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The Harlequin Rasbora is a small fish belonging to the Family Cyprinidae, subfamily Rasborinae, which was originally given the scientific name of Rasbora heteromorpha. The specific name heteromorpha, translating from the Greek as “differently shaped”, alludes to the fact that the body shape of this species differs from that of other members of the genus Rasbora, and as a consequence of this and assorted other factors (including the difference in breeding technique), the fish was assigned to a new genus, Trigonostigma, and is thus now known to science as Trigonostigma heteromorpha. The common name for this fish, Harlequin Rasbora, alludes to the black triangular patch upon the body, which is reminiscent of the patterns found on the costume of a Harlequin.
The Harlequin Rasbora is a native of Malaysia, Singapore, Sumatra and southern Thailand. It is an inhabitant of streams and other waterbodies that are located in peat swamp forests (more details covered in the Habitat section below).
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 further 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 shape that approximates to that of a Stillson wrench, and fishes with this marking were once known as “Lamb Chop Rasboras” because the marking was fancifully thought to resemble in appearance the item of meat in question.
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