Taxonomy & naming
Aphyosemion aureum was described by the Austrian killifish specialist Arthur Christian Radda in 1980, published in the journal Aquaria (volume 27, issue 11, pages 201–205). The type locality is the Lolo River approximately 47 kilometres southwest of Koulamoutou, in the Du Chaillu Massif of southern Gabon, at an elevation of around 470 metres. Eschmeyer's Catalog of Fishes (Fricke, Eschmeyer & Fong) treats Aphyosemion aureum Radda, 1980 as the valid name.
The species belongs to the large and taxonomically intricate genus Aphyosemion, the African 'flag-fish' killifish of West and Central Africa. Aphyosemion encompasses several hundred species distributed from Senegal and Guinea in the west across to Uganda and the Congo basin in the east, and species-level boundaries within the genus have been extensively revised by molecular and morphological work since Radda's time. A. aureum is placed in the Aphyosemion (Chromaphyosemion) or equivalent species-group by some authors, but the exact subgeneric placement varies by authority; the American Killifish Association documents multiple wild-collected populations under collection codes including GEB, EBT, PEG, and TDK series, reflecting the fine-grained locality structure that characterises the genus in the hobby.
Meristic data from the American Killifish Association give D=13 and A=16–17, consistent with the moderate-finned profile typical of forest-stream Aphyosemion.
Morphology
Adult males reach approximately 2 in total length; FishBase records a maximum of 2 in TL. Females are somewhat shorter and noticeably less colourful. The body is moderately elongate and laterally compressed, typical of the genus, with a slightly rounded dorsal profile and a pointed snout.
Male colouration is the defining feature of the species and the root of its common name. The flanks carry a warm golden-yellow to amber base that grades into reddish-orange or brownish-red tones on the upper body; red or brownish spotting and streaking is superimposed on the flanks. The unpaired fins are elaborately patterned — the dorsal and caudal fins typically show bands or margins of red, blue, and yellow in combinations that vary between populations, and the caudal fin may be slightly lyrate in shape. The American Killifish Association records that subsequent generations bred in captivity tend to lose some of this intensity, an observation consistent with the selective pressure of wild conditions on colouration.
Females are plain by comparison: brownish-grey on the dorsum, paler on the flanks, with minimal fin ornamentation. This pronounced sexual dichromatism is characteristic of Aphyosemion and reflects strong intersexual selection pressure in the wild.
Habitat
Aphyosemion aureum is known only from the upper Lolo and Yaou river drainages in the Du Chaillu Massif, a highland massif of southern Gabon. The type locality on the Lolo River southwest of Koulamoutou sits at roughly 470 metres elevation, and the species has been found in both the Lolo and Yaou systems and their tributaries within this small geographic area. The known range is narrow — the American Killifish Association characterises distribution as restricted to the upper portions of these two drainages — and the population appears naturally fragmented by the topography of the massif.
The habitat is small, fast-flowing mountain forest streams: clear, cool, and well-oxygenated, running through intact equatorial rainforest. FishBase records water temperatures of 64–72 °F and pH 5.8–7.2. These values reflect the cool, moderately acidic conditions typical of high-altitude Central African forest water — significantly cooler than the 77–82 °F of lowland West African killifish streams. The current is meaningful, and the streambed is likely sand, gravel, and leaf-litter with marginal vegetation providing cover and spawning substrate. The fish is benthopelagic, spending time in the water column and near the substrate rather than at the open surface.
Feeding
Like virtually all Aphyosemion, A. aureum is a micro-predator in the wild, taking small invertebrates — aquatic insect larvae, small crustaceans, worms, and zooplankton — from the water column and substrate of its stream habitat. The cool, fast-flowing, well-oxygenated stream environment supports a rich invertebrate fauna that provides the dietary basis for these small killifish.
In captivity the species readily accepts live and frozen foods: Daphnia, Artemia nauplii, bloodworm (chironomid larvae), Tubifex, and equivalent micro-invertebrate substitutes. Dry foods of appropriate size may be accepted by established fish, but a diet rich in live or frozen invertebrates is recommended to maintain colour, condition, and willingness to spawn. Given the comparatively cool water preference (64–72 °F), feeding activity is typically measured rather than frantic.
Mating
Aphyosemion aureum is a non-annual killifish. It inhabits permanent, perennial streams rather than seasonal pools and does not produce diapausing eggs buried in substrate. Instead, it is a plant or mop spawner: the pair spawn over fine-leaved aquatic vegetation, plant roots, or synthetic spawning mops, depositing individual adhesive eggs that adhere to plant matter and incubate in water without a dry period or diapause.
Male courtship follows the typical Aphyosemion pattern of lateral display, spreading and quivering the elaborately patterned fins toward the female while pursuing her through the vegetation. Receptive females are led to a spawning site where the pair press side by side and deposit a single egg, repeating the sequence over multiple days or weeks rather than in a single mass-spawning event. The AKA notes that the species is not particularly prolific; the slow, serial spawning rate is characteristic of forest-stream non-annuals. The American Killifish Association also notes that fish should be sensitively handled since the species is susceptible to velvet (Oodinium).
Breeding
In the aquarium, A. aureum is best bred in a dedicated species tank furnished with fine-leaved plants (Java moss, hornwort, or equivalent) or synthetic wool mops placed both at the surface and near the substrate. A trio of one male and two females, or a pair, is the standard approach. Eggs are deposited singly on the spawning medium over an extended period; the AKA recommends a well-planted tank and regular water changes to maintain egg-laying activity.
Eggs may be collected periodically from the mop or plants and incubated separately in shallow water at approximately 68 °F. Incubation takes around 14–21 days depending on temperature, after which the larvae hatch as comparatively large, free-swimming fry capable of taking baby brine shrimp nauplii and micro-worm from the outset. First foods of very fine particle size are needed in the initial days. The AKA reports that subsequent captive generations tend to produce fewer fry and lose colour intensity, suggesting the importance of periodic refreshing of the breeding stock from wild or wild-caught-descended lines. Sexual maturity is reached at approximately six months.
In the aquarium
Aphyosemion aureum is regarded as manageable for an experienced killifish keeper but requires closer attention to water temperature than most hobbyist killies. Its mountain-stream origin means it thrives at 64–72 °F — temperatures that most tropical aquarium fish would find uncomfortably cool — and keeping it at the 77–82 °F standard for tropical tanks is stressful and shortens its lifespan. A cool, shaded room or a small chiller is the appropriate solution for those in warm climates.
The species does well in a small, densely planted species aquarium with gentle, well-oxygenated filtration replicating the flow of its forest stream habitat. pH in the range 6.0–7.0 and soft to moderately hard water are appropriate. It is peaceful toward similarly sized fish and conspecifics provided males have enough visual cover to avoid constant aggression; a single male with one or two females is the simplest configuration. Décor of fine-leaved plants, driftwood, leaf litter, and spawning mops suits the species well and encourages breeding behaviour.
In commerce, A. aureum circulates mainly through killifish specialist clubs and societies — the American Killifish Association and British Killifish Association both document it, and the GEB 94-9 'Boudianguila' collection is among the better-known hobby strains. It is not a mainstream tropical fish shop item. The sensitivity to velvet noted by the AKA means that quarantine protocols and minimal stress at introduction are important.
Conservation
The IUCN Red List assesses Aphyosemion aureum as Vulnerable under criteria B1ab(iii)+2ab(iii), assessed on 16 February 2009. The B-criterion assessment reflects both a small estimated extent of occurrence and area of occupancy — the species is confined to the upper Lolo and Yaou drainages of the Du Chaillu Massif — combined with ongoing decline in the quality of its forest-stream habitat. The specific sub-criterion (iii) points to declining habitat quality as the driver.
The Du Chaillu Massif lies in a region of Gabon subject to ongoing deforestation and forest degradation, including selective logging, agricultural clearance, and associated changes to stream hydrology and water quality. The cool, fast-flowing, oxygen-rich mountain brooks that A. aureum depends on are sensitive to riparian forest removal, which raises water temperatures, increases sedimentation, and reduces the invertebrate prey base. A fish with a naturally tiny range confined to a single highland drainage system has almost no capacity to recolonise from unaffected populations if local conditions deteriorate.
Gabon retains large intact forest tracts relative to many West and Central African nations, and some of the Du Chaillu region falls within or adjacent to protected areas, but enforcement of forest protection is imperfect and the trajectory of habitat quality in the Lolo–Yaou headwaters is uncertain. No ex-situ conservation programme is known for this species, though it is maintained in the killifish hobby and periodic re-collection to refresh captive stocks has taken place.