Taxonomy & naming
Aphyosemion decorsei was formally described by the French ichthyologist Jacques Pellegrin in 1904 on the basis of material collected in the southern Central African Republic. The authority is presented in parentheses because the species has since been transferred from the original genus; the valid combination, as listed in Eschmeyer's Catalog of Fishes, is Aphyosemion decorsei (Pellegrin, 1904). Within the large and taxonomically complex genus Aphyosemion, it is placed in the subgenus Aphyosemion and assigned to the elegans species group, a grouping of West and Central African forest killies sharing a broadly similar body plan and overlapping ranges in the Congo basin watersheds.
The genus name Aphyosemion is a compound of the Greek aphyo (small fish, fry) and semeion (banner or flag), a reference to the elaborately coloured and often lyre-shaped caudal fins that characterise many of its members. The specific epithet decorsei is a patronym honouring Dr Gaston-Jules Decorse (1873–1907), a French army physician and naturalist who collected zoological specimens during service in Central Africa and who died in the field at the age of 34. The common name Decorse's lyretail — used in killifish society literature and the AKA reference library — reflects both the patronym and the distinctive caudal fin shape of adult males.
The type locality is Bessou, in the Lobaye prefecture of what is now the southern Central African Republic, within the Ubangi (Oubangui) River basin. The American Killifish Association database records 19 journal articles in JAKA (Journal of the American Killifish Association) referencing the species, indicating a sustained, if modest, hobbyist literature since its rediscovery in the aquarium trade.
Morphology
Aphyosemion decorsei is a slender, moderately compressed killifish in which males reach approximately 2 in total length; females are slightly smaller. Meristic counts from the WAK reference library give dorsal fin rays D 8–9, anal fin rays A 13–14, and lateral-line scale counts of 25–28 in earlier accounts and 28–32 (mean 30) in later, more complete series — a variability common in Aphyosemion where geographic populations sometimes differ in scale number.
As with other members of the elegans group, males carry the elaborate finnage that defines the genus at its most ornate: a lyre-shaped or banner-like caudal fin with extended lobes, and unpaired fins edged in colour. The specific colour pattern — ground colour of the flanks, the arrangement of red or blue spots and dashes, and the pigmentation of the fin margins — follows the lineage's typical West–Central African Aphyosemion palette, though detailed published colour descriptions are sparse in the open literature for this particular species, reflecting its relative rarity in the trade. Females are plainer, with rounded unpaired fins and subdued colouration, as is typical across the genus.
Habitat
The middle Oubanghi (Ubangi) River drainage in the southern Central African Republic is the confirmed range of Aphyosemion decorsei, with the type locality at Bessou (Lobaye prefecture) and additional collection records from sites including Bangui, Kapou, Bombala, and Gilima noted in AKA literature. The range likely extends into the northern Republic of Congo and the northern Democratic Republic of Congo, though surveys confirming presence there are lacking.
FishBase characterises the habitat as savanna brooks in permanent or near-permanent water — an important distinction from the seasonal pools used by annual killifish in the same broad region. The WAK reference library specifies rainforest streams, small brooks, and swampy areas, indicating the species exploits shaded, slow-flowing or still water with dense riparian vegetation typical of the forest-savanna transition zone of the northern Congo basin. Such streams are typically soft and acidic, coloured by leaf litter and organic acids, warm (72–79 °F in the habitat), and low in dissolved minerals — the same conditions that characterise the water requirements of most West and Central African Aphyosemion.
Because the Ubangi drainage basin and its southern tributaries in the CAR receive limited ichthyological survey effort relative to the better-studied Lower Congo and Cameroon watersheds, the full distributional range of this species remains uncertain. The IUCN notes it is known with certainty only from one location, though it is suspected to be more widespread.
Feeding
Like other members of the genus, Aphyosemion decorsei is a microinvertebrate predator in the wild, feeding on the small aquatic invertebrates — insect larvae, zooplankton, worms, and small crustaceans — available in the shaded forest and savanna streams it inhabits. No detailed dietary study of this species has been published in the open literature, but the feeding ecology of Aphyosemion as a group is well characterised: these are opportunistic surface and mid-water predators that locate prey visually and take it by a quick strike.
In captivity, Decorse's lyretail responds well to small live and frozen foods — Daphnia, Artemia nauplii, bloodworm, and similar invertebrate prey — which are also important for bringing the fish into breeding condition. FishBase notes the species can be difficult to maintain in aquaria, suggesting that live or high-quality frozen foods are likely more important for its long-term health than for more adaptable Aphyosemion species.
Mating
Aphyosemion decorsei is a non-annual, plant-spawning killifish. Unlike the annual killifish of West African savanna pools, which deposit diapausing eggs in the substrate to survive dry-season desiccation, Decorse's lyretail inhabits permanent or near-permanent water and reproduces as a continuous plant or mop spawner: pairs deposit individual adhesive eggs on fine-leaved aquatic vegetation, plant roots, or fibrous spawning mops, with spawning activity distributed over days or weeks rather than concentrated in a single event.
Male Aphyosemion in this group display actively to females with spread fins, intensified colour, and lateral posturing. Spawning follows individual pairings rather than group or communal mating; the male courts a single female and the pair enter a brief embrace near the spawning substrate, releasing egg and sperm simultaneously. Eggs are deposited singly or in very small numbers per embrace, and the adults do not guard or attend the eggs once laid. This mating system — distributed pairing, plant attachment, no diapause — is the defining feature distinguishing non-annual Aphyosemion from the annual killifish with which they sometimes co-occur at the edge of their ranges.
Breeding
Eggs of Aphyosemion decorsei are adhesive and hatch in water without any dry-storage phase. The WAK reference library records incubation of 12–21 days at 72–75 °F — a range reflecting the variability typical in Aphyosemion depending on temperature and individual clutch. Newly hatched fry are immediately free-swimming and large enough to accept Artemia nauplii as a first food; the WAK library confirms Artemia as a standard first food for this species. Young fish grow steadily under good conditions and are reported to reach sexual maturity at around 6 months of age.
In the aquarium, conditioning the pair on a varied diet of live or frozen invertebrates before introducing a spawning mop or clump of fine-leaved plants (Java moss, Ceratophyllum, or synthetic mop fibres) is the standard approach. Eggs can be collected and incubated in small containers of water at 72–75 °F with gentle aeration, or the spawning medium can simply be transferred to a rearing vessel and the fry harvested as they hatch. Because the adult fish will consume eggs and small fry, either the eggs or the adults should be removed from the spawning tank within a few days of the first spawn. No peat burial, diapause, or dry incubation is required or appropriate for this non-annual species.
In the aquarium
Aphyosemion decorsei is a specialist's fish rather than a beginner's killifish. FishBase flags it explicitly as difficult to maintain in the aquarium — an assessment that likely reflects sensitivity to water quality and the precision required in matching its forest-stream habitat conditions. A small, well-planted tank of 10–15 US gal, heavily shaded with floating plants or a dark lid, kept at 72–77 °F, soft, and mildly acidic (pH 6.0–7.0) approximates the conditions of its native Ubangi tributary streams. Filtration should be gentle — this is not a species of turbulent water — and water changes must be regular but moderate to avoid sudden parameter shifts.
The fish is best maintained in a species tank or with other small, peaceful killies of similar size and water requirements. Males may be territorial toward one another; a ratio of one male to two or more females reduces aggression and spreads spawning effort. The AKA reference library notes multiple collection localities across the CAR range, suggesting some geographic variation in optimum conditions; hobbyists working with identified collection-locality stock should attempt to match the specific water parameters of that provenance if data are available.
Because the species is rarely imported commercially and the wild range is remote and poorly surveyed, most aquarium specimens derive from hobbyist-to-hobbyist transfers within killifish societies. The 19 JAKA citations in the AKA database attest to a sustained specialist interest in the species, and it is through the killifish society network — rather than retail channels — that live specimens and husbandry knowledge are most reliably accessed.
Conservation
The IUCN Red List assessed Aphyosemion decorsei as Data Deficient (DD) in February 2009 (Moelants, T., assessment version 2025-2). The DD designation reflects insufficient information to determine whether the species meets the criteria for any threatened category — not a finding of security. Assessors noted that the species is confirmed from only a single location (the Bessou type locality in the Lobaye prefecture, CAR), though it is suspected to be more widespread in the Ubangi basin and possibly into northern Congo and the DRC.
The primary documented threat, noted in the IUCN assessment, is commercial aquarium harvesting — a pressure that is meaningful in the context of a species with a small, poorly known range and no formal population monitoring. The broader pressures on forest and savanna streams in the southern CAR — deforestation for agriculture and charcoal production, small-scale mining in the Lobaye and Sangha drainages, and the general deterioration of water quality in a country with limited environmental governance capacity — add to the background risk without being quantified for this species specifically.
The remoteness of the Ubangi tributaries in southern CAR has, until now, offered some incidental protection, but it has equally made systematic survey difficult. A targeted assessment of the species' actual distribution, population density, and habitat condition in the Lobaye, Sangha, and adjacent drainages would be the necessary first step toward a meaningful threat evaluation and is the standing conservation need for Aphyosemion decorsei.