Taxonomy & naming
Trigonostigma espei was described by the German ichthyologist Herbert Meinken in 1967, originally placed within the sprawling genus Rasbora as Rasbora espei. The species was transferred to Trigonostigma by Kottelat and Witte in 1999, when they erected the genus to accommodate the harlequin-type rasboras — species that share a distinctive lateral blotch pattern and a leaf-spawning reproductive mode that differs from the egg-scattering typical of Rasbora. The Catalog of Fishes (Eschmeyer, CAS) records the valid combination as Trigonostigma espei (Meinken, 1967), the parenthetical authority noting the move from Meinken's original genus.
The genus Trigonostigma currently contains four closely related species, of which T. heteromorpha (the harlequin rasbora) is the most familiar in the hobby. Trigonostigma espei is the smallest of the group and is distinguished by its more elongate blotch shape and somewhat brighter body colouration compared to T. heteromorpha. Older aquarium literature and some hobbyist sources will still list the species as Rasbora espei; both names refer to the same fish. GBIF taxonomic records confirm the current placement and note no additional synonyms.
Morphology
Trigonostigma espei is a small, laterally compressed cyprinid with a streamlined but deep-bodied form. Maximum standard length is around 1–1.5 in, with FishBase recording a maximum of 1 in SL and Seriously Fish noting up to 1.5 in in wild specimens and slightly larger in well-fed captives. The body is warm coppery-orange to pinkish-gold on the flanks, shading to a pale silvery belly.
The defining mark is the dark lateral blotch — a roughly triangular wedge of blackish-brown pigment beginning at the mid-body and tapering to a fine point at the caudal peduncle, giving the characteristic lambchop or boomerang outline that distinguishes the species from the broader, more club-shaped blotch of the harlequin rasbora. Fins are largely translucent with a slight orange or red flush at the base of the dorsal and caudal fins. The mouth is terminal and small, suited to picking tiny prey from the water column.
Sexual dimorphism is mild. Females tend to be slightly deeper-bodied and more rounded in the belly, particularly when gravid, while males are slimmer and often display more intense colouration.
Habitat
Trigonostigma espei inhabits slow-moving and still waters across a relatively modest range in mainland Southeast Asia. The type locality lies in south-western Thailand (Krabi province) and south-eastern Thailand near the Cambodian border (Chanthaburi province); the species also occurs in the Prek Tuk Sap river basin in Cambodia and has been recorded from Phu Quoc island off the southern Vietnamese coast.
Its preferred habitats are ponds, pools, marshes, swamps and seasonally flooded areas with dense submerged or emergent vegetation. In Krabi province these include limestone-influenced forest pools and shaded stream margins, where the water can range from soft and acidic to marginally harder depending on local geology. In Thailand and Cambodia it is associated with the kind of slow, heavily vegetated lowland waters — often shaded and tannin-stained — that typify much of Sundaland and adjacent mainland Southeast Asia. Water temperatures across its range span approximately 73–82 °F. The Aqua-Fish reference notes that it has been found in both slightly acidic forest streams and limestone-influenced pools in Krabi province, indicating some tolerance for locally variable water chemistry.
Feeding
Trigonostigma espei is a micropredator in the wild, taking small insects, aquatic invertebrates and microcrustaceans from the water column and water surface in the shallow, densely vegetated habitats it frequents. Its small, terminal mouth suits it to picking off minute prey items rather than pursuing larger quarry.
In the aquarium it is undemanding and accepts a wide range of foods. Seriously Fish recommends a varied diet of small live and frozen foods — bloodworm, Daphnia and brine shrimp (Artemia) — supplemented with quality dried flakes or micro-granules appropriate to its small mouth size. Like most small cyprinids it benefits from variety, and frequent offerings of live or frozen invertebrates help maintain condition and colouration. Its small size means it can be outcompeted by larger or faster tankmates at feeding time; calm, similarly-sized community fish are better companions.
Mating
Trigonostigma espei is a leaf-spawner — the characteristic reproductive mode of the genus Trigonostigma — rather than a free egg-scatterer like most danionins. Courtship involves a driving male pursuing a ripe female closely through vegetation, with spawning triggered by the pair discovering and circling a suitable broad-leaved plant, typically in shallow water.
The actual spawning act involves the pair inverting beneath the chosen leaf, pressing together briefly, and depositing a small clutch of adhesive eggs directly onto the underside of the leaf surface. The adhesive eggs remain attached rather than falling to the substrate. No pair bond persists beyond spawning and no parental care of any kind is given — both fish return to normal shoaling behaviour immediately after, and will consume eggs they encounter if given the opportunity.
Breeding
To breed Trigonostigma espei in the aquarium, a soft, warm and slightly acidic environment is essential. Seriously Fish records successful spawning in water with temperatures of 73–82 °F (most reports place optimum spawning temperature toward the upper part of this range), pH 5.5–7.5 and moderate softness. A dedicated spawning tank with broad-leaved aquatic plants — Cryptocoryne species are the classic choice and closely mirror the natural spawning substrate — should be provided.
Spawning takes place in shallow water as low as 6–8 in depth according to FishBase, typically early in the day. The pair press together beneath a leaf and deposit adhesive eggs on its underside in small batches across multiple visits to the plant. Eggs hatch in approximately 24–48 hours. The larvae become free-swimming around one week after hatching, at which point they require very fine first foods — infusoria, commercially prepared fry foods or freshly hatched brine shrimp nauplii as they grow. Adults should be removed after spawning to prevent egg predation. Growth is relatively slow compared to egg-scattering danios.
In the aquarium
Trigonostigma espei is a peaceful, undemanding community fish that repays a thoughtful setup. It is a genuine shoaling species and should be kept in groups of at least six — ideally more — to keep the fish confident and to allow the natural schooling and display behaviour that brings out its best colouration. Kept in small numbers it becomes shy and subdued. It suits tanks of 10–15 US gal or more with ample planting.
Soft, slightly acidic water (pH 5.5–7.0 is ideal, though Seriously Fish notes tolerance up to pH 7.5) with a temperature of 73–82 °F best matches its lowland Southeast Asian origins. Subdued lighting, dark substrate and the addition of dried leaves or other tannin-releasing materials to simulate its shaded, humus-rich natural habitat will bring out its colouration and keep it at ease. It pairs naturally with other small, peaceful Southeast Asian species sharing similar water requirements — small Boraras, peaceful Corydoras-type catfishes and dwarf gouramis of compatible temperament. It is entirely safe with plants and is too small and gentle to bother any tankmate above a few millimetres.
Conservation
Trigonostigma espei is assessed as Least Concern (LC) on the IUCN Red List (assessment version 2022.2). The IUCN notes a stable population with a broad enough geographic range across Thailand, Cambodia and Vietnam to preclude listing in a threatened category under current data.
Nonetheless, its preferred habitat — shallow, densely vegetated lowland pools and seasonally flooded marshes — is subject to ongoing pressure across the Indochina lowlands. Agricultural conversion, wetland drainage, pesticide runoff and urban expansion all degrade the slow-water swamp and pond habitats on which the species depends. The Krabi province sites in southern Thailand, where the species occurs alongside limestone geology, face additional pressure from tourism development. The species is collected for the ornamental fish trade, though most fish available commercially are captive-bred. Monitoring of wild populations, particularly at the edges of its range, is warranted as lowland wetland loss accelerates across mainland Southeast Asia.