Taxonomy & naming
Nothobranchius korthausae was described by Hermann Meinken in 1973, with the holotype SMF 12273 — a 1 in TL male — collected from Mafia Island, Tanzania, at the coordinates 07°58'S, 39°44.5'E. Eschmeyer's Catalog of Fishes (Fricke, Eschmeyer & Fong) recognises the valid name as Nothobranchius korthausae Meinken, 1973, in the family Nothobranchiidae.
Within the genus, the species is placed in the subgenus Adiniops and assigned to the guentheri species group — an East African radiation characterised by a relatively robust build and the possession of transverse bars or reticulations in the caudal fin. The guentheri group contains several ecologically and geographically parallel annual species distributed across coastal Tanzania, Kenya, and the adjacent islands, and N. korthausae is one of the very few members of the entire genus endemic to an island rather than the African mainland.
The species epithet honours Edith Korthaus (1923–1987), a German aquarist, killifish enthusiast, and longtime editor of the magazine Das Aquarium, who contributed substantially to popularising annual killifish in the German-speaking hobby.
Morphology
Males reach a maximum total length of approximately 2 in (FishBase; holotype 1 in TL), though aquarium specimens commonly mature at 1.5–2 in. The build is typical for the guentheri group: moderately deep-bodied, with a rounded snout and a slightly upturned mouth suited to surface and mid-water feeding. The body is scaleless on the head but fully scaled on the trunk, with the relatively large, lustrous scales typical of the genus.
Male colouration is striking. The flanks are intense iridescent blue to blue-green, overlaid with red or orange-red markings that vary in extent between individuals and populations. The dorsal and anal fins carry a white distal margin, and the caudal fin is patterned with distinctive transverse bars — a key identification feature of this species within the guentheri group. A blue blotch is present on the pectoral fin. Females and juveniles are plain: brownish-grey with no significant patterning, the caudal fin rounded and unbarred.
Sexual dimorphism is pronounced. Males are always more colourful, with the extended unpaired fins and transverse caudal banding that make them among the more photogenic Nothobranchius available to hobbyists. Females are distinguishable at a glance by their uniform colouration and smaller size.
Habitat
Nothobranchius korthausae is known exclusively from Mafia Island, an island of roughly 270 mi² lying about 75 mi south of Dar es Salaam in the Indian Ocean. Its documented extent of occurrence is 75–124 mi² and area of occupancy 30–62 mi², spread across five recognised threat-based locations. These metrics — both qualifying thresholds for Endangered status — underline just how narrowly the species' survival depends on a small cluster of seasonal freshwater pools.
The pools it inhabits are ephemeral: they form during the rainy season (typically October–December and March–May on Mafia) on flat or gently undulating ground under open savanna-like vegetation and gallery forest edges. The substrate is characteristically dark grey to black vertisol mud — a swelling clay soil that cracks and hardens completely during the dry season, preserving diapausing eggs within it. Water chemistry recorded at collection sites spans pH 5.79–7.50 and conductivity 102–186 µS (TDS 51–93 ppm), typically turbid with suspended clay particles. Temperatures measured in situ range from approximately 84–92 °F in the field during the wet season, though FishBase records the thermal tolerance more broadly as 73–79 °F — both are likely valid depending on pool depth, time of day, and season. Syntopic association with Nothobranchius insularis has been documented at shared sites at about 5% relative abundance.
Feeding
Like virtually all Nothobranchius species, N. korthausae is a carnivore, feeding on small invertebrates encountered at or near the water surface and in the water column. In the ephemeral pools it inhabits, prey is seasonally abundant: the same rains that fill the pools hatch aquatic insect larvae, zooplankton, ostracods, and terrestrial invertebrates that fall or are washed in. The upturned mouth is well suited to taking prey at the surface film.
In the aquarium, the species accepts a wide range of live and frozen meaty foods: Daphnia, Artemia nauplii and adults, chironomid larvae (bloodworm), Tubifex, and small terrestrial insects such as fruit flies (Drosophila). Dried and pellet foods are generally accepted after acclimatisation but should not be the staple diet. Feeding to satiation once or twice daily maintains condition and promotes breeding. Care should be taken not to overfeed in small aquaria, where waste accumulates quickly; uneaten food should be removed promptly.
Mating
Nothobranchius korthausae follows the annual killifish reproductive plan with precision: males compete aggressively for breeding access, displaying to females through lateral body presentations that show off the iridescent flanks and barred caudal fin. When a female is receptive she adopts a slightly lowered posture and moves toward the male. The pair then swim in close parallel formation toward the substrate.
At the moment of spawning the male curves his body around the female in a characteristic T-embrace, and both fish drive briefly into the soft mud or peat, releasing eggs and sperm simultaneously into the substrate. Each embrace deposits a small number of eggs — typically a handful at a time — buried centimetres below the surface where they will be protected from desiccation within the clay matrix. Multiple spawning events occur over days or weeks while the pool persists. Syntopic presence of N. insularis in shared pools may intensify male display, as inter-specific competition for females has been documented in some Nothobranchius co-occurrences.
Breeding
The eggs of N. korthausae are deposited into soft dark mud and enter diapause as the pools dry out — arrested embryonic development that allows the embryo to survive the months-long dry season sealed in a desiccating clay substrate. The embryonic development period in the hobby is approximately 2–3 months in moist peat moss at room temperature, though the IUCN assessment notes a semi-annual life cycle, suggesting that under natural conditions incubation may be shorter than for some other Nothobranchius and that fish can complete two generations per year if conditions allow.
To breed the species in the aquarium, hobbyists maintain a layer of peat or coconut coir 2–3 in deep on the tank floor. After several weeks of conditioning the adults on live and frozen foods, the peat is carefully removed and sealed damp in a plastic bag or container labelled with the date. After 8–10 weeks the peat is re-wetted with soft, slightly acidic aged water: fry typically emerge within 24–48 hours of wetting if development is complete. Any eggs that fail to hatch may be re-dried and re-wetted a second time. Fry are small but able to take Artemia nauplii from the first day. Growth is rapid; males show colour within 4–6 weeks and the fish reach sexual maturity in about 8–10 weeks from hatching.
In the aquarium
Nothobranchius korthausae is a well-established species in the killifish hobby, valued both for the male's vivid colours and for the manageable logistics of its annual breeding cycle. A small, well-planted aquarium of 5.5–10 US gal per pair is adequate; a dark substrate (or a layer of peat over sand) to receive eggs is essential. The tank should be covered, as killifish are capable jumpers. Keeping multiple males together leads to fin-nipping and chasing; a species tank with one male to two or three females is the standard approach, or a pair per tank if space is limited.
Water quality should be soft and slightly acidic — pH 5.8–7.0 and hardness below about 6 dH, matching the natural profile. Temperature around 73–79 °F suits the species without the metabolic stress of excessive warmth, though short-term excursions to 82 °F are tolerated. The short natural lifespan (typically under a year in the wild) means colony turnover is essential; hobbyists cycle eggs continuously and maintain multiple cohorts. The species is not recommended for community aquaria because of its need for specific water conditions and its tendency to be out-competed by more aggressive fish.
Because N. korthausae is Endangered in the wild and not commercially wild-caught, all aquarium stock is captive-bred. Good record-keeping of collection locality or strain origin is valued by serious killifish keepers, as different island populations may carry distinct genetic material worth preserving. The Killifish Association networks (KFN, AKA, EKA affiliates) maintain studbooks and egg-passing programs for threatened Nothobranchius species.
Conservation
The IUCN Red List assessed Nothobranchius korthausae as Endangered under criterion B1ab(iii)+2ab(iii) in December 2018 (published 2019), reflecting its tiny geographic range — EOO 75–124 mi², AOO 30–62 mi² — and the continuing decline in the quality of its seasonal pool habitat. Assessors were B. Nagy and L. Watters; the assessment was reviewed by J. Snoeks. The status represents an upgrade from Vulnerable in the 2006 assessment, reflecting a sharpened understanding of how small and fragile the species' foothold on Mafia Island actually is.
The principal threats identified in the IUCN assessment are residential and commercial development and agricultural cultivation. As Mafia Island develops economically — largely driven by tourism, fishing infrastructure, and smallholder farming — the flat, seasonally flooded ground that supports the vertisol pools is progressively drained, converted, or degraded. The pools themselves are not obvious biodiversity hotspots and receive no formal protection; there are no protected areas covering N. korthausae's range. Population trend is listed as unknown, meaning that even the current rate of decline cannot be quantified from existing data.
Mafia Island is an official Marine Park and Ramsar-listed wetland area, but this designation covers the coastal and marine environment, not the inland seasonal pools where the killifish lives. Targeted freshwater surveys, habitat mapping, and engagement with local communities and land-use planners are the most pressing conservation needs. The captive hobby population, while not a substitute for wild conservation, provides a genetic reservoir and has generated most of the biological knowledge currently available for the species.