Taxonomy & naming
Nothobranchius willerti was described by Rudolf Wildekamp in 1992. The type locality is recorded as Kenya, Mnanzini, a residual pool of a small stream flowing to the lower Tana River, at approximately 01°58.9'S, 40°06.5'E, at an elevation of 52–92 ft. The species is placed in the subgenus Adiniops within Nothobranchius, a grouping of East African annual killies broadly characterised by their compact size and occupation of short-lived coastal or lowland savanna pools.
Eschmeyer's Catalog of Fishes (Fricke, Eschmeyer & Fong) records the valid name as Nothobranchius willerti Wildekamp, 1992. The genus name Nothobranchius combines the Greek nothos (false or spurious) with branchia (gill), an allusion to the reduced or modified gill apparatus that early workers observed in these fish. The specific epithet willerti honours Manfred Willert, a German aquarist who was among the co-discoverers of the species and who contributed substantially to the early exploration of East African killifish populations.
At its type locality N. willerti occurs syntopically with two congeners, N. jubbi and N. patrizii — a reminder that the seasonal pools of the eastern Kenya coast can support multiple annual killi species in the same water body, presumably partitioning microhabitat or breeding season.
Morphology
Nothobranchius willerti is a tiny fish. The holotype, the largest specimen measured at time of description, reaches only 1 in standard length (SL), placing it at the diminutive end of the genus. Like other members of Nothobranchius, males are conspicuously more colourful than females, displaying the bold, contrasting livery typical of annual killies that must find mates quickly within a compressed breeding window.
In overall body plan the species follows the stout, somewhat compressed Nothobranchius template: a robust body relative to length, a large, slightly upturned mouth adapted for surface and mid-water prey capture, and relatively large fins. Female colouration is cryptic — dull olive-brown or greyish — consistent with the typical pattern across the genus and providing camouflage in leaf-litter and detritus-rich substrates.
Habitat
The species is confined to temporary pools on the coastal plains of eastern Kenya, within the lower Tana River catchment. These pools form in low-lying depressions during seasonal rainfall, filling with warm, shallow water over soft substrates of mud, leaf litter, and accumulated organic detritus. The pools are ephemeral: when the rains end, water levels fall rapidly and the pools dry completely, leaving the substrate baked by sun and wind for the duration of the dry season.
Water chemistry measurements taken at the type locality recorded a temperature of 81 °F, a pH of 7.2, and a hardness of 5–6 degrees DGH — soft to moderately soft water, essentially neutral, enriched with dissolved organics from decaying vegetation. The surrounding landscape is coastal lowland savanna and bush; the pools are unshaded or lightly shaded by scrub vegetation. Syntopic Nothobranchius species (N. jubbi, N. patrizii) suggest the pools represent a characteristic East African coastal killi habitat supporting multiple annual species.
Feeding
Like other members of Nothobranchius, N. willerti is a micropredator. The genus as a whole feeds on small aquatic crustaceans, insect larvae, worms, and zooplankton — prey items abundant in the productive, warm, seasonally flooded pools they inhabit. The fish's compressed life schedule demands high energy throughput: individuals must grow rapidly from hatchling to breeding adult within a single rainy season, placing a premium on calorie-rich live prey.
In captivity, established protocol for the genus recommends live food as the dietary foundation. Artemia nauplii, Grindal worms (Enchytraeus buchholzi), planktonic crustaceans, and mosquito larvae are standard offerings. Frozen red mosquito larvae can serve as a supplement when live food is unavailable, though hobbyists report that the best growth rates and reproductive success come from sustained live-food feeding. The small adult size of N. willerti means appropriately scaled prey is essential, particularly for recently hatched fry.
Mating
Nothobranchius willerti is a classic annual killifish: its entire mating strategy is shaped by a narrow window between pool formation and pool desiccation. Once a pool fills, males display immediately and compete vigorously for access to females, using bright colouration and lateral displays. Pair bonds are brief and functional — spawning occurs quickly once a willing female is located.
Spawning takes place in the substrate: the male courts a female, the pair align side by side, and they dive together into the soft mud or leaf litter, depositing eggs at a depth of a few centimetres. Each dive produces a small number of individually buried eggs. The pair may spawn repeatedly across many sessions throughout the pool's active phase, spreading their egg production through the substrate across the season. Males in Nothobranchius typically display intense sexual dichromatism — vivid, species-specific colour patterns that function as species-recognition signals in multi-species pools like those inhabited by N. willerti.
Breeding
The fertilised eggs of N. willerti enter diapause — a state of arrested embryonic development — and remain buried in the substrate as the pool dries and the adult fish perish. The desiccation-resistant eggs are encased in a chorion that retards water loss, allowing them to survive months of dry-season heat in the cracked, dry mud. When seasonal rains return and the pool refills, the eggs hatch and the new generation begins the cycle again.
Hobbyist incubation data for this species indicates an embryonic development period of approximately 3–4 months in moist peat moss kept at room temperature — the standard annual killi technique in which eggs are removed from the spawning container while still buried in peat or coir, the medium is lightly dried and stored in a sealed container, and then re-wetted after the incubation period to trigger mass hatching. FishBase records a bottom-spawner incubation time of approximately 3 months. Because wild pools are noted to have a semi-annual cycle at the type locality, the species may experience two wet–dry cycles per year, potentially shortening the effective incubation window. Fish raised in captivity are short-lived: annual Nothobranchius rarely survive beyond 12–18 months even under optimal conditions, because longevity beyond a single wet season offers no fitness advantage in the wild.
In the aquarium
Nothobranchius willerti is not a common aquarium fish and FishBase notes it is difficult to maintain — a rating applied to many specialist Nothobranchius rather than an indication of any unusual fragility. The challenge lies in the specialist husbandry the annual cycle demands rather than in water-chemistry sensitivity. The fish does best in a species-only or single-pair setup in a small aquarium (10–15 US gal is ample for a pair or trio), with a substrate of soft peat or coconut coir to receive the spawning dives, dim lighting, and gentle filtration that does not disturb the bottom layer.
Water parameters should approximate the type locality: temperature around 79–82 °F, pH close to neutral, and softness maintained by using reverse-osmosis or rainwater adjusted to approximately 5–6 DGH. Live food is the cornerstone of good maintenance. Spawning medium should be harvested periodically, lightly dried, and stored for 3–4 months before re-wetting over a shallow container of warm, soft, slightly acidic water to stimulate hatching. Fry are small and require infusoria or paramecia as first foods before graduating to Artemia nauplii.
The species' appeal to killifish specialists lies in its rarity and restricted range, its placement in the historically important Tana River killi fauna, and its compact size and manageable care requirements compared with larger Nothobranchius species. Within the killifish hobby it is primarily maintained by members of specialist clubs such as the American Killifish Association (AKA) and its affiliated national organisations, which circulate eggs and peat among members through organised swapping programmes.
Conservation
The IUCN Red List assessed Nothobranchius willerti as Vulnerable under criterion D2 in 2019 (Nagy & Watters), confirming an earlier Vulnerable assessment by Hanssens in 2006. The D2 criterion reflects an extremely restricted area of occupancy (or number of locations) combined with a plausible threat that could drive the species to Critically Endangered or Extinct within a very short period. The type locality — a single cluster of temporary pools on the eastern Kenya coastal plain near Mnanzini — represents effectively the entire known range of the species.
The eastern Kenya coastal lowlands are under sustained agricultural, pastoral, and settlement pressure. Ephemeral pools of the kind inhabited by N. willerti are easily drained, filled, or converted; they attract few legal protections and are not visible to most conservation planning processes focused on permanent wetlands. The species' restricted range means that any infrastructure development, drainage project, or water abstraction affecting its small cluster of pools could eliminate the entire wild population in a single event. Climate variability that shifts the timing or intensity of seasonal rainfall poses an additional risk to a fish whose embryonic development is calibrated precisely to local dry-season length.
The syntopic presence of N. jubbi and N. patrizii at the same locality suggests the site has conservation value for multiple threatened taxa, but formal protected-area coverage of these coastal-plain pools appears absent. Maintenance of captive populations through the killifish hobby provides a limited buffer, but the species' obscurity means hobby stocks may not be broad enough to serve as a meaningful insurance population without deliberate effort by specialist organisations.