Taxonomy & naming
Aphyosemion herzogi was described by the Austrian killifish specialist Arthur C. Radda in 1975. Eschmeyer's Catalog of Fishes (Fricke, Eschmeyer & Fong) recognises the valid combination as Aphyosemion herzogi Radda, 1975. The species is placed in the subgenus Mesoaphyosemion, a grouping within the sprawling genus Aphyosemion that encompasses many of the non-annual, forest-stream killies of West and Central Africa. Several names have been treated as synonyms in the literature, including Aphyosemion batesii, Aphyosemion bochtleri, and Aphyosemion calliurum ahli, though these identifications have been debated by specialists over the decades.
The genus Aphyosemion belongs to the family Nothobranchiidae, the nothobranchid killifishes, which together with the Aplocheilidae and the New World Rivulidae form the core of the killifish radiation. Within Nothobranchiidae, Aphyosemion is the largest genus, with well over a hundred described species distributed across West and Central Africa from Senegal to the Congo basin. The subgenus Mesoaphyosemion groups the species that inhabit the cool, well-oxygenated streams of the Cameroon Highlands and the Lower Guinea forest belt, a setting that selects for non-annual life histories and longer lifespans than the seasonal-pool species of the drier lowlands.
Morphology
Aphyosemion herzogi is a slender, laterally compressed killifish reaching a maximum total length of 2 in (FishBase). The body plan is typical of the non-annual Aphyosemion group: a pointed snout, a slightly upturned mouth adapted for surface-feeding, and a single dorsal fin set well back on the body, roughly opposite the anal fin. The caudal fin in males tends to be rounded to slightly lyrate.
Coloration in males is vivid: the body ground colour is typically bluish-green to greenish-blue, overlaid with a pattern of red spots or dashes that vary in density and arrangement across collection localities — a characteristic of the genus that makes locality strains visually distinct and a source of considerable interest to specialist hobbyists. Fin coloration is similarly complex, with red, blue, and yellow elements in the unpaired fins. Females are considerably plainer: brownish or olive with faint spotting and rounded, transparent fins. This strong sexual dichromatism is typical of non-annual Aphyosemion and is used by both males in rival displays and by females in mate assessment.
Habitat
Herzog's killi is native to the Ntem, Middle Okano, Middle Mvoung, upper Ivindo, and upper Benito river systems, which together span northern Gabon, Equatorial Guinea, and adjacent southern Cameroon. The IUCN assessment places it within freshwater wetlands (inland), both permanent and seasonal/intermittent rivers and streams — though the FishBase habitat description emphasises clear, flowing brooks in mountainous rainforest on the inland plain, suggesting a preference for cooler, better-oxygenated headwater environments over sluggish lowland channels.
Water conditions in these streams are relatively mild for a West African forest killi. FishBase records a pH of 6.5–7.2 — somewhat less acid than the blackwater streams that host the most demanding members of the genus — and a temperature range of 64–72 °F reflecting the elevated, shaded character of highland forest brooks. These cooler temperatures are significant: they set Herzog's killi apart from many lowland Aphyosemion and mean that standard tropical aquarium temperatures (75–79 °F) are often too warm for long-term health. The water is described as clear rather than heavily tannin-stained, though some humic influence is typical of any Central African forest catchment.
Feeding
Like most small nothobranchiid killifishes, Aphyosemion herzogi is an opportunistic micro-predator in the wild. Its upturned mouth and position in the water column indicate a preference for surface and mid-water prey: small invertebrates — insect larvae, microcrustaceans, worms, and terrestrial insects that fall onto the surface — make up the bulk of the diet in nature.
In the aquarium the species is straightforward to feed. Live and frozen foods of appropriate size are accepted enthusiastically: Daphnia, Artemia nauplii, bloodworm (Chironomus larvae), small Tubifex, and blackworm are all suitable. Micro-pellets and high-quality dried foods can supplement the diet but should not form its entirety, as this group tends to condition and colour up best on varied, protein-rich live or frozen prey. Feeding small amounts twice daily is preferable to one large daily feed, both for digestive health and to keep aggression between males manageable.
Mating
Aphyosemion herzogi is a non-annual killifish that spawns in the plant-spawner mode: pairs or trios deposit adhesive eggs individually onto fine-leaved plants, java moss, roots, or synthetic spawning mops over an extended period, rather than burying a clutch in substrate and entering diapause. There is no diapausing embryonic phase — eggs develop continuously in water. This reflects the species' origin in permanent or near-permanent streams rather than seasonal pools that dry up annually.
Male Aphyosemion herzogi are active and persistent courters. A dominant male will display continuously to females, raising and spreading his fins to show colour, and will drive rivals away from a preferred patch of cover. Rival males in close proximity display to each other with flared fins and lateral body turns; outright fighting can occur in cramped quarters. The system appears to be one of male-biased polygamy with no lasting pair bond: a single male may spawn with several females on the same day.
Breeding
Spawning takes place over fine-leaved vegetation or mop fibres near the substrate or at mid-water, with the pair briefly aligning side-by-side at the moment of egg and sperm release. Eggs are small, adhesive, and transparent; they are deposited singly or in small clusters rather than in a mass. Incubation in water at the natural temperature range (64–72 °F) takes approximately one month (FishBase), a relatively long incubation consistent with the cool highland stream origin. At warmer temperatures development accelerates but longevity may be reduced.
Hobbyists typically use one of two production methods for this subgenus. In the mop-pull method, spawning mops are checked daily or every few days and ripe eggs removed to a shallow container of clean, slightly acidic water for incubation away from adults. Alternatively, the spawning tank is densely planted and the fry allowed to grow out among the adults once free-swimming — this works in larger tanks with ample cover and moderate feeding. Newly hatched fry are small but capable of taking Artemia nauplii from the first day of free-swimming; growth is moderate at the cooler temperatures this species prefers.
In the aquarium
Herzog's killi is a species for the specialist rather than the general community tank. Its modest size (to 2 in) and striking male coloration make it appealing, but its requirement for cool water — 64–72 °F — is the primary management challenge in regions where ambient room temperature exceeds this range. A small chiller, a fan directed at the water surface, or a basement or climate-controlled fish room is often necessary in summer months. A small, well-planted tank of 10–15 US gal is adequate for a pair or trio; an abundance of fine-leaved plants and floating cover reduces male aggression and gives females retreat space.
Water quality should be pristine: the species comes from clear-flowing streams and is sensitive to nitrate accumulation. Small, frequent water changes with soft to moderately soft, slightly acidic water (pH 6.5–7.2) matching the natural range are recommended. Tankmates, if any, should be peaceful species of similar size that tolerate the same cool temperature — many of the popular tropical community fish are inappropriate. Several Aphyosemion specialists keep the species alone or with other cool-water forest killies.
The American Killifish Association maintains an active record of this species, with at least 18 locality entries and a substantial body of hobbyist literature in JAKA (Japanese killifish publications), reflecting the interest it has received from killifish specialists in both North America and Japan.
Conservation
The IUCN Red List assessed Aphyosemion herzogi as Least Concern (LC) in February 2009, published 2010 (ver. 3.1). The assessment noted no information on specific threats and no known conservation actions in place. The population trend was listed as unknown. The broad distribution across multiple river systems — the Ntem, Okano, Mvoung, Ivindo, and Benito basins — spanning three countries provides a degree of resilience that justified the LC outcome at the time of assessment.
Nevertheless, the species is a Lower Guinea endemic, and the forests of northern Gabon, Equatorial Guinea, and southern Cameroon face ongoing pressure from logging concessions, mining, and road-building that can alter stream hydrology, raise siltation levels, and degrade the clear-water habitat on which this fish depends. The cool, well-oxygenated highland-stream microhabitat is particularly sensitive: even localised canopy removal raises water temperature and reduces oxygen saturation. Most Aphyosemion of the Mesoaphyosemion subgenus have restricted ranges and occupy similar fragile forest-stream systems; while herzogi fares better than many single-locality congeners, continued monitoring of its highland stream populations would be warranted as extraction pressure in the Lower Guinea forests intensifies.