Taxonomy & naming
Parosphromenus juelinae was described by Shi, Guo, Haryono, Hong & Zhang in 2021, in the journal Zootaxa (volume 5061, issue 1, page 72). The holotype (MZB 25116) was deposited at the Museum Zoologicum Bogoriense and originates from a blackwater peat swamp on Bangka Island, Indonesia. Eschmeyer's Catalog of Fishes lists the name as valid within the family Osphronemidae, subfamily Macropodusinae.
The species was first collected in 1993 but went unrecognised for more than two decades, having been misidentified as the closely related P. bintan from the Riau Islands. It was the Parosphromenus Project — an international hobbyist and researcher network dedicated to licorice gouramis — that flagged the Bangka population as distinct, ultimately prompting the formal 2021 description. The specific epithet juelinae honours a contributor to that collaborative effort.
Parosphromenus is a genus of small, often brilliantly coloured labyrinth fishes (order Anabantiformes) found across the Sunda shelf — Peninsular Malaysia, Sumatra, and Borneo — with most species inhabiting acidic, tannin-stained blackwaters. The genus currently contains around 25 described species, many of which are similarly narrow endemics in threatened peat-swamp systems.
Morphology
Like its congeners, P. juelinae is a small fish; the genus typically reaches 0.5–1 in in total length, and available data do not record a measurement outside this range for juelinae. The body is slender and torpedo-shaped, laterally compressed, with rounded fins that are elaborated into colourful filamentous extensions in courting males — a defining feature of the genus.
Males display the vivid fin patterning typical of Parosphromenus: dorsal, caudal, and anal fins marked with bands or patches of iridescent blue, red, or orange set against darker borders, used in close-quarters courtship displays. Females are cryptically coloured — brownish to beige with a faint lateral stripe — allowing them to remain inconspicuous in the root tangles and leaf litter of their peat-swamp habitat. As with other licorice gouramis, the suprabranchial labyrinth organ is developed, permitting the fish to breathe atmospheric air and survive in the oxygen-depleted waters of intact peat domes.
Habitat
Parosphromenus juelinae is known only from freshwater peat swamps on Bangka Island, a low-lying tin-mining island in the Java Sea between Sumatra and the Belitung archipelago. The type locality consists of blackwater swamp with dense stands of Cryptocoryne and the carnivorous aquatic plant Utricularia, shaded by peat-swamp forest canopy. The water is clear but deeply stained — black-tea coloured — from dissolved humic and fulvic acids leached from the accumulating peat, with extremely low conductivity and pH typical of intact peat domes: analogous data from sister species P. phoenicurus cite pH around 5.25 and conductivity near 7 µS/cm.
These conditions — high tannin content, near-zero hardness, low pH, high temperature (typically 79–82 °F), and naturally low dissolved oxygen — define the microhabitat in which Parosphromenus evolved. The labyrinth organ is an adaptation to exactly this oxygen-limited environment. Structurally complex micro-habitats of submerged roots, decaying leaf litter, and aquatic macrophytes are critical for shelter, territory, and cave-spawning sites.
The total area of suitable habitat remaining on Bangka Island is estimated at less than 31 mi², and is fragmented by decades of open-cast tin mining, agricultural conversion to oil palm, and deliberate drainage for development. This restricted and shrinking footprint is the direct basis for the species' Critically Endangered status.
Feeding
Parosphromenus juelinae is a micropredator, as are all known members of the genus. In the wild it feeds on small aquatic invertebrates — zooplankton, tiny crustaceans, mosquito larvae, and other minute organisms that inhabit the leaf litter and water column of peat-swamp habitats.
In captivity the species can be maintained on appropriately sized live and frozen foods: micro-worms, Artemia nauplii, Daphnia, Moina, and similar small invertebrate prey. Like other licorice gouramis it may initially refuse prepared or dry foods, and a diet centred on live foods is recommended both to ensure acceptance and to bring fish into breeding condition. Feeding frequency should be small and regular to match the fish's diminutive size and metabolic pace.
Mating
Parosphromenus juelinae follows the cave-spawning reproductive strategy characteristic of the genus. Courtship begins with the male adopting a head-down posture — a distinctive angled orientation seen across licorice gourami species — while displaying the full spread of his coloured fins at close quarters to the female. The iridescent fin markings function as species-recognition signals in addition to quality indicators, which is significant given that sympatric or near-sympatric Parosphromenus species can co-occur on Bangka Island.
The male leads the female toward a chosen spawning site — a cave, crevice, the underside of a leaf, or another enclosed shelter — and display intensifies near the entrance to the site. Spawning is preceded by extended and sometimes elaborate mutual inspection, and the female may reject multiple sites or males before accepting. Reported courtship in this species aligns with the broader genus description: orientation is horizontal to head-down, and fin display is the primary signal.
Breeding
Spawning in P. juelinae, as in other cave-spawning Parosphromenus, takes place inside or directly beneath a sheltered structure: a small cluster of eggs is deposited on the ceiling of the cave or the underside of a surface, typically accompanied by a small bubble raft that helps hold the eggs in place. The male assumes sole responsibility for guarding the clutch and the subsequent larvae, fanning the eggs and defending the site.
The species has been bred in captivity, and hobbyist reports indicate it is considered slightly more demanding than some congeners. Soft, very acidic blackwater conditions are essential: pH in the range of 4.5–6.0, conductivity below 50 µS/cm, and temperatures around 79–82 °F give best results. Leaf litter (Indian almond leaves are widely used) and appropriate spawning caves — small clay caves, coconut shell halves, or sections of bamboo — are necessary furnishings. Clutch sizes are small, as is typical for the genus, and the fry are correspondingly tiny, requiring infusoria-grade first foods before graduating to Artemia nauplii.
In the aquarium
Parosphromenus juelinae is a specialist species suited only to aquarists prepared to maintain authentic blackwater conditions. A dedicated species tank or a carefully curated community of equally small, acid-tolerant species is strongly preferred; any boisterous or larger tankmates will stress or outcompete these fish. A tank of 10–15 US gal with a tight-fitting lid (labyrinth fishes must access warm, humid air at the surface), very gentle filtration, and heavy planting with fine-leaved plants and floating vegetation provides an appropriate environment.
Water chemistry cannot be compromised: very soft, acidic water with pH 4.5–6.0, prepared with reverse-osmosis or rainwater and conditioned with peat or botanical materials, is the standard maintained by serious keepers. Tannin-stained water from Indian almond leaves, alder cones, or peat also provides the natural cues the fish associate with safety and spawning readiness. Lighting should be dim; this is not a fish for the brightly lit community aquarium.
Given the species' Critically Endangered status, hobbyist breeding programmes coordinated through the Parosphromenus Project carry genuine conservation significance. Wild-caught specimens should not be sought; the focus should be on obtaining captive-bred stock and participating in cooperative breeding efforts to maintain genetically diverse captive populations of this highly threatened species.
Conservation
Parosphromenus juelinae is assessed as Critically Endangered (CR) under IUCN criteria B2ab(iii), reflecting both its extremely restricted area of occupancy (less than 31 mi²) and the continuing decline in the extent and quality of its peat-swamp habitat. The primary threats are open-cast tin mining — historically the dominant industry on Bangka Island — combined with conversion of peat swamp forest to oil-palm plantation and agricultural drainage. These pressures have operated since at least 2018 and show no sign of abating; much of Bangka's lowland peat swamp has been transformed or degraded beyond recovery.
The species' single-island endemic distribution leaves it with no demographic buffer: there is no alternative population elsewhere to which individuals can disperse or from which recruitment could occur following a local catastrophe. The formal description in 2021 came after more than two decades of the species existing in collections without a name, underscoring how poorly known the peat-swamp fish fauna of the Sunda shelf remains — and how quickly habitat loss can overtake taxonomy.
Conservation action at present is concentrated in the hobbyist sphere, primarily through the Parosphromenus Project's captive-breeding network. This network maintains living populations of multiple licorice gourami species under threat and advocates for in-situ protection of the remaining peat swamps. For P. juelinae in particular, the captive population represents an important safeguard against extinction given the ongoing habitat destruction on Bangka Island.