Taxonomy & naming
Trigonostigma truncata was formally described by Heok Hee Tan in 2020, making it one of the most recently erected members of its genus. The species description was published in a scientific archive (Zenodo, 2020) and documented through the novataxa species-new-to-science network. The Catalog of Fishes (Eschmeyer, CAS) is the authority for the current valid name, Trigonostigma truncata Tan, 2020.
The genus Trigonostigma was erected by Kottelat & Witte in 1999 to receive the harlequin rasboras, which had previously been placed in the large catch-all genus Rasbora. The defining character of Trigonostigma is the black axial blaze — triangular or wedge-shaped — running along the posterior flank, combined with a stocky body form and the leaf-spawning reproductive strategy shared by all members. The genus currently contains four or five recognized species: the classic harlequin T. heteromorpha, the similar T. espei, the small T. hengeli, and now T. truncata, plus the Borneo endemic T. somphongsi.
The epithet truncata (Latin: cut off, shortened) refers to the distinctively shaped black marking on the flank, which differs from the longer, more sweeping wedge seen in T. heteromorpha and T. espei — though the precise morphological character to which Tan applied the name should be confirmed in the formal description.
Morphology
Trigonostigma truncata is a small, laterally compressed cyprinid reaching approximately 1 in standard length in type specimens, with trade fish recorded to about 1.5 in SL; FishBase gives a total-length range of 1.5–2 in. The body profile is slightly less deep than in the closely related T. heteromorpha, which tends to be rather high-bodied.
The most striking features are the black axial marking on the posterior flank — which gives the genus its name (Greek trigono = triangle, stigma = spot or mark) and is distinctively shaped in this species — paired with a bluish-lilac iridescent sheen across the flanks and bold orange-red coloration in the anal fin. A light blue iridescent stripe is also visible. These colour elements set T. truncata apart from congeners and make it recognisable even in trade stock. The general body plan, including the typical harlequin body depth and small upturned mouth, follows the genus pattern.
Habitat
Trigonostigma truncata is native to the east coast of the Malay Peninsula, from Narathiwat province in the extreme south of Thailand southward into Terengganu state in peninsular Malaysia. The type locality falls within coastal swamp-forest terrain draining into the South China Sea. The species inhabits lowland freshwater acid swamp-forest streams — slow or near-still, shallow channels running through peat-swamp and coastal forest, heavily stained with humic and fulvic acids leached from decaying leaf litter and peat.
These blackwater habitats are chemically extreme: the water is typically very soft with negligible buffering capacity, strongly acidic (pH commonly 4–6), warm, and rich in dissolved organic compounds. Conductivity is extremely low. Marginal and submerged vegetation, including Cryptocoryne species, occurs in the shallower margins where these rasboras congregate. The light is diffuse and filtered through the forest canopy, giving the fish the subdued, tannin-tinted environment in which their iridescent body sheen is presumably an important intraspecific signal.
Feeding
Like other small Trigonostigma species, T. truncata is almost certainly a micropredator and opportunistic omnivore in the wild. In the still or gently flowing blackwaters it inhabits, the diet likely consists of small insects and their larvae (particularly chironomid midges and surface-fallen terrestrial invertebrates), microcrustaceans, zooplankton, and fine organic detritus — the typical resource base of small-bodied cyprinids in Southeast Asian peat-swamp streams.
In the aquarium T. truncata will accept standard small-particle foods: high-quality micro-pellets and flake, with live or frozen daphnia, cyclops, and baby brine shrimp providing enrichment and supporting colour and condition. Given the species' soft-water, low-nutrient native habitat, overfeeding and the resulting water-quality degradation are more damaging than in species from richer, harder waters.
Mating
Trigonostigma truncata, like all members of its genus, is a leaf-spawning cyprinid. Courtship follows the pattern established for T. heteromorpha and T. hengeli: a driving male pursues a gravid female through the water column and among submerged or broad-leaved vegetation, swimming persistently alongside and beneath her. When both fish are ready, they invert together beneath the chosen leaf — typically a broad, smooth-surfaced plant such as a Cryptocoryne — and press their bodies against the underside of the leaf as eggs and milt are released simultaneously.
Pairs may revisit the same leaf and the same spawning site multiple times during a session, depositing eggs in small batches across the underside of the leaf. There is no pair bond beyond the spawning act and no guarding of eggs or fry at any stage; the adults show no interest in the eggs once spawning is complete.
Breeding
Breeding T. truncata in captivity should follow the well-established protocol for T. heteromorpha and its relatives, adapted to the species' more extreme water-chemistry requirements. A soft, warm, acidic environment is essential: temperature in the 75–81 °F range, pH 5.0–6.5, and very low hardness are the conditions that both trigger spawning and ensure reasonable egg viability. Broad-leaved plants — Cryptocoryne species are the classic choice — must be present for the eggs to be placed on.
After spawning the adults should be removed, as they will eat any eggs within reach. The adhesive eggs are attached to the leaf underside and hatch in roughly 24–36 hours at warm temperatures; the larvae are tiny and require appropriately fine first foods (infusoria, rotifers, or commercial liquid fry food) before advancing to newly hatched brine shrimp nauplii. No parental care is given at any stage. Because this species comes from very low-nutrient, oligotrophic blackwaters, water quality must be kept pristine throughout the breeding and rearing period.
In the aquarium
Trigonostigma truncata has appeared in the specialist trade, though it is not yet as widely available as T. heteromorpha or T. hengeli. It is suited to a dedicated soft-water or blackwater biotope aquarium — a heavily planted tank with a dark substrate, driftwood, leaf litter, and subdued lighting. Maintaining the correct water chemistry (very soft, pH 5–7, 75–81 °F) is non-negotiable: this is not a species that adapts well to hard, alkaline tap water, and attempts to keep it in unsuitable chemistry result in poor colour, susceptibility to disease, and failure to spawn.
Like all Trigonostigma species it is a peaceable, mid-water shoaling fish that should be kept in groups of at least six to eight individuals. Its companions should be other small, non-aggressive species suited to the same blackwater conditions — other rasboras, small pencilfish, pygmy corydoras or small Otocinclus that share the same water-chemistry requirements. The fish's bluish-lilac sheen and orange-red anal fins are most visible under indirect or simulated forest-canopy lighting over a dark background.
Conservation
Trigonostigma truncata has not yet been formally assessed by the IUCN Red List, having been described only in 2020; its conservation status is therefore Not Evaluated (NE). However, the habitat to which it is restricted — lowland peat-swamp and coastal swamp-forest streams on the east coast of the Malay Peninsula — is among the most threatened freshwater environments in Southeast Asia. Peninsular Malaysia and southern Thailand have lost the majority of their peat-swamp forests to oil palm and rubber plantation development, drainage for agriculture, and coastal infrastructure over the past several decades.
The narrow geographic range of T. truncata (Narathiwat to Terengganu, a stretch of east-coast coastal lowland) places it squarely within one of the regions where these pressures are most acute. The species' dependence on intact swamp-forest hydrology — the specific chemistry of undisturbed blackwater draining through peat — means that habitat degradation, even without outright conversion, can render streams unsuitable. A formal IUCN assessment, when conducted, will need to account for these ongoing pressures and the limited extent of remaining suitable habitat.