harlequin rasboras, stamped in 14k gold, graced the cover of the first edition of William T. Innes's classic Exotic Aquarium Fishes, and remained through all 19 editions.
Etymology and taxonomy The species was originally classified into the genus Rasbora, and given the specific name heteromorpha (Greek, "differently shaped") to alludes to the fact that its body shape differed from other members of that genus. The common name alludes to the black triangular patch on the body, reminiscent of the patterns found on the costume of a harlequin.
Description The harlequin rasbora is a fish that has an approximately lozenge-shaped body, whose basal colour from the head to the caudal peduncle is an orange-pink, the exact hue varying depending upon such factors as water conditions and the original population from which the fish was obtained. The posterior half of the body is overlaid with a large, roughly triangular black marking, that tapers toward the terminal end of the caudal peduncle, and begins approximately below the midpoint of the attachment of the dorsal fin (commonly called a "black wedge"). In common with all cyprinids, the articulation of the pectoral and pelvic fins follows a familiar pattern, the pectoral fins being located immediately posterior to the operculum or gill cover, whilst the pelvic fins are located some way farther back along the ventral portion of the body, in this case almost directly in a vertical line drawn through the dorsal fin. This relationship between the pectoral and pelvic fins is seen in characins as well as cyprinids, and is a feature of all the ostariophysans (fishes possessing an auxiliary mechanism for detecting sound consisting of a set of internal bones called the Weberian ossicles). The dorsal, anal, caudal and pelvic fins are all tinted red, the caudal fin being forked, with the red colour concentrated in the outermost rays, the inner section of the tail fin being more hyaline. It is possible to confuse this fish with two similar species that were indeed originally considered to be subspecies of T. heteromorpha, namely Trigonostigma espei and Trigonostigma hengeli. These fishes are more slender in body shape than T. heteromorpha, and the black marking, instead of being approximately triangular, has a horizontal stripe which tapers towards the caudal peduncle, and is greatly thickened and extended downwards below the dorsal fin. Due to this fishes with this marking are commonly known as "lamb chop rasboras" due to its perceived resemblance to the butchery cut known as a lamb chop. A fully mature individual harlequin rasbora attains a standard length of 4 cm～4.5 cm, the maximum length can reach two inches (5.1 cm). Males are cited as possessing a slightly larger black body patch than females, with the section adjoining the anal fin being more rounded in males. Ripe females are noticeably fuller in body outline.
Distribution and habitat
The harlequin rasbora is a native of the Malay Peninsula, Singapore, Sumatra and Borneo. Populations distributed in southern Thailand were found to be of the related species Trigonostigma truncata in 2020. It inhabits streams and other watercourses characterised principally by low mineral content, high concentrations of dissolved humic acids, a consequence of those waters flowing through peat swamp forests. The waterlogged soils of these forests inhibit the complete decay of leaf litter, resulting in the formation of peat, which leaches humic acids and related compounds into the watercourses flowing through these forests. The conditions thus resemble those found on a different continent, namely the blackwater habitats of South America, and the water chemistry of the Asian habitats of the harlequin rasbora is accordingly similar to that of the Rio Negro.
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