Taxonomy & naming
Trigonostigma heteromorpha was described by Georg Duncker in 1904 from the Malay Peninsula, originally as Rasbora heteromorpha. The Catalog of Fishes (Eschmeyer, the authority for valid names) records the valid combination as Trigonostigma heteromorpha (Duncker, 1904), the parenthetical authority marking the move out of Rasbora.
The genus Trigonostigma was erected by Kottelat and Witte in 1999 for a small group of deep-bodied, wedge-marked "rasboras" — the harlequin and its relatives T. espei (the lambchop rasbora), T. hengeli (the glowlight rasbora), T. truncata and T. somphongsi — distinguished from true Rasbora by body shape, the triangular flank mark, and above all by their leaf-spawning reproduction. The harlequin is therefore a textbook case of the fuzzy danio–rasbora boundary: long sold and written about as a Rasbora, it is now placed in its own genus, while the celestial pearl "galaxy rasbora" went the other way, into Danio.
Morphology
The harlequin is a small, deep-bodied and laterally compressed cyprinid reaching about 1.5–2 in in length, much taller-backed than a typical slim rasbora. The body is a warm pinkish-copper to orange, often with a faint pink iridescence, and the defining feature is a single solid black marking — a blunt triangle or "pork-chop" wedge — covering the rear half of the body from roughly below the dorsal fin to the base of the tail. The fins are flushed with red-orange in well-conditioned fish.
Sexual dimorphism is subtle but readable. Males are slightly smaller and slimmer with a more intense, more extensive black wedge, the lower front edge of which is often rounded or extended into a small point; females are deeper and rounder, especially when carrying eggs, with a more straight-edged wedge. Colour intensity rises sharply with good condition, soft water and the security of a large group.
Habitat
Trigonostigma heteromorpha is native to Southeast Asia, on the Malay Peninsula (peninsular Malaysia and southern Thailand), Singapore and Sumatra. Its classic habitat is blackwater — peat swamp forests and the slow, shaded streams that drain them, where tannins from decaying leaf litter stain the water the colour of weak tea and dissolved minerals are very low.
This is soft, acidic water: pH values well below neutral, often in the 5–6.5 range, very low hardness, and warm tropical temperatures broadly in the mid-to-high twenties Celsius. Such habitats are dim, still and rich in submerged wood, leaf litter and broad-leaved plants — including the Cryptocoryne whose leaves the fish uses to spawn. Much of this original peat-swamp habitat has been lost to drainage and conversion, so the species is now more reliably encountered in the trade than in pristine wild blackwater.
Feeding
The harlequin rasbora is an omnivorous micropredator. In the wild it feeds on small invertebrates — insect larvae, tiny crustaceans and zooplankton — taken from the water column, along with some algae and fine organic material. Its small, upturned mouth suits picking small prey from open water and surfaces rather than grubbing in the substrate.
In the aquarium it is easy to feed and accepts good-quality flakes and micro-pellets readily, but its colour and condition are best maintained with regular small live and frozen foods such as daphnia, cyclops, baby brine shrimp and finely chopped bloodworm. As a small, gentle fish it feeds calmly and can be outpaced by boisterous tankmates, so it does best among similarly peaceful companions.
Mating
The harlequin is the notable exception to the danio-and-rasbora rule of scatter-and-abandon. Instead of broadcasting adhesive eggs over plants or gravel, a courting pair selects a broad-leaved plant — classically a Cryptocoryne — and the female deposits her eggs on the underside of the leaf, where the male fertilises them. The pair perform a characteristic trembling, side-by-side dance beneath the leaf, the female inverting to press her vent against its surface.
The eggs are adhesive and stick to the leaf rather than falling away. There is still no extended parental care — the adults do not guard the clutch and will eat eggs and fry if able — but the choice of a specific, sheltered spawning site sets the harlequin and its Trigonostigma relatives apart from the true scatterers, and is one of the traits that defines the genus.
Breeding
Breeding harlequins is a rewarding step up from the easy scatterers and demands attention to water chemistry. A separate, dimly lit spawning tank with very soft, acidic water (low hardness, pH around 5.5–6.5) and broad-leaved plants such as Cryptocoryne is the classic setup. A well-conditioned pair, brought up on live foods, is introduced, and spawning follows the under-leaf dance described above.
A female lays a modest clutch — typically a few dozen eggs, placed in small batches on the leaf undersides over the course of a spawning. The adults are removed afterwards, as they will eat the eggs. The eggs hatch in roughly a day at tropical temperatures, and the larvae become free-swimming after a few more days, when they are fed infusoria and then newly hatched brine shrimp. Success hinges far more on soft, clean, acidic water than on any special trick, which is why the species was long considered a connoisseur's fish to breed.
In the aquarium
The harlequin rasbora is a hardy, peaceful and beautifully coloured community fish, and one of the best choices for a calm, well-planted tank. It is a shoaling species and should be kept in a group of at least eight to ten, where it colours up, schools loosely and stays confident; in small numbers it is drab and shy. Its even temperament makes it an ideal companion for other gentle fishes — small tetras, other rasboras and danios, dwarf cichlids, gouramis and Corydoras.
It looks its best and stays healthiest in soft, slightly acidic, tannin-tinted water in the mid-to-high twenties Celsius, over a dark substrate among plants and wood — conditions that echo its blackwater home and deepen its red. While it tolerates a range of ordinary tap-water conditions and is undemanding to keep alive, the difference between a hard-water survivor and a soft-water harlequin in full colour is striking. It is long-lived for a small cyprinid when well kept.
Conservation
Trigonostigma heteromorpha is listed as Least Concern (LC) on the IUCN Red List. It remains widespread across the Malay Peninsula, Singapore and Sumatra and is abundant in the aquarium trade, the bulk of which is now supplied by commercial captive breeding rather than wild collection, limiting pressure on natural populations.
The real threat is habitat-based. The peat-swamp forests and blackwater streams the species depends on are among the most heavily destroyed freshwater habitats in Southeast Asia, drained and cleared for agriculture, plantations and development; the harlequin is regarded as locally threatened where these waters have been lost, including in Singapore. Its overall security rests on its wide range and its established place in cultivation rather than on the health of its shrinking wild blackwaters.