Taxonomy & naming
Microctenopoma nigricans was described by Norris in 1995 and is placed in the family Anabantidae. The genus Microctenopoma is an African genus of labyrinth fishes closely related to Ctenopoma, and was separated from that broader grouping on the basis of meristic and morphological characters. Eschmeyer's Catalog of Fishes treats Microctenopoma nigricans Norris, 1995 as the valid name.
The specific epithet nigricans is Latin, meaning 'becoming black' or 'blackish', a reference to the dark pigmentation that characterises the species. Within Microctenopoma, nigricans is distinguished by its fin-ray and vertebral counts: dorsal spines 15–18, anal spines 7–9, and 26–27 vertebrae. The genus as a whole encompasses a set of small, largely secretive swamp-dwelling anabantids distributed across sub-Saharan Africa, the Congo basin being a major centre of diversity.
Morphology
Microctenopoma nigricans is a small, compressed anabantid. Males reach a maximum of 2.5 in standard length; females are notably smaller, reaching around 2 in SL — a degree of sexual size dimorphism that is typical of the genus. The body is deep relative to its length and laterally flattened, well suited to manoeuvring through dense vegetation and tangled root systems in swampy habitats.
The species carries the dark, often mottled colouration implied by its name. The flanks are typically dusky with darker barring or mottling, and the fins may show additional patterning. As in all Microctenopoma, the dorsal fin carries the serrated ctenii on the scales that give the genus its name. Females are generally plainer and smaller than males. The suprabranchial labyrinth organ is well developed, as in all anabantids, allowing the fish to survive in water with very low dissolved oxygen.
Habitat
In the wild, Microctenopoma nigricans is known from the Kasai and Sankuru drainages of the middle Congo River basin, in south-central Democratic Republic of Congo. Related material has been recorded from the Lulua River, a tributary of the Kasai, where typical water conditions include a slightly acidic pH averaging around 6.65, with values dropping as low as 5.5 in some habitats — characteristic of the Congo basin's blackwater and swamp systems.
The species is benthopelagic in freshwater, occupying the still or slow-moving margins of swamps, bogs, and floodplain pools where surface access for air-breathing is unobstructed and emergent or floating vegetation provides shelter. These environments are often low in dissolved oxygen, a condition the labyrinth organ allows the fish to tolerate with ease. Temperature across the Kasai basin is tropical and broadly stable year-round, typical of equatorial lowland Congo habitats.
Feeding
Microctenopoma nigricans is a predatory insectivore, as is typical of the genus. In the wild it forages among vegetation and leaf litter for small invertebrates — aquatic insect larvae, small crustaceans, worms, and any other invertebrate prey of appropriate size. The compressed, moderately deep body and relatively large mouth are characteristic of an ambush-oriented or stalking predator that waits among cover rather than pursuing prey in open water.
In captivity the species accepts a range of small live and frozen foods readily: bloodworm, Daphnia, brine shrimp, and similar invertebrate prey. Prepared foods may be accepted by settled fish but live or frozen items are preferred and help maintain condition. As with many small anabantids, feeding little and often suits the species better than infrequent large meals.
Mating
Microctenopoma nigricans is a bubble-nester, following the reproductive strategy widespread among African and Asian anabantids. The male constructs a floating raft of mucus-bound air bubbles at the water surface, typically anchored among floating vegetation or plant debris. Courtship involves the male displaying to the female, spreading his fins and intensifying any colour patterning to signal fitness and territorial possession of the nest site.
As in other bubble-nesting anabantids, the male takes the dominant role in site selection and nest construction, and it is his display that triggers the female's willingness to spawn. The pair embrace beneath or alongside the bubble nest in the characteristic anabantid nuptial clasp, with the male wrapping around the female to fertilise eggs as they are released.
Breeding
Following spawning, the buoyant fertilised eggs float up into the bubble nest, where they are caught and maintained by the male. He actively tends the nest, retrieving any eggs that sink or fall clear and repairing the bubble raft as needed. Incubation is brief at tropical temperatures — a matter of one to two days — after which tiny larvae hang in the nest before becoming free-swimming.
The male guards the nest and early fry, and in well-managed aquarium breeding setups the female is typically removed after spawning to prevent aggression from the guarding male. First foods for the fry must be very small: infusoria or commercial liquid fry foods in the earliest days, followed by newly hatched brine shrimp nauplii and micro-worm as the fry grow. Data on clutch size and full fry-rearing timelines for this specific species are sparse in the available literature, but the pattern follows that of congeners closely.
In the aquarium
Microctenopoma nigricans is an uncommon species in the hobby, better known among specialists in African anabantids than in the wider fishkeeping community. It is best suited to a species tank or a very carefully chosen community of similarly sized, peaceful fishes from comparable blackwater environments. The small adult size — under 3 in for males, smaller for females — means it can be housed in a modest aquarium, though it needs dense planting and floating cover to feel secure and display natural behaviour.
Water conditions should lean toward the soft, slightly acidic chemistry of its native Kasai basin habitats: pH in the range of 6.0–7.0 and low to moderate hardness. Temperature should be maintained in the tropical range typical of equatorial Congo lowlands. Filtration should produce minimal surface turbulence, as the fish must be able to access the surface freely to breathe air; a tight-fitting lid is important to prevent cold air from chilling the fish as they surface-breathe. Live or frozen invertebrate foods are the dietary staple. The species is not aggressive toward dissimilar tankmates but may quarrel with conspecifics or closely related species, particularly males; a bonded pair or a single specimen is the safest approach.
Conservation
Microctenopoma nigricans is assessed as Least Concern on the IUCN Red List, with the most recent assessment recorded in 2009. This reflects the species' occurrence across the Kasai and Sankuru drainages of the middle Congo basin — a large, biologically rich, and relatively intact river system — and the absence of evidence for major population declines or acute habitat threats at the time of assessment.
The Congo basin as a whole faces increasing pressure from artisanal and industrial activity, agricultural expansion, and deforestation, and swamp-dwelling specialists like Microctenopoma nigricans may be affected by habitat loss in the longer term even where they are currently secure. However, within the broader context of the basin's extent and the species' apparent tolerance of the blackwater swamp habitats that remain widespread across the Kasai region, no immediate conservation concern is flagged. The species' rarity in the hobby means that wild populations are not subject to significant collecting pressure for the ornamental trade.