Taxonomy & naming
Sphaerichthys acrostoma was described by the German aquarist and ichthyologist Jörg Vierke in 1979. It belongs to the small genus Sphaerichthys — the chocolate gouramis — within the family Osphronemidae, the same family that encompasses the giant gourami, the paradise fish, and the fighting fish. Eschmeyer's Catalog of Fishes records the valid name as Sphaerichthys acrostoma Vierke, 1979.
The genus Sphaerichthys currently contains four recognised species, all confined to the Greater Sunda Islands and the Malay Peninsula. S. acrostoma is distinguished from its congeners by its larger adult size and by a subtly different facial profile and colour pattern. Together the Sphaerichthys species share a body plan adapted to still, deeply tannic forest waters: a laterally compressed, elongate form, a small upturned mouth, and a cryptic pattern of fine horizontal lines and muted browns that blends into leaf litter and submerged roots.
Morphology
S. acrostoma is the largest of the chocolate gouramis, reaching approximately 2.5–3 in standard length (some sources cite up to 2 in SL for typical adults, with the Seriously Fish figure of 2.5–3 in representing maximum recorded size). The body is elongate and strongly compressed, with a relatively small, terminal to slightly upturned mouth.
Coloration is cryptic: a warm brown to tan ground color overlaid with narrow, darker horizontal streaks running the length of the body. The fins are similarly subdued, with the dorsal and caudal showing faint edging. Sexual dichromatism is present but subtle; males tend to be slightly slimmer with a more angular snout profile, while females in breeding condition develop a fuller, deeper belly. Like all labyrinth fishes, S. acrostoma possesses a suprabranchial labyrinth organ — a vascular, folded accessory breathing structure that allows the fish to extract oxygen directly from atmospheric air, essential for survival in the oxygen-depleted waters it inhabits.
Habitat
Sphaerichthys acrostoma is native to southern Borneo, where it inhabits peat swamp forests and associated blackwater streams — environments among the most chemically extreme in freshwater. The water is dark brown, deeply stained by humic and fulvic acids leached from decaying plant matter, with very low mineral content, negligible buffering capacity, and pH values commonly between 3.5 and 5.0. Dissolved oxygen levels are often low, which is where the labyrinth organ becomes critical: the fish must have access to the water surface to breathe air directly.
These peat swamp habitats are highly stable in chemistry and temperature under natural conditions, rarely exceeding 77–79 °F and sometimes cooler in shaded forest streams around 70 °F. The water is virtually ion-free, with conductivity near zero and hardness immeasurably low. Submerged leaf litter, root tangles, and the dense shade of the forest canopy define the microhabitat. Bornean peat swamps are among the most threatened forest ecosystems on the island; drainage for agriculture and plantation development has destroyed vast areas, fragmenting the populations of all peat-swamp specialists including S. acrostoma.
Feeding
In the wild, Sphaerichthys acrostoma is a micropredator. Its diet consists of small invertebrates including zooplankton, small crustaceans, aquatic worms, and insect larvae. The small, upturned mouth is suited to picking prey from the water column and from the surface of submerged substrates and leaf litter rather than from open water.
In captivity the species can be conditioned to accept a range of small live and frozen foods — Daphnia, micro-worms, mosquito larvae, baby brine shrimp, and Cyclops are all suitable. Prepared foods such as micro-pellets may be accepted once the fish is settled. Because the species is shy and easily outcompeted for food, feeding in a species-appropriate tank with no faster or more assertive tankmates is important to ensure adequate nutrition.
Mating
Sphaerichthys acrostoma is a paternal mouthbrooder: unlike the majority of chocolate gourami species in which the female incubates the eggs, in S. acrostoma it is the male who broods. This distinction has been confirmed by observers of spawning behaviour in both wild habitats and in captivity, making it an exception within a genus otherwise characterised by maternal brooding in some species.
Courting is initiated by the female, who pursues the male and displays alongside him. After spawning, the fertilised eggs are gathered into the male's mouth. The male abstains from feeding throughout the brooding period and should not be disturbed or startled, as stressed brooders may swallow or expel the eggs prematurely. Breeding condition is achieved through careful water-chemistry matching, slow acclimatisation, and a diverse diet of live foods prior to spawning attempts.
Breeding
Once eggs are taken into the male's mouth, the brooding period lasts approximately 7 to 20 days before the fry are released. The wide range reflects temperature and individual variation; cooler conditions within the species' natural range tend to extend development. The male guards the fry for a short period after release before parental interest wanes.
Fry are tiny at release and require appropriately small first foods: infusoria, rotifers, or commercial liquid fry preparations. Newly hatched brine shrimp can be introduced as the fry grow. Success in breeding S. acrostoma in captivity depends almost entirely on reproducing the species' extreme water conditions — ultra-soft, acidic water with pH below 6 and ideally approaching 4.5–5.5, minimal water movement, and the absence of any chemical contamination — and on providing the male with a quiet, stress-free environment during brooding.
In the aquarium
Sphaerichthys acrostoma is among the most demanding of the ornamental gouramis, suited only to aquarists willing to commit to recreating genuine blackwater conditions. The tank water must be very soft (near-zero hardness), acidic (pH 4.5–6.5 at most, ideally lower), warm (72–79 °F), and darkened with Indian almond leaves, peat filtration, or commercially produced blackwater extract. Tap water must be treated or replaced entirely with RO or rainwater; even minor mineral contamination or a pH above 7 can stress or kill the fish over time.
A quiet, heavily planted tank with low flow, subdued lighting, and a deep layer of leaf litter most closely approximates the natural environment. S. acrostoma is peaceful and shy; it should not be kept with assertive or faster-moving species. A species-only setup, or a community restricted to equally small and placid blackwater specialists from the same region, is safest. Surface access must always be available so the labyrinth organ can function. Given its restricted range, threatened habitat, and the difficulty of maintaining it well, sourcing captive-bred specimens is strongly preferred over wild-caught fish.
Conservation
The IUCN Red List assessed Sphaerichthys acrostoma as Vulnerable (VU) in January 2019, under criteria B1ab(iii), reflecting a restricted extent of occurrence combined with continuing decline in the quality and extent of its peat-swamp forest habitat. Southern Borneo's peat swamps have been subject to extensive drainage, burning, and conversion to oil palm and other plantation agriculture, eliminating large areas of the species' potential range.
As a peat-swamp specialist with no ability to tolerate degraded or altered water chemistry, S. acrostoma cannot persist in modified or secondary habitats. Population fragmentation increases vulnerability to local extinction events. The ornamental trade has historically collected the species, and because it is difficult to breed reliably in captivity, wild-caught specimens still appear in the trade. Aquarists are encouraged to seek verified captive-bred stock and to support conservation efforts aimed at protecting remaining intact peat swamp forest in Borneo.