Taxonomy & naming
Nothobranchius cyaneus was formally described in 2015 by van der Zee, Sonnenberg and Mipingo, making it among the most recently added members of one of Africa's most intensively studied killifish genera. The type locality is in Kenya. Eschmeyer's Catalog of Fishes records the valid name as Nothobranchius cyaneus van der Zee, Sonnenberg & Mipingo, 2015, within the family Nothobranchiidae — the African rivulines — a group of egg-laying Cyprinodontiformes entirely restricted to sub-Saharan Africa.
The genus Nothobranchius currently contains over 40 described species distributed across sub-tropical and tropical East and southeastern Africa, concentrated in the coastal lowlands and interior rift-valley drainages of Tanzania, Kenya, Mozambique, Zambia, Zimbabwe, and neighbouring countries. The group is the subject of intense taxonomic scrutiny: new species continue to be described at a rate of several per decade as field collectors work remote seasonal pools, and molecular phylogenies are progressively refining species boundaries and relationships within the genus.
The specific epithet cyaneus is Latin for 'dark blue' or 'cyan', almost certainly an allusion to the male's blue or blue-green body colouration, which is the hallmark of many Nothobranchius species described from Kenyan and coastal East African localities. The original description appeared in a specialist killifish context and was noted by the British Killifish Association, which published an article on the species by Dave Astbury in Killi-News around the same period.
Morphology
Nothobranchius cyaneus conforms to the characteristic Nothobranchius body plan: a relatively short, deep, laterally compressed body with a rounded profile, large eyes, and an upturned mouth suited to surface-feeding on small invertebrates. Males reach approximately 1.5–2.5 in total length, typical for the genus, which rarely exceeds 3 in. The body is robust relative to length, with the dorsal fin set well back and broad unpaired fins that males display during territorial and courtship interactions.
Sexual dichromatism is pronounced, as in virtually all Nothobranchius. Males are vividly coloured — blue or blue-green on the flanks with the patterned finnage of red, orange, or contrasting darker margins that characterises the species — the specific epithet directly referencing this blue colouration. Females are cryptically coloured in drab grey-brown, a common pattern across the genus correlated with the energetic cost of egg production and the advantage of camouflage while foraging in shallow open pools. Both sexes have the soft, rounded scales and relatively delicate integument typical of annual killifishes.
The male's striking coloration intensifies during the breeding season and varies somewhat between populations and under differing water chemistry; captive individuals may show reduced intensity compared to wild fish photographed at collection sites.
Habitat
Nothobranchius cyaneus occupies the seasonal floodplain pools and ephemeral wetlands of its Kenyan range — habitats that are as defining for the species as any morphological character. These pools arise on flat-lying savanna or bushland when seasonal rains collect in shallow depressions, ditches, and the margins of floodplains. The substrate is typically fine, sticky black mud or clay that retains moisture and organic matter from year to year, providing the medium in which eggs are deposited and survive the dry season.
Water conditions in such pools are variable but often lean toward warmer temperatures, as shallow equatorial water bodies heat rapidly in the African sun. The American Killifish Association notes that most Nothobranchius prefer alkaline, moderate-to-hard water, though the genus as a whole spans a range of water chemistries at the species level; an aquarium optimum of approximately 73–77 °F (around 74–75 °F) is cited for the genus. Individual pools may experience significant chemistry shifts as they concentrate through evaporation during the late dry season and dilute rapidly at the season's onset.
At sites hosting multiple Nothobranchius species, niche partitioning is documented: species may split the available habitat by pool position (central deeper areas versus vegetated edges), microhabitat preference, or thermal regime. The specific microhabitat preferences of N. cyaneus within the Kenyan pool systems it inhabits have not been extensively published in the English-language hobbyist literature.
Feeding
Nothobranchius cyaneus is a carnivore, feeding on the small aquatic invertebrates that bloom abundantly in seasonal pools at the onset of the rains. In the wild, the diet is dominated by microcrustaceans (copepods, ostracods, cladocerans), insect larvae — particularly mosquito and chironomid larvae — and any other small invertebrate prey within reach of the upturned mouth. This food base is richest immediately after pool filling, when decomposing plant material fuels rapid zooplankton growth and the fish, newly hatched, must grow to sexual maturity within a matter of weeks.
The fast pace of annual killifish biology is tightly coupled to feeding rate: unlike fish in stable environments, Nothobranchius have no option to grow slowly. Wild fish must reach spawning condition before the pool dries, which constrains them to efficient, high-energy feeding strategies from first feeding onward.
In the aquarium, N. cyaneus is readily maintained on live and frozen foods. The American Killifish Association recommends mosquito larvae, brine shrimp, white worms, and daphnia as primary foods for the genus, with frozen alternatives accepted once the fish are settled. A varied, protein-rich diet is important for maintaining condition and colouration in males and for supporting the high egg output of females.
Mating
The mating system of Nothobranchius cyaneus follows the well-documented Nothobranchius pattern: polygynous, with males establishing and defending territories over the deepest, most mud-rich areas of the pool — the preferred spawning substrate. Territory holders perform vivid lateral displays toward rival males and toward females that enter the area, the intensity of male colour functioning as an honest signal of quality in the compressed time available for mate assessment.
Spawning is initiated by the male positioning alongside the female and then rapidly wrapping around her body as she dips toward the substrate. The pair drive into the soft mud together, releasing a single egg per embrace at a depth sufficient to protect it from desiccation during the dry season. This spawning act is repeated many times throughout the brief adult lifespan; WildNothos describes daily spawning throughout the adult phase as the norm for the genus, reflecting the urgency of maximising reproductive output before the pool dries.
Females assess male quality through display interactions and may exercise mate choice even within the short adult lifespan. Males are persistently active in courtship and may harass females intensively, particularly in confined aquarium conditions where females cannot escape easily.
Breeding
The reproductive biology of Nothobranchius cyaneus is that of a true annual killifish: the eggs are buried in the pool substrate and must survive a complete dry season in diapause before hatching. Diapause in Nothobranchius is a genetically programmed developmental arrest that can occur at multiple embryonic stages; WildNothos reports incubation periods ranging from 6 to 48 weeks across the genus depending on species, population, and the duration of simulated dry season. This variability is an adaptation to unpredictable rainfall — staggered hatching reduces the risk of total population loss if an early rain is followed by re-drying.
In the hobby, annual killifish are bred on moist peat or coconut coir pressed into a shallow spawning container. After a productive spawning period of a few weeks, the peat is removed, lightly dried (not bone-dry), sealed in a plastic bag, and stored at room temperature for the species-appropriate incubation period. The eggs are then re-wetted by placing the peat in shallow, soft, slightly acidic water, and fry emerge within hours. Initial growth is rapid; sexual maturity is typically reached within 3–4 weeks, mirroring wild conditions.
Fry require infusoria or commercial micro-fry foods at first, advancing quickly to baby brine shrimp nauplii. Given the short adult lifespan — wild fish may live only a few months before the pool dries, and aquarium individuals rarely survive more than a year even under ideal conditions — dedicated breeders often maintain rolling incubating batches to keep the line alive between generations.
In the aquarium
Nothobranchius cyaneus is a rewarding species for specialist killifish keepers with experience managing annual fishes, though it is not recommended for the general community aquarium. The short lifespan and specific breeding requirements place it firmly in the specialist category. A small aquarium of 10–15 US gal is adequate for a pair or small group; the tank should be covered tightly, as killies are accomplished jumpers.
The American Killifish Association recommends slightly alkaline, moderately hard water for most Nothobranchius, with a temperature of approximately 73–77 °F. Décor can be simple — a dark substrate, floating plants for refuge, and a peat moss or coconut coir layer for spawning. The water should be changed regularly: the AKA notes that Nothobranchius are susceptible to Oodinium (velvet disease), a ciliate parasite, and clean water combined with modest hardness and non-iodized salt (approximately 1 gram per litre) significantly reduces outbreak risk.
Males must be monitored carefully: they are vigorous in pursuit of females and can injure or stress a female if she has nowhere to retreat. Keeping two females per male, or providing dense surface cover, helps distribute male attention. Given that adults naturally live only a few months, the keeper's focus quickly shifts to managing the next generation through the peat storage and hatching cycle — a rhythm that specialist killifish hobbyists find compelling and that connects directly to the biology of the wild fish.
Conservation
Nothobranchius cyaneus has not been evaluated by the IUCN Red List as of the date of this article (status: Not Evaluated, NE). This is not unusual for recently described Nothobranchius: the genus was the subject of a wave of assessments in the 2010s and 2020s, but new species described close to or after those assessment cycles have not yet been individually reviewed. Many congeners have received IUCN assessments — N. cooperi, for example, is listed as Vulnerable (2018) based on three locations in Zambia — and the conservation framework for the genus is well developed.
The threats facing N. cyaneus are those common to all African annual killifish occupying seasonal pools in East Africa: drainage and conversion of floodplain grasslands and seasonal wetlands to agriculture or settlement; alteration of hydrology through irrigation and water extraction that can cause pools to dry earlier or fail to refill; and the introduction of mosquitofish (Gambusia) and other exotic species that compete for food and may predate eggs and fry. Because many Nothobranchius species are single-drainage or even single-pool endemics, the loss of even one habitat can constitute local extinction of the entire wild population.
The specialist killifish hobby plays a meaningful informal conservation role for this group: captive populations maintained by collectors in killifish societies such as the AKA and BKA represent living insurance against wild population loss. For a species as recently described as N. cyaneus, whose range and population structure remain incompletely documented, formal IUCN evaluation and field surveys of its Kenyan localities are a conservation priority.