Taxonomy & naming
Parosphromenus filamentosus was described by the German aquarist and ichthyologist Jörg Vierke in 1981 in Senckenbergiana Biologica (vol. 61, pp. 363–367). The type locality is the blackwater swamp region near Banjarmasin, the capital of South Kalimantan province on the Indonesian part of Borneo. The valid name as recognised by Eschmeyer's Catalog of Fishes is Parosphromenus filamentosus Vierke, 1981.
The genus Parosphromenus belongs to the family Osphronemidae (the gouramis) and represents the licorice gouramis, a clade of diminutive cave-spawning blackwater specialists endemic to the Malay Peninsula, Sumatra, and Borneo. Vierke had described the closely related P. parvulus from Borneo two years earlier in 1979; both species are considered among the least-derived members of the genus. Schindler and Linke (2012) in Vertebrate Zoology treated the genus in detail, confirming the Bornean origins of P. filamentosus and placing it within the broader radiation of peat-swamp anabantoids.
The specific epithet filamentosus is Latin, meaning 'thread-bearing' or 'filamentous', and refers to the elongated filamentous extensions on the caudal fin that give the fish its common name of spiketail gourami. A variant collection from the Ampah area, between Ampah and Muarateweh in southeast Kalimantan, is traded as Parosphromenus cf. filamentosus (Ampah), indicating geographic colour or morphological variation within the species.
Morphology
Parosphromenus filamentosus is a small, slender labyrinth fish reaching approximately 2 in total length, with males typically reaching 1.5 in TL. The body is laterally compressed and elongate, tapering to a pointed caudal region — the form typical of the genus. The caudal fin displays the filamentous extensions that give the species its common name, with central rays drawn out into spike-like projections, particularly pronounced in displaying males.
The Parosphromenus Project notes that P. filamentosus is regarded as one of the least specialised members of its genus, sharing with P. parvulus a comparatively generalised body plan relative to the more derived Bornean species. Courting males develop a dark, smoky coloration — a deep, dusky overlay across the flanks — contrasting with iridescent scale highlights. This darkening is a display state rather than a fixed pattern; males outside of courtship appear plainer. Females are generally less intensely coloured and lack the exaggerated caudal filaments of adult males.
Habitat
In the wild, Parosphromenus filamentosus inhabits the jungle river systems and associated blackwater swamps in the vicinity of Banjarmasin, south-central Kalimantan, Borneo. The species is syntopic with P. parvulus at some sites, both occurring in the same blackwater habitats. Natural water chemistry recorded at Parosphromenus sites in the region is extremely acidic: the Parosphromenus Project AMAZONAS article documents pH values as low as 3.7, typical of the tannin-stained, humic-acid-rich blackwater of Bornean peat swamps.
The genus as a whole is restricted to shallow, heavily vegetated habitats with little current — areas 19.5–39.5 in deep with dense aquatic and marginal plant growth. The blackwater environment is characterised by very low dissolved mineral content, extremely low pH, low bacterial and microbial loads, and high dissolved organic carbon from decomposing peat and leaf litter. These conditions are oxygen-poor relative to clear running water, but the licorice gouramis are labyrinth fishes equipped with a suprabranchial accessory breathing organ that allows atmospheric air to supplement gill respiration — a critical adaptation for life in still, warm, organically loaded water.
FishBase records the type locality temperature at 70 °F, though field measurements in Bornean peat swamps often fall in the low-to-mid 20s depending on season and canopy cover. The geographic variant from Ampah (southeast Kalimantan) indicates the species has some distributional breadth within south-central Borneo, though all known populations fall within a habitat type now severely threatened by deforestation.
Feeding
Parosphromenus filamentosus is a micropredator in the wild, feeding on small invertebrates encountered among leaf litter, root tangles, and aquatic vegetation in its blackwater habitat. The natural prey items include tiny crustaceans, worm larvae, insect larvae, and other small invertebrates available in the low-productivity peat-swamp environment.
In the aquarium, the species shares the general feeding habits of the genus and accepts small live and frozen foods — newly hatched brine shrimp (Artemia nauplii), Daphnia, Moina, micro-worms, and black mosquito larvae are all suitable. Like most Parosphromenus, it is a deliberate, slow feeder that should not be kept with faster, more aggressive feeders that might outcompete it at the surface. Small, frequent offerings suit its temperament better than large meals, and the extremely soft, acidic water it requires means that the microorganism community of a well-aged blackwater aquarium can provide supplemental live food.
Mating
Parosphromenus filamentosus breeds as a cave spawner. Courtship begins with the male staking out a cave or sheltered overhang — typically a small hollow, half-coconut shell, bamboo tube, or similar enclosure — and engaging in head-down displays toward females. The Parosphromenus Project notes that courting males adopt the genus's characteristic courtship posture with the body angled head-downward, and that males develop a dark, smoky coloration during display. Males are reported to show the most distinct foam nest of any species in the genus, building a relatively large foam cluster inside the cave to which eggs will be attached.
The species is described by the Parosphromenus Project as producing up to 200 eggs per spawning, though typical clutch sizes are 50–70 eggs. Females approach a receptive male inside the cave or just at its entrance, and spawning takes place in the shelter of the cave interior, away from the open water.
Breeding
After spawning, the male of Parosphromenus filamentosus tends the egg cluster attached to the foam nest on the cave ceiling, guarding and fanning. The female typically does not participate in parental care and may be driven away by the male. Clutch sizes of 50–70 eggs are typical, with exceptional spawnings producing up to around 200. Incubation and fry development follow the pattern typical of the genus in warm, very soft, very acidic water.
Successful captive breeding requires close attention to water chemistry. The EAZA Best Practice Guidelines for Parosphromenus confirm that breeding outcomes depend significantly on water parameters, and the AMAZONAS research article documents the extremely acidic conditions (pH as low as 3.7) of natural sites. Breeders typically aim for pH in the range of 4.0–5.5 with very low conductivity (under 50 µS/cm) to prompt spawning and ensure egg viability. The fry are initially tiny and require infusoria or other microscopic first foods before graduating to Artemia nauplii. Gentle, near-still filtration (sponge filtration or even none initially) is standard practice, as the fry cannot cope with current and the adults require the calm conditions of their natural habitat.
In the aquarium
Parosphromenus filamentosus is one of the more approachable species in the licorice gourami genus — the Parosphromenus Project rates it as among the least specialised — but it is still an exacting fish by general hobbyist standards. The non-negotiable requirement is very soft, very acidic blackwater: pH 4.0–5.5, conductivity below 100 µS/cm (ideally lower), temperature in the low-to-mid 20s Celsius. Water of this quality is best achieved with reverse osmosis or rainwater base, acidified with peat, dried leaves (Indian almond, oak), or alder cones, and conditioned with natural tannin sources to replicate the humic-rich environment of Bornean peat swamps.
The tank should be small — a dedicated species tank of 10–15 US gal suits a pair or trio well — with dense planting or leaf litter, subdued lighting, and multiple cave structures (small terracotta pots, coconut shells, PVC fittings, or commercial breeding caves). Filtration should be minimal and gentle; a small sponge filter run slowly is ideal. The labyrinth organ requires unobstructed access to the surface, and the air above the water should be kept humid and warm to avoid chilling the labyrinth tissue during breathing.
Parosphromenus filamentosus should be kept as a species-only tank or at most with small, non-competitive companions such as microrasboras (Boraras spp.) or small Sundadanio. Aggressive or boisterous tankmates stress these fish into chronic colour-suppression and breeding failure. Given its Endangered status and the conservation importance of captive assurance populations, sourcing tank-bred stock from specialist breeders — through the Parosphromenus Project's network or similar licorice gourami enthusiast groups — is strongly preferred over wild-caught imports.
Conservation
Parosphromenus filamentosus is assessed as Endangered (EN) on the IUCN Red List (assessed 14 January 2019). The threat is primarily habitat destruction: the peat-swamp forests of Borneo have undergone catastrophic conversion to oil-palm plantations and other agricultural land over recent decades, eliminating or fragmenting the blackwater habitats that the species requires. The type locality near Banjarmasin lies in a region of Kalimantan that has experienced among the highest rates of forest clearance in Indonesia. The Parosphromenus Project notes that the threat level for this species is likely very high given the scale of forest destruction in its range.
Like all Parosphromenus, P. filamentosus is entirely dependent on intact peat-swamp blackwater — it cannot persist in converted agricultural drainage ditches or disturbed streams with altered pH and sediment loads. The Schindler and Linke (2012) Vertebrate Zoology paper on the genus emphasises that taxonomy and species delineation in Parosphromenus directly support conservation prioritisation, since splitting cryptic populations into recognised species strengthens the case for protecting individual sites.
Captive assurance populations maintained by specialist hobbyists represent a meaningful conservation contribution for this and other licorice gouramis. The Parosphromenus Project coordinates a network of dedicated breeders and maintains studbook-style records for the genus; aquarists interested in keeping P. filamentosus are encouraged to obtain stock through this network and to participate in its breeding programmes rather than rely on wild-caught specimens.