Taxonomy & naming
Aphyosemion loennbergii was described by the Belgian-British zoologist George Albert Boulenger in 1903, based on specimens from the Kienké River (then commonly known as the Kribi River) in southwestern Cameroon. Boulenger's original placement was in the genus Fundulus, and the species circulated for many decades under the synonyms Fundulus loennbergii and Fundulus pappenheimi before being transferred to Aphyosemion — a genus erected to accommodate the diverse annual and non-annual killies of sub-Saharan Africa. The authority appears in parentheses because the species has been moved from the genus in which Boulenger originally described it. The valid combination recognised by Eschmeyer's Catalog of Fishes is Aphyosemion loennbergii (Boulenger, 1903).
Within Aphyosemion, the species is placed in the non-annual clade — permanently running-water forms that retain adhesive eggs and plant-spawning behaviour ancestral to the genus, as opposed to the derived annual life-history strategy seen in West African savanna species. The genus Aphyosemion itself belongs to the family Nothobranchiidae (or within Rivulidae sensu lato depending on the authority consulted), the dominant family of African killifishes.
Morphology
Aphyosemion loennbergii is a slender, laterally compressed killifish reaching approximately 2 in total length (FishBase), with Seriously Fish placing practical adult size at 2 in standard length. The body plan is typical of the forest-stream Aphyosemion guild: an elongate torpedo shape, a slightly upturned terminal mouth adapted for surface and near-surface feeding, and large, reflective scales.
The dorsal fin carries 11–14 soft rays and the anal fin 12–16, ray-counts consistent with Boulenger's original description and FishBase. As in many Aphyosemion, males are more vividly coloured and carry more elaborate finnage than females; the specific epithet and common name 'lyretail' allude to the extensions of the caudal fin that develop in mature males, giving the tail a lyre-shaped outline. Colouration details vary by population and captive lineage, but the species is generally regarded as one of the more subdued members of the genus compared to the vivid blues and reds of, for example, A. australe.
Habitat
In nature, Aphyosemion loennbergii inhabits brooks, pools, and small, slow-running streams within primary rainforest on basement (non-laterite) soils in southwestern Cameroon. The Kienké (Kribi) River drainage is the type locality; records also extend to the Lokundje and Nyong drainages. These are permanent watercourses, not seasonal pools — a critical distinction from the annual killifish of the adjacent savanna zones, whose waters dry out completely each year.
Forest-stream water in this region is typically very soft, gently acidic, and heavily tannin-stained by leaf litter and decaying wood. Seriously Fish records temperature in the range 64–72 °F and pH 6.0–7.0 with hardness of 18–90 ppm — notably cooler and softer than the water requirements of many tropical aquarium fishes, and markedly different from the warm, harder water that suits the annual species. The low, dappled light of rainforest canopy and the organic substrate create conditions that strongly influence the species' husbandry requirements in captivity.
Feeding
Like most small Aphyosemion, A. loennbergii is a micropredator in the wild, hunting small invertebrates — aquatic insect larvae, microcrustaceans, worms, and surface-fallen insects — in the slow-moving, dimly lit waters of the forest floor. The upturned mouth is well adapted for taking prey at or near the water surface.
In captivity the fish accepts live and frozen foods readily: Daphnia, Artemia nauplii, bloodworm, Grindal worm, and micro-worm are all suitable staples. Dried and prepared foods may be accepted once the fish is settled, but live and frozen items remain the foundation of good condition and colour. Overfeeding in a small, soft-water tank can rapidly compromise water quality, so modest, regular feeding is preferable to large infrequent meals.
Mating
Aphyosemion loennbergii is a non-annual, plant-spawning killifish. Rather than producing a diapausing egg buried in dried peat, the species scatters individual adhesive eggs on fine-leaved plants, submerged roots, or spawning mops over an extended period — a few eggs per day or per spawning episode across days and weeks. This continuous trickle-spawning strategy is characteristic of the permanent-water Aphyosemion and contrasts sharply with the mass single-event spawning of annual species.
Spawning is initiated by the male's courtship display — lateral spreading of fins, colour brightening, and persistent following of the female. The pair come alongside each other near a plant or mop substrate and deposit a single adhesive egg at a time. Because adults can and will eat their own eggs if given the opportunity, the spawning substrate (mop or plant bundle) is typically removed and replaced every few days so eggs can be incubated separately.
Breeding
Eggs collected from a spawning mop can be incubated in one of two ways. The most common method is water incubation: eggs are placed in small containers of soft, slightly acidic water (pH 6.0–6.5) at a temperature of 64–72 °F and monitored for infertile or fungused eggs, which are removed. Hatching occurs after approximately two to four weeks depending on temperature. Alternatively, eggs can be incubated in slightly damp peat or synthetic medium, though A. loennbergii is not a true annual and does not require a dry diapause phase — the moist-medium approach is simply a convenience for the aquarist.
Fry hatch at a size small enough to require infusoria, paramecia, or very fine commercial fry powder as first foods, graduating to micro-worm and Artemia nauplii within the first week or two. Growth is steady in clean, soft water at the lower end of the temperature range. Because this species is described as 'difficult to maintain' (FishBase), maintaining stable, cool, soft water throughout the rearing period is critical. Water quality is arguably more important than food variety; the species is intolerant of accumulating organic waste and does poorly if temperature is allowed to climb above approximately 75 °F for extended periods.
In the aquarium
Aphyosemion loennbergii carries a reputation for being more demanding than many Aphyosemion, and it is typically recommended for experienced killifish keepers rather than beginners. The primary challenge is its preference for cool, very soft, and acidic water — conditions common in the European killifish hobby (where rainwater or RO water is standard) but less easily achieved in regions with alkaline tap water or warm ambient temperatures. Seriously Fish records an aquarium temperature range of 64–72 °F, which is towards the cool end for tropical fishkeeping and may require chilling in summer in warm climates.
The species is best housed in a small, heavily planted or mop-equipped species tank of 10–15 US gal. A pair or trio (one male, two females) is a practical group. The water should be soft (below 90 ppm hardness), gently acidic (pH 6.0–7.0), and lightly filtered — a sponge filter or air-driven box filter providing gentle flow without strong current suits the fish's natural habitat. Subdued lighting and a dark substrate or tannin-stained water will bring out the best colour and reduce stress. The tank must be covered securely, as killifishes are practiced escape artists.
Compatible tankmates should be peaceful, similarly sized, and tolerant of the same soft, cool water. Other small, non-aggressive killifishes or small rasboras and microdevario can work in sufficiently large systems, though a species-only setup is simpler. Avoid robust or nippy species that might damage the males' elaborate finnage.
Conservation
The IUCN Red List assessed Aphyosemion loennbergii as Least Concern (LC) in 2009 (assessor: T. Moelants), on the basis of a distribution range across multiple river drainages in southwestern Cameroon (Kienké, Lokundje, and Nyong systems) and the absence of evidence of a population decline large enough to warrant a threatened category. The LC assessment reflects the species' occurrence across more than one watershed rather than strict abundance data.
However, the species inhabits primary rainforest streams, an ecosystem under sustained pressure from deforestation, agricultural expansion (particularly oil palm and rubber), and logging in the coastal lowlands of southwestern Cameroon. Rainforest killifishes with restricted regional ranges are demonstrably sensitive to habitat disturbance — siltation, canopy removal, and water-temperature increases that accompany forest clearance can rapidly render previously suitable streams unsuitable. The broader Kienké–Lokundje catchment is not comprehensively protected, and continued land-use change in the region warrants monitoring even for species currently assessed as Least Concern.