Taxonomy & naming
Trigonostigma somphongsi was originally described by Meinken in 1958. The Catalog of Fishes (Eschmeyer, CAS — the authority for valid names) places it in genus Trigonostigma, erected by Kottelat & Witte in 1999 to accommodate the harlequin-type rasboras that had previously been housed in a broad, polyphyletic Rasbora. The species was at one point placed in Microrasbora, and some older literature (including early FishBase records) uses that combination, but the valid current name is Trigonostigma somphongsi (Meinken, 1958).
Trigonostigma is a small genus of compact, laterally compressed cyprinids united by their preference for soft, acidic, tannin-stained Sundaland waters and their distinctive leaf-spawning reproductive strategy. T. somphongsi is the sister to the better-known harlequin rasboras T. heteromorpha and T. espei, and — like them — is a true Sundaland-affiliated small cyprinid rather than a danio or a 'danionin' rasbora in the more recent molecular sense.
Morphology
T. somphongsi is a small, laterally compressed fish reaching approximately 1 in SL (about 1 in standard length), making it decidedly tiny even among the miniature Trigonostigma. The body is deep-bodied and rounded in cross-section as in other harlequin-type rasboras, with a subterminal mouth and no prominent barbels.
The species shares the characteristic dark lateral wedge or blotch of the genus, though the precise colour pattern — body ground colour, intensity and extent of the lateral marking — distinguishes it from T. heteromorpha and T. espei. Sexual dimorphism is moderate: females are generally deeper-bellied and more rounded when gravid; males are slimmer and may show more vivid colouration during courtship. Detailed published morphometric data are sparse, reflecting the extreme rarity of live material.
Habitat
The species is considered endemic to the Mae Klong river basin in western Thailand, with the principal known locality near Ratchaburi province. It inhabits heavily vegetated, shaded streams and river margins where the water is deeply stained with tannins and other humic compounds from leaf litter and decaying organic matter. FishBase records a pH range of 6.0–6.5 for the species, and Seriously Fish reports 5.0–7.0 with a temperature band of 72–79 °F, consistent with the seasonally stable lowland streams of western Thailand.
The habitat is characterised by soft, acidic to weakly acidic water, dense aquatic and marginal vegetation, a dark water column, and abundant submerged leaves and root masses — the signature 'blackwater-adjacent' environment of Sundaland-affiliated peat-margin and forest stream fishes. Degradation of this habitat through agricultural conversion, pollution and water extraction has been severe.
Feeding
T. somphongsi is a micropredator in nature, feeding on small aquatic and terrestrial invertebrates — insect larvae, small crustaceans, worms and similar prey — taken from the water column and surface in the sheltered, vegetated habitats it occupies.
In captivity it accepts small live and frozen foods readily: bloodworm, daphnia, and artemia nauplii are reported to suit it well, along with quality micro-pellets and dried flakes of an appropriate size. Because of its very small adult size, food particle size matters; overly large items are ignored or rejected. Feeding a varied diet of live and frozen invertebrate prey keeps condition and reproductive readiness at their best.
Mating
Trigonostigma somphongsi follows the leaf-spawning strategy characteristic of its genus. Unlike the egg-scattering majority of danionins and rasboras, harlequin-type Trigonostigma attach their adhesive eggs to the underside of a broad, smooth leaf — in nature most likely Cryptocoryne or a similarly broad-leafed aquatic plant that grows in the same soft-water streams. Spawning most commonly occurs in the morning hours.
Courtship involves a driving male pursuing and pressing against a gravid female, the pair eventually inverting together beneath a selected leaf to deposit eggs. A female may lay a small batch of adhesive eggs in a session. There is no pair bond and no parental involvement after spawning; the adults are best removed to prevent egg predation.
Breeding
Captive breeding of T. somphongsi requires replicating the species' soft, acidic, warm and heavily vegetated natural habitat as closely as possible. A well-planted, dimly lit breeding tank with very soft, acidic water (pH 5.5–6.5, low hardness), a temperature of around 75–79 °F and broad-leaved plants such as Cryptocoryne provides the conditions under which spawning is most likely. Tannin-stained water achieved via Indian almond leaves or peat filtration can further encourage the fish.
Once a pair or group is conditioned on live and frozen foods, spawning attempts will occur at or around dawn. Adhesive eggs are deposited on leaf undersides and are not tended by the parents. The eggs and resulting larvae are tiny and the fry require infusoria or commercial microfeeds at first. Given the species' extreme rarity and conservation status, captive breeding is actively encouraged by specialist organisations as a hedge against wild extinction.
In the aquarium
T. somphongsi is a specialist fish requiring soft, acidic, warm water and will not thrive in the hard, neutral or alkaline conditions that suit the majority of community-tank inhabitants. A dedicated biotope-style setup recreating western Thai forest streams — with dark water, soft substrate, abundant leaf litter and broad-leaved plants, and minimal current — is the appropriate environment. Its diminutive size (under 1 in) means it should only be combined with equally small, non-aggressive species that share its water chemistry requirements.
The species was considered potentially extinct for approximately twenty years before a trio of individuals arrived in Germany in 2006 mixed into a consignment of Boraras urophthalmoides — an indication of how vanishingly rare it is in the trade. Specimens that enter the hobby come almost exclusively from the dedicated specialist community and captive-breeding efforts. Any keeper fortunate enough to acquire this species carries a genuine conservation responsibility.
Conservation
Trigonostigma somphongsi is assessed as Critically Endangered (CR) on the IUCN Red List (assessed February 2011, criteria A2ac), reflecting a severe and ongoing decline in both population size and extent of occurrence driven primarily by habitat loss and degradation. The Mae Klong basin of western Thailand — the only known range of this species — has been heavily impacted by agricultural conversion, pollution, water extraction and general lowland forest loss.
The species was widely presumed extinct for roughly two decades before three individuals were detected in a German aquarium import in 2006. This rediscovery confirmed the species still exists in the wild but underlines how critically reduced its populations have become. Captive breeding in specialist collections is the primary conservation hedge currently available, and the species is considered a high priority for coordinated captive-breeding and potential reintroduction programmes.