Taxonomy & naming
Aphyosemion cognatum was described by the German aquarist-ichthyologist Hermann Meinken in 1951, based on specimens that had arrived in the German aquarium trade via a commercial shipment handled by the Dutch importer Aquarium Hamburg, believed to have originated from Leopoldville (today Kinshasa), in what was then the Belgian Congo. The original description appeared as 'Aphyosemion cognatum eine neue Zahnkarpferart aus dem Congo Gebiet' in Die Aquarien und Terrarien Zeitschrift, IV(10):255–257. Eschmeyer's Catalog of Fishes treats Aphyosemion cognatum Meinken, 1951 as the valid combination; authority is not parenthesised because the species has remained in the genus Aphyosemion since its original description.
Meinken named the new species cognatum — Latin for 'related' or 'kindred' — because he believed it to be closely related to A. calliurum and A. striatum, two other brightly coloured West and Central African rivulines already known to the hobby. The genus name Aphyosemion combines the Greek aphyē (a small fish) with sēmeion (a sign or mark), referring broadly to the bold markings characteristic of the group. Further commercial shipments were received in 1954, and specimens were acquired by the killi specialist Joerg Scheel, who contributed substantially to the hobby literature on this and related species.
Aphyosemion cognatum belongs to the family Nothobranchiidae (African rivulines) as recognised by current molecular phylogenetics, which separated this clade from the New World rivulines previously grouped together under Aplocheilidae. Within Nothobranchiidae the genus Aphyosemion is large and taxonomically active; cognatum is placed in no formally defined species group by all authorities, but has historically been associated with the calliurum group of non-annual plant-spawning Congo basin species.
Morphology
Aphyosemion cognatum is a slender, moderately compressed killifish reaching approximately 2 in total length, with FishBase recording a maximum of 2 in TL. The body plan is typical of the genus: a slightly flattened dorsal profile, a pointed snout, a terminal mouth directed slightly upward, and all fins positioned toward the posterior half of the body. The caudal fin is rounded, and the dorsal and anal fins are symmetrically placed and relatively large, giving breeding males ample display area.
Sexual dichromatism is pronounced. Males are among the more richly coloured species in the middle Congo Aphyosemion fauna: the ground colour of the flanks is a vivid blue-green to teal, overlaid with a pattern of red to orange-red spots and short streaks that become denser toward the posterior. The unpaired fins — dorsal, caudal, and anal — continue the red spotting against a background of blue and yellow, with the fin margins often edged in a contrasting hue. The intensity of colouration varies with condition, age, and the individual population of origin. Females are considerably more subdued: an olive to yellow-brown ground colour with faint spots or mottling and largely translucent unpaired fins, an effective camouflage strategy for a small fish that carries and guards developing embryos in vegetation.
Habitat
The native range of Aphyosemion cognatum covers the lower and middle Congo River basin, spanning the Republic of Congo and the Democratic Republic of the Congo. The type locality is Leopoldville (Kinshasa), at the Pool Malebo reach of the lower Congo, placing cognatum among the Congo basin killies rather than the coastal forest streams inhabited by many other Aphyosemion species from Cameroon, Gabon, or Nigeria.
Within this basin the species occupies permanent or near-permanent slow-moving waters: lagoons, backwaters, flooded forest pools, and the margins of minor tributaries where current is minimal and vegetation abundant. FishBase records a water temperature of 72 °F and Seriously Fish notes 75 °F as an optimum, consistent with the stable warmth of equatorial lowland forest water. Hardness is low — FishBase gives 12 dGH and Seriously Fish gives approximately 8 dGH — reflecting the soft, poorly buffered character of water draining off the ancient, mineral-poor Congolian basement. pH centres around 6.5, mildly acidic, typical of water shaded by forest canopy with some input of tannins from leaf litter. Unlike the savanna-draining streams that host annual killies, the water bodies cognatum inhabits do not dry out seasonally, which correlates directly with the species' non-annual reproductive strategy.
Feeding
Aphyosemion cognatum is an opportunistic predator of small invertebrates, as is typical for killies of its size. In nature the diet consists primarily of small insects and their larvae, microcrustaceans, worms, and other zooplankton available at the water surface and in the upper water column, where the upturned mouth is best positioned to intercept prey.
In the aquarium the species is straightforward to feed. It accepts live and frozen small invertebrates — Daphnia, Artemia nauplii, bloodworm, and similar fare — with enthusiasm, and most established individuals will also take high-quality micropellets or flake of an appropriate particle size. A varied diet with a strong invertebrate component supports the best colouration and breeding condition in males. As with all small killifish, care should be taken not to offer food items large enough to be a choking hazard or to allow uneaten food to foul the water chemistry.
Mating
Aphyosemion cognatum is a non-annual killifish and a plant-spawner — the pair deposits adhesive eggs on fine-leaved plants, submerged roots, or floating spawning mops rather than burying eggs in substrate. This reproductive mode matches the permanence of the species' natural habitat: because the water bodies it inhabits do not dry completely between seasons, there is no selective pressure to produce diapausing eggs capable of surviving in dry mud, and the eggs hatch in water over a matter of weeks rather than entering a dormant phase measured in months.
During spawning, males display vigorously to females, erecting their fins to show the full extent of their colouration and engaging in the characteristic S-shaped lateral display common to Aphyosemion. The pair aligns closely — the male curving alongside the female in the typical killifish spawning embrace — and a small number of eggs is deposited and fertilised at each pass. The adhesive eggs attach to the chosen substrate where they incubate in water. Spawning occurs over days to weeks in a well-maintained pair, with the female continuing to produce eggs as long as conditions are favourable and the male maintains interest. Healthy, well-conditioned fish will spawn readily and repeatedly in captivity.
Breeding
Eggs of Aphyosemion cognatum are adhesive and small, adhering to the fibres of spawning mops or to the leaves and roots of fine-leaved plants such as Java moss. Incubation takes approximately two to six weeks in water at appropriate temperatures, a range typical of non-annual Aphyosemion; development is temperature-dependent and slower at the cooler end of the species' range. The eggs can also be removed from the mop and incubated on slightly damp peat or paper towelling for a period of one to three weeks before being returned to water, a technique hobbyists use to control fungus and separate clutches.
Fry are small at hatching but free-swimming from the start and capable of taking appropriately sized live foods — micro-worms, freshly hatched Artemia nauplii, or vinegar eels — within the first days of life. They grow steadily and reach sexual maturity within several months under good conditions. Unlike annual killies, whose pairs are often kept together only briefly for spawning before separation, cognatum pairs can be maintained together continuously; the low per-spawn egg count and the adhesive nature of the eggs reduces the risk of significant egg predation compared with non-adhesive egg-scatterers, though some removal of eggs to a separate rearing container is common practice to maximise fry survival.
In the aquarium
Aphyosemion cognatum is a well-established aquarium fish with an uninterrupted hobby history stretching back to the early 1950s, and it is considered one of the more accessible non-annual Aphyosemion species for beginners to the killifish hobby. A small species tank of 10–15 US gal is adequate for a pair or small group; the water should be soft and slightly acidic — targeting pH 6.0–6.8, hardness under 10 dGH, and temperature in the range of 72–79 °F — in keeping with the chemistry of the lower Congo basin. Peat filtration or the use of botanicals (dried leaves, seed pods) to add tannins both buffers the chemistry and provides a visual and chemical environment that promotes natural behaviour and colour.
The tank should be planted with fine-leaved species and furnished with floating spawning mops to give the fish appropriate spawning sites and shelter. The species is not aggressive but is small and delicate; tankmates, if any, should be of comparable size and peaceful disposition, and care should be taken not to combine it with larger or more boisterous fish. Males will display and occasionally spar, but serious injury is uncommon. A tight-fitting lid is essential — killifish are accomplished jumpers and will exploit any gap.
Colouration in males is best maintained with varied live and frozen foods, stable and clean water chemistry, and avoidance of overstocking. The species has been maintained in continuous cultivation by killifish clubs and specialist enthusiasts worldwide, making captive-bred stock readily available through the American Killifish Association, British Killifish Association, and European equivalents. Wild-caught importation is infrequent; hobbyist-bred lines are the norm.
Conservation
The IUCN Red List assessed Aphyosemion cognatum as Least Concern (LC) in 2009, reflecting that the species has a reasonably broad distribution across the lower and middle Congo basin and is not considered to face immediate extinction risk at a range-wide level. Threats identified in the IUCN assessment include wetland loss, disturbance from shipping lanes on the Congo River, fishing and harvesting pressure, and domestic and urban wastewater inputs — the latter of particular relevance given the type locality's proximity to Kinshasa, one of Africa's most populous cities.
The lower and middle Congo basin is one of the world's most biodiverse freshwater systems, but it is subject to significant and growing anthropogenic pressure: rapid urban expansion around Kinshasa and Brazzaville, deforestation in the surrounding basin, and increasing industrial and artisanal exploitation of river resources all threaten the slow-moving, vegetated habitats that Aphyosemion cognatum depends on. The species' reliance on permanent, shallow, heavily vegetated backwaters makes it sensitive to land clearance and sedimentation. The LC assessment is now more than fifteen years old; a reassessment incorporating more recent data on habitat change in the Kinshasa/Pool Malebo region would be welcome. In captivity the species is secure, with self-sustaining populations maintained by killifish enthusiasts around the world independent of wild collection.