Taxonomy & naming
Parasphaerichthys lineatus was formally described by Ralf Britz and Maurice Kottelat in 2002, in the journal Ichthyological Exploration of Freshwaters (volume 13, number 3, pages 243–250). The type locality is the Ayeyarwaddy delta floodplain in southern Myanmar, at approximately 22°N, 96°E. The valid name, as recorded in Eschmeyer's Catalog of Fishes, is Parasphaerichthys lineatus Britz & Kottelat, 2002.
The genus Parasphaerichthys is placed within the family Osphronemidae (the gouramis and allies), in the subfamily sometimes called the chocolate gouramis. It is a small genus whose other member, P. ocellatus, is notably larger — reaching around 2 in — and is more frequently encountered in the hobby. The two species share a general build but differ strikingly in adult size and colour pattern. Parasphaerichthys is a labyrinth fish, possessing the suprabranchial air-breathing organ that defines the group, though in so small a species the labyrinth is correspondingly reduced.
The specific epithet lineatus derives from the Latin lineatus, meaning striped or lined, and refers to the bold, dark mid-lateral stripe that runs along the flank and is one of the species' most recognisable features.
Morphology
Parasphaerichthys lineatus is strikingly small: males and unsexed individuals reach a maximum of approximately 0.5 in standard length, making it one of the smallest labyrinth fishes described. In build it is a compact, somewhat rounded fish with the typical gourami body plan — laterally compressed, with the elongated pelvic fin rays that characterise the family. The dark mid-lateral stripe, running from behind the eye to the caudal peduncle, is the most consistent pigment feature across sexes and conditions.
Males in breeding condition undergo a dramatic colour transformation: the base colouration shifts to a warm honey-yellow to orange, and the head takes on a contrasting black mask that covers much of the facial region. Outside of breeding condition, both sexes are more subdued, and separating males from females can be difficult. The scale pattern is relatively fine for a fish of this size, giving a jewelled appearance under good light. As a labyrinth fish it can access atmospheric oxygen at the surface, but its small size means it can be outcompeted for surface access by larger tankmates.
Habitat
Parasphaerichthys lineatus is known from peat-swamp forests and associated blackwater habitats on the Ayeyarwaddy delta floodplain in southern Myanmar. These environments are characterised by highly stained, tannin-rich water that is naturally soft, acidic, and low in dissolved minerals — conditions typical of the peat-swamp biotope across mainland Southeast Asia. FishBase records a temperature of 77 °F and a pH of 6.5 for the type locality, consistent with the blackwater peat-swamp character of the habitat.
Like most labyrinth fishes, P. lineatus inhabits still or very slow-moving water where surface oxygen access is straightforward and where dense aquatic vegetation and leaf litter provide cover and feeding opportunities. The species appears to have a naturally restricted range even within Myanmar, and its blackwater peat-swamp habitat is among the most threatened in the region, subject to rapid conversion for agriculture, plantation forestry, and other land uses. In the aquarium, Seriously Fish notes a broader tolerated range of 63–86 °F and pH 6.5–8.0, though conditions closer to the natural blackwater parameters are likely to produce the best results with a species this specialised.
Feeding
Parasphaerichthys lineatus is a micropredator in the wild, subsisting on small aquatic crustaceans, worms, insect larvae, and zooplankton. Its diminutive mouth is sized appropriately for prey in this size class, and keepers report that it can be selective and reluctant to accept prepared foods in the aquarium.
In captivity the species is best offered live and frozen micro-foods: baby brine shrimp (Artemia nauplii), micro-worms, Daphnia, Cyclops, and similar invertebrates sized to its small mouth. Some individuals can be weaned onto high-quality micro-pellets over time, but live or frozen food should form the backbone of the diet — particularly for conditioning breeding pairs. The species' rarity in the hobby means that maintaining condition through good nutrition is especially important; a fish that is not feeding confidently will not breed.
Mating
Parasphaerichthys lineatus is a bubble-nest builder. Detailed aquarium observations confirm that males construct a bubble nest, with spawning behaviour centred on the nest site. The male's dramatic breeding colouration — honey-yellow to orange body with a black facial mask — develops fully in the lead-up to and during spawning, and this colour shift is one of the clearest cues that a male is in condition and actively courting.
Spawning has been observed both in direct association with a constructed nest and, occasionally, on the tank bottom with eggs subsequently being taken up — behaviour that superficially resembles intermediate steps toward mouthbrooding but which stops short of oral incubation. This variability may reflect the species' placement at a phylogenetic boundary within the subfamily, but confirmed oral incubation has not been reported. The male is the principal partner in nest construction and initial care. Given the fish's small size and sensitivity, a well-planted, calm environment with minimal disturbance and appropriate water chemistry is essential for natural mating behaviour to be expressed.
Breeding
Successful captive breeding of Parasphaerichthys lineatus has been achieved by a small number of specialist keepers, though it remains rare enough that each success is notable within the hobbyist community. The male builds a bubble nest — accounts describe a perfect nest constructed in the arch of a Java fern leaf — and spawning follows in typical osphronemid fashion: the pair embrace, eggs are released, and the male gathers and tends the eggs within the nest structure.
Eggs hatch within approximately two days. The fry are extremely small on emergence, consistent with the adult size, and require infusoria or other micro-first-foods in the initial days before progressing to Artemia nauplii. Early reports note that fry are initially retained in or near the nest and then disperse — often into fine floating plants such as Riccia — as their mobility increases. The male guards the nest through the early post-hatch period. Given the rarity of the species and the small number of keepers holding it, captive populations are maintained through informal fish-passing networks, and each successful breeding cohort is significant for the hobby's gene pool.
In the aquarium
Parasphaerichthys lineatus is not a fish for the general community aquarium. Its minute size (under 1 in), delicacy, and specific needs place it firmly in the specialist category, and it is rarely available in the trade — when it does appear, it is usually through hobbyist-to-hobbyist contact rather than retail channels. Even within the Myanmar-fish import trade, it was noted as a rarity as recently as 2010.
A dedicated species tank is strongly recommended: a small, heavily planted aquarium with very gentle filtration, soft and acidic water (pH 6.5 or lower, low conductivity), tannin staining from Indian almond leaves or peat filtration, and no tankmates large enough to intimidate or outcompete the fish. Surface access must not be obstructed, as it is a labyrinth fish. Water temperature in the mid-20s Celsius suits the type locality conditions, though the species has been reported to tolerate a wide range in captivity. Feeding exclusively on live and frozen micro-foods is the practical norm. Any keeper who obtains this species has an obligation — in the view of the community that has kept it alive — to attempt breeding and pass offspring forward.
Conservation
Parasphaerichthys lineatus has not been formally assessed by the IUCN Red List, and its status is recorded as Not Evaluated (NE). The original description by Britz and Kottelat (2002) notes that the Ayeyarwaddy delta peat-swamp forests, from which the species is known, are under severe threat from deforestation, agricultural conversion, and plantation development. These habitat pressures are consistent with those affecting peat-swamp fishes throughout mainland Southeast Asia, and the species' apparently restricted range amplifies the risk.
Within the aquarium hobby, a consensus has formed that P. lineatus may be practically extinct or extremely depleted in the wild, with captive populations maintained by a small network of keepers being the primary safeguard for the species. This view, while not yet backed by a formal IUCN assessment, underscores the importance of treating each captive individual as a conservation asset. Collectors and importers who encounter the species in Myanmar should exercise caution, and hobbyists who obtain it are encouraged to prioritise breeding and distribution of offspring within the keeper community rather than treating it as a display animal alone.