Taxonomy & naming
Parosphromenus phoenicurus was formally described by Ingo Schindler and Horst Linke in 2012, in the journal Vertebrate Zoology (62(3):399–406). The type locality is recorded as Langgam, Sungai Kampar, Riau, Indonesia (approximately 0°11′N, 101°38′E), a site within the broader Sungai Kampar river drainage in central Sumatra. The holotype measured 1 in standard length.
The specific epithet phoenicurus is derived from the Greek phoinix (crimson, or the mythical firebird) and oura (tail), a reference to the vivid red or crimson colouration of the caudal fin in adult males. The genus Parosphromenus belongs to the family Osphronemidae and encompasses the licorice gouramis — a group of small, often brilliantly coloured labyrinth fishes confined to the acidic peat swamps and blackwater streams of Southeast Asia, most of them on the Malay Peninsula, Sumatra, and Borneo. Eschmeyer's Catalog of Fishes (Fricke, Eschmeyer & Fong) lists the valid name as Parosphromenus phoenicurus Schindler & Linke, 2012.
The species is distinguished from its congeners most reliably by the rhombic (diamond-shaped) caudal fin of adult males, in contrast to the round or lanceolate tails of most relatives, combined with a distinctive dark triangular patch on the caudal and the narrow iridescent light-bluish bands bordering the crimson fin colouration. Females are paler throughout, with grey-brown fins lacking the male's bright markings.
Morphology
Parosphromenus phoenicurus is a tiny fish; the wild form reaches approximately 1 in standard length, though aquarium-raised individuals — presumably benefiting from stable food supply and absence of predators — can grow to around 1.5 in total length. The body form is typical for the genus: slender, compressed, with an elongated head and a small upturned mouth suited to feeding at and just below the surface film. Fin ray counts are Dorsal XI–XIII, 6–7 and Anal XI–XIII, 8–11, within the range typical of Parosphromenus.
Sexual dichromatism is pronounced. Adult males in display develop intense crimson or scarlet banding across the dorsal, anal, and caudal fins, overlaid with narrow iridescent bluish-white margins. The caudal fin takes on a rhombic or diamond outline in mature males — the most reliable morphological distinction from similar species. A nearly solid dark triangular patch occupies much of the central caudal. Females lack these ornaments entirely, showing dark grey to brownish fins against a pale body; juveniles of both sexes resemble females until males begin to colour at sexual maturity.
Habitat
The only confirmed wild population of Parosphromenus phoenicurus comes from blackwater swamp habitat within the Sungai Kampar drainage of Riau Province, Sumatra, Indonesia. Water measurements recorded at the type locality document extreme soft-water, acid conditions: pH 4.1 (original description) to pH 5.25 (FishBase), conductivity approximately 7 µS/cm, and temperature around 80–84 °F. These are classic peat-swamp blackwater parameters — water stained dark brown by dissolved humic and fulvic acids leaching from decomposing vegetation, almost devoid of dissolved salts, and buffered to low pH by the organic acids of the peat substrate.
Peat swamps of this type are among the most threatened freshwater habitats in Southeast Asia. Standing or very slowly flowing, heavily shaded by the canopy, and rich in surface litter and submerged woody debris, they provide the dim, still, acidic environment that Parosphromenus species require. The suprabranchial labyrinth organ — the air-breathing structure shared by all osphronemid gouramis — is particularly important in such habitats, where dissolved oxygen levels can be very low in the warm, stagnant water beneath the leaf litter. The species is known from a single locality and has not been collected elsewhere.
Feeding
In the wild, Parosphromenus phoenicurus almost certainly feeds on small invertebrates encountered at and just below the water surface and in the leaf litter: zooplankton, small insect larvae, microcrustaceans such as copepods and small Daphnia, and worms. All Parosphromenus are micro-predators of this type, and the small upturned mouth is adapted to taking tiny prey items.
In the aquarium, the species is readily maintained on live and frozen micro-foods: baby brine shrimp (Artemia nauplii), microworms, Daphnia, Moina, and similar small live cultures. Dried or prepared foods are accepted by some aquarium-reared individuals but should not be relied upon as the sole diet; live foods sustain better condition, colour, and breeding readiness. Feeding small amounts twice daily is preferable to a single large meal for fish of this size.
Mating
Like all Parosphromenus, P. phoenicurus is a cave spawner rather than an open bubble-nester. Courtship is elaborate for such a small fish. Males display to females with a distinctive head-down posture reported as typical of the genus, fanning their brilliantly coloured fins fully extended while oriented at a steep downward angle. This posture emphasises both the rhombic caudal and the crimson fin banding, and the intensity of the display is a reliable signal of male condition and readiness.
A receptive female is led toward a suitable spawning site — a small cave, crevice, hollow, or overhang — where the pair will eventually spawn. Males may spend considerable time preparing and investigating prospective sites before inducing a female to enter. As with other Parosphromenus, the male plays the dominant role in site selection and defence.
Breeding
Spawning takes place inside a cave or confined shelter, with the eggs typically deposited on the ceiling or inner wall of the cavity, held in place by a small cluster of bubbles produced by the male. Clutch sizes in Parosphromenus are small — usually a few dozen eggs — and the male tends the eggs alone after spawning, fanning them and defending the entrance to the cave. The female is best removed after spawning to reduce stress on the guarding male.
The Parosphromenus Project notes that P. phoenicurus is comparatively easy to breed relative to some of the most demanding species in the genus, and that several small captive stocks have been established since the first import in 2008 and more significant commercial import in 2014. At typical breeding temperatures in the high 20s Celsius, eggs hatch within a few days, and the fry become free-swimming shortly after. First foods are infusoria or similarly fine live foods, progressing to newly hatched brine shrimp nauplii. Because the wild population is considered critically threatened and known only from a single locality, maintenance of captive breeding groups carries genuine conservation significance.
In the aquarium
Parosphromenus phoenicurus is a specialist fish suited to experienced keepers prepared to replicate its demanding water-chemistry requirements. The aquarium should closely approximate the species' native blackwater conditions: very soft, acid water (pH 4.0–5.5, conductivity under 50 µS/cm, ideally much lower), warm temperatures in the 79–84 °F range, very low flow, and dim lighting. Filtration through peat or the use of leaf litter (dried catappa or beech leaves) helps establish the humic-rich environment the species needs. Strong currents and hard, alkaline tap water are incompatible with long-term health.
Tanks for this species need not be large — a dedicated breeding group can be housed in a well-planted 10–15 US gal aquarium with multiple small caves (coconut shell halves, short lengths of narrow PVC pipe, or ceramic cave ornaments) at low light levels, with a dark substrate of fine sand or peat. Tankmates should be limited to other very small, equally soft-water-adapted species, or the fish kept as a species tank. The labyrinth organ requires unobstructed access to the surface; a tight-fitting lid prevents both escape and cold draughts that can damage the organ.
Given the Critically Endangered status and the fact that the species is known from a single wild locality, hobbyists are strongly encouraged to source captive-bred stock and to engage with the Parosphromenus Project or similar specialist groups to support coordinated breeding efforts.
Conservation
Parosphromenus phoenicurus is assessed as Critically Endangered (CR) on the IUCN Red List under criterion B2ab(iii), assessed 12 January 2019. The listing reflects both the extreme geographic restriction of the known population — a single locality within the Sungai Kampar drainage — and the ongoing severe decline of peat-swamp habitats throughout Sumatra and the broader Sundaland region.
Sumatran peat swamps have been converted at massive scale for oil-palm and pulpwood plantations, drained by canals, and repeatedly burned in land-clearing fires. The blackwater habitats on which Parosphromenus depend are among the ecosystems most comprehensively transformed: many sites that once held suitable fish habitat have been entirely eliminated. A species with such a narrow range and such extreme habitat specialisation has essentially no capacity to shift locations in response to local habitat loss.
The small captive population maintained by specialist hobbyists and the Parosphromenus Project represents a genuine insurance against the extinction of this species in the wild. First collected in 2008 and imported in meaningful numbers only from 2014, P. phoenicurus has been established in enough breeding groups to remain securely held in cultivation even as its native habitat continues to diminish. Hobbyist participation in the captive breeding network — sourcing only aquarium-bred stock, documenting lineages, and exchanging fish among breeders — is the most practical form of conservation available for a fish of this kind.