Taxonomy & naming
Sphaerichthys selatanensis was described by the German aquarist and ichthyologist Jörg Vierke in 1979, the same year that Vierke described the larger Sphaerichthys acrostoma. The species belongs to the small genus Sphaerichthys — the chocolate gouramis — within the family Osphronemidae. Eschmeyer's Catalog of Fishes records the valid name as Sphaerichthys selatanensis Vierke, 1979.
The genus Sphaerichthys currently contains four recognised species, all confined to the Greater Sunda Islands and the Malay Peninsula. S. selatanensis is endemic to southern Borneo (South and Central Kalimantan provinces of Indonesia) and Belitung Island, in the southern Sunda Shelf drainage. The specific epithet selatanensis is derived from the Indonesian word selatan, meaning 'south', a reference to the southern Bornean origin of the type material. Together the Sphaerichthys species share an elongate, compressed body, a small upturned mouth, and a cryptic pattern suited to life among leaf litter and root tangles in dark forest streams.
Morphology
S. selatanensis is a small, slender, laterally compressed fish reaching approximately 1.5–2 in standard length. The body profile is elongate with a slightly arched dorsal outline and a characteristically small, upturned mouth adapted to picking prey from the water column and from submerged surfaces. The fins are rounded and relatively small.
Coloration follows the chocolate gourami pattern: a warm brown to reddish-brown or olive ground, overlaid with faint horizontal streaks or rows of spots, and often with a pale lateral stripe or series of blotches along the midline. The fins may show subtle banding or edging. Sexual dichromatism is present but subtle; females in breeding condition typically develop a more rounded, fuller belly, while males may show a slightly slimmer profile. Like all members of the family Osphronemidae, S. selatanensis possesses a suprabranchial labyrinth organ — a vascularised, folded accessory respiratory structure that allows the fish to breathe atmospheric air directly at the water surface, an adaptation essential for survival in the low-oxygen peat-swamp waters it inhabits.
Habitat
Sphaerichthys selatanensis inhabits peat swamp forests and associated blackwater streams across South and Central Kalimantan (southern Indonesian Borneo) and Belitung Island. These environments are among the most chemically extreme in freshwater ecology: the water is deeply stained dark brown by humic and fulvic acids leached from decomposing peat and forest litter, virtually ion-free, and highly acidic. Reported pH values approach 4.0 in typical habitat, with temperatures in the range of 77–86 °F. Dissolved oxygen is frequently low, making the labyrinth organ critical — surface breathing is not optional but necessary.
The microhabitat is defined by dense submerged leaf litter, root tangles, and overhanging riparian forest that casts the water in perpetual shade. The still or near-still water has negligible buffering capacity and conductivity close to zero. Bornean peat swamp forests are among the most threatened terrestrial and freshwater ecosystems in Southeast Asia; drainage, burning, and conversion for oil palm and timber plantations have eliminated vast tracts of this habitat across Kalimantan, fragmenting the populations of all peat-swamp specialists including S. selatanensis.
Feeding
In the wild, Sphaerichthys selatanensis is a micropredator. Its diet consists of small invertebrates — zooplankton, small crustaceans, aquatic worms, and insect larvae — picked from the water column and from submerged substrates and leaf litter. The small, upturned mouth is suited to precise picking rather than pursuit of faster prey.
In captivity the species can be conditioned to accept a range of appropriately small live and frozen foods: Daphnia, micro-worms, baby brine shrimp, mosquito larvae, and Cyclops are all suitable. Prepared micro-pellets may be accepted by settled specimens. As with all Sphaerichthys, S. selatanensis is shy and easily outcompeted at feeding; a quiet species-appropriate setup without assertive or faster-moving tankmates is necessary to ensure the fish feeds reliably and remains in good condition.
Mating
Sphaerichthys selatanensis is a mouthbrooder. Research published by Zworykin and colleagues in 2024 — focused primarily on S. osphromenoides but with implications for the genus — resolved long-standing confusion about sex identification in chocolate gouramis and established that in Sphaerichthys it is the male, not the female, who performs oral incubation of the eggs. Hobbyist breeding accounts corroborate mouthbrooding behaviour in S. selatanensis specifically.
Courting is typically initiated or intensified by the female. The pair spawn in open water or near the substrate, and fertilised eggs are taken into the male's mouth. The male does not feed during the brooding period and requires a quiet, stress-free environment: disturbance or startling a brooding male may cause it to swallow or eject the clutch prematurely. Achieving spawning in captivity requires careful replication of the species' natural water chemistry — ultra-soft, highly acidic blackwater — combined with a conditioning diet of live foods.
Breeding
Once the fertilised eggs are gathered into the male's mouth, the brooding period lasts approximately 7–20 days before free-swimming fry are released; the range reflects temperature and individual variation. Cooler conditions within the natural range tend to prolong development. After release, the male may guard the fry briefly before parental interest wanes.
Fry are very small at release and require appropriately small first foods: infusoria, rotifers, or commercial liquid fry preparations for the first days, graduating to newly hatched brine shrimp as the fry grow. Success depends almost entirely on water quality: ultra-soft water of near-zero hardness, pH in the range of 4.0–6.0, minimal water movement, and a stress-free environment for the brooding male are all essential. Because the species is rarely bred in captivity, hobbyist accounts of successful spawning — such as those documented by dedicated keepers of this genus — represent a meaningful contribution to understanding of its reproductive biology.
In the aquarium
Sphaerichthys selatanensis is a specialist fish for aquarists willing to commit to genuine blackwater conditions. The aquarium water must be very soft (near-zero hardness), highly acidic (pH 4.0–6.5, ideally approaching 4.5–5.5), warm (77–86 °F), and darkened with Indian almond leaves, peat filtration, or commercially produced blackwater extract. Tap water is unsuitable without extensive treatment; reverse osmosis or rainwater is necessary, and even minor mineral contamination or a pH rise above 7 will stress the fish. Water movement should be very gentle, and the tank must provide open surface access at all times for the labyrinth organ to function.
A quiet, heavily planted or leaf-litter aquarium with subdued lighting most closely approximates the natural peat-swamp environment. The species is shy and peaceful; it should not be kept with assertive or significantly larger fish. A species-only setup or a community restricted to equally placid blackwater specialists from the same biogeographic region — such as other small Sphaerichthys species or compatible Parosphromenus — is the safest approach. Given its restricted range, threatened habitat, and the rarity of captive-bred specimens, sourcing tank-bred fish is strongly preferred over wild-caught individuals.
Conservation
The IUCN Red List assessed Sphaerichthys selatanensis as Near Threatened (NT) in January 2019, under criterion B2ab(iii), reflecting a moderately restricted area of occupancy combined with a continuing decline in the quality and extent of its peat-swamp forest habitat. Southern Borneo's peat swamps have been subject to widespread drainage, peat fires, and conversion to oil palm and other plantation crops, eliminating large areas of the species' potential range.
As an extreme peat-swamp specialist, S. selatanensis cannot persist in modified, drained, or chemically altered habitats; its tolerance for change is essentially zero. Population fragmentation across the heavily modified landscapes of South and Central Kalimantan increases vulnerability to local extinction events. Belitung Island represents an additional population unit, but the island's peat habitats face similar pressures. The species is rarely encountered in the ornamental trade, and when it does appear, wild-caught origins are likely given the difficulty of captive breeding. Conservation of intact peat swamp forest in the Indonesian Borneo interior remains the only effective protection for this and other peat-dependent specialists.