Taxonomy & naming
Aphyosemion franzwerneri was described by Jørgen Jens Scheel in 1971, and the name is recorded in Eschmeyer's Catalog of Fishes (Fricke, Eschmeyer & Fong) under that authority. The species belongs to the large and complex genus Aphyosemion, the West and Central African forest killies, within the family Nothobranchiidae.
Within the genus, A. franzwerneri is sometimes grouped informally with the so-called 'goby killies' — small, benthic, relatively poor-swimming Aphyosemion that behaviorally and morphologically parallel gobies to a degree unusual within the genus. The American Killifish Association notes that the species is sympatric with A. riggenbachi at certain collection points in the lower Sanaga drainage, suggesting a close ecological and possibly phylogenetic relationship with other small western Cameroon Aphyosemion.
Several collection codes and locality designations are in circulation in the killifish hobby, including Bonépoupa, Cellucam, Ndokama, Song Makak, CMG 13/4, and C 89/23, reflecting the species' restricted but multi-locality distribution in western Cameroon.
Morphology
Aphyosemion franzwerneri is a small killifish reaching approximately 2 in total length, with FishBase giving a maximum of 2 in TL and the original Scheel (1971) description citing 2 in. The body form is typical of the goby-like Aphyosemion grouping: stocky, somewhat depressed rather than laterally compressed, giving the fish a vaguely mudskipper-like profile suited to life in very shallow water over sandy or root-laced bottoms.
Colouration in males is the richer of the sexes, as is typical in Aphyosemion, with iridescent patterning on the flanks and fins. Females are plainer, brownish, and recognizable by their rounder body outline. Sexual dimorphism in colour is pronounced, with males displaying the bold fin and flank patterning characteristic of the genus. Exact colour descriptions for live specimens should be verified against photographs from known-locality aquarium strains.
The near-benthic, shallow-water lifestyle is reflected in the body plan: these fish are described as poor swimmers that tend to rest on or near the substrate, using the bottom and root tangles as cover rather than holding position in open water.
Habitat
The natural range of Aphyosemion franzwerneri is restricted to western Cameroon, in the lower drainages of the Sanaga and Wuri rivers. The type locality is recorded as approximately 9 mi north of the Douala–Edea–Yabassi road junction, in high humid forest over Cretaceous sedimentary substrate. The species has not been recorded outside Cameroon.
Habitat is strikingly specialized. The AKA reference literature describes microhabitats of extraordinary shallowness — pools and brook margins with water depth of approximately 1 in (around one inch), over sandy substrate densely interlaced with submerged tree roots. These are not fast-flowing streams but slow seepages and ephemeral-to-semi-permanent pools in the forest understory. FishBase records the habitat as 'very small pools, shallows in swamps and shallow swampy parts of brooks in rainforest'.
Water chemistry in such forest-floor microhabitats is typically soft and mildly acidic. FishBase gives pH 6.0–7.2 and a temperature range of 72–79 °F. Hardness figures are not precisely established. The habitat is benthopelagic in nature, with the fish spending most of their time at or near the substrate surface in water that is itself barely deeper than the fish's own body height.
Feeding
Like all Aphyosemion, A. franzwerneri is a carnivore, feeding on small invertebrates encountered at or near the substrate surface in its shallow rainforest-pool habitat. In nature the diet almost certainly consists of microcrustaceans, midge larvae, worms, and other small aquatic invertebrates characteristic of forest-floor seepages and shallow pools.
In the aquarium the species accepts a range of live and frozen foods. Small live foods — Daphnia, Artemia nauplii, small bloodworm, Grindal worm, and micro-worm — are favored and help maintain condition and coloration. The small adult size (under 2 in) means prey items must be appropriately scaled. Some captive individuals can be weaned onto high-quality small prepared foods, though the species is not known as an especially enthusiastic dry-food taker.
Mating
Aphyosemion franzwerneri is a non-annual killifish. Unlike the annual species of seasonally desiccating habitats, it inhabits pools and seepages that persist through much of the year, and it reproduces accordingly — laying eggs directly on the bottom substrate (or among roots and leaf litter at the base of the water column) in shallow water rather than burying them in peat to wait out a dry season.
The AKA literature describes eggs being laid on the bottom in shallow water. Males are noted to be aggressive, consistent with the territorial and display-oriented behavior typical of Aphyosemion generally. Pair interaction follows the typical killifish pattern of male courtship display and a brief paired spawning act at or near the substrate. The absence of a diapause requirement distinguishes this species clearly from annual killifish of drier savanna habitats.
Breeding
Eggs of A. franzwerneri are deposited on the substrate in very shallow water, where they incubate directly in the water column without diapause. The incubation period is approximately 12–16 days at typical aquarium temperatures in the lower part of the species' range (around 72–75 °F). Growth of juveniles is described as slow relative to many killies, and sexual maturity is not reached until approximately 8–10 months of age — a notably extended juvenile period for a small killifish.
The species is not considered prolific. Clutch sizes per spawning event are modest, and breeding success in captivity requires close attention to water quality and appropriate microhabitat. Given the species' natural preference for extremely shallow water with sandy substrate and root cover, spawning setups that approximate these conditions — shallow tanks or trays with fine substrate or a mop placed at the bottom — are most likely to elicit natural spawning behavior. Eggs and fry should be managed carefully given the low productivity and the species' conservation status.
In the aquarium
Aphyosemion franzwerneri is an uncommon but periodically available species in the specialist killifish hobby, traded primarily through killifish associations under identified locality or collection codes (Bonépoupa, Ndokama, CMG 13/4, and similar designations). Maintaining provenance records for captive strains is particularly important for a species with an Endangered IUCN status and a restricted wild range.
The species' microhabitat preferences argue for a species-only setup. A shallow aquarium or tank fitted out with fine sandy substrate, dense cover of roots, driftwood pieces, and leaf litter, with dim lighting and soft, slightly acidic water (pH 6.0–7.0, 72–77 °F), best meets its needs. Given males' reported aggression, keeping a single pair or a trio (one male, two females) is advisable unless the tank is large enough to establish clear territories. The fish are benthic in habit and not active open-water swimmers; surface cover is not required in the same way as for bubble-nesting anabantoids, but tight-fitting lids are always good practice for killifish.
Breeders note the species is not easy to breed prolifically and patience is required during the 8–10 month maturation period. Small live foods significantly improve condition and breeding success. The combination of rarity, modest size, and remarkable habitat specialization make this a species of appeal to the serious killifish keeper interested in the full range of Aphyosemion biology.
Conservation
The IUCN Red List assessed Aphyosemion franzwerneri as Endangered (EN) in February 2009, under criteria B1ab(iii)+2ab(iii), which relate to restricted geographic range combined with a continuing decline in area, extent, or quality of habitat. The assessment reflects the species' narrow endemic range within the lowland rainforest of western Cameroon — specifically the lower Sanaga and Wuri drainages — and the ongoing pressure on its forest-pool and seepage habitats from deforestation and land conversion.
The species' microhabitat — inch-deep forest pools and seepages over sandy substrate in intact Cretaceous-sediment rainforest — is among the most fragile and easily destroyed freshwater habitat types. Logging, agricultural clearance, and road construction in western Cameroon all directly affect the shallow-pool systems on which this and other micro-endemic killies depend. Because the species is not a seasonal annual, it cannot recolonize dried or disturbed pools by resting in buried eggs; once a population's pool is eliminated, that local population is lost.
Maintaining captive populations through the killifish hobby — with documented locality records and careful husbandry — is one of the few practical conservation measures available for a species at this level of endemism and rarity. Habitat protection of the lower Sanaga and Wuri forest zones is the essential long-term need.