Taxonomy & naming
Nothobranchius cardinalis was described by Berkenkamp and Etzel in 1993. The type locality is given as a seasonal pool along the Mbezi River between Mkuranga and Kibiti, Pawani District, coastal Tanzania — a locality that situates the species squarely within the Mbwemkuru River drainage system. The valid name is registered in Eschmeyer's Catalog of Fishes under the authority Berkenkamp & Etzel, 1993, and no subsequent synonyms or revisions have displaced it.
The genus Nothobranchius belongs to the family Aplocheilidae within the order Cyprinodontiformes. Tanzania is the global centre of diversity for Nothobranchius: of the roughly 94 species recognised at the time of the most recent diversity surveys, 47 occur there, many of them range-restricted endemics confined to a single drainage or a cluster of adjacent seasonal pools. N. cardinalis is one of the narrower-ranged members of this coastal cluster and is closely related to N. rubripinnis, from which it is distinguished primarily by the combination of the red-flushed snout and the distinctive colouration of the pectoral and anal fins in males.
The specific epithet cardinalis is Latin for 'cardinal' — a reference to the vivid crimson tones of the male's unpaired and paired fins that recall the plumage of a cardinal bird.
Morphology
Nothobranchius cardinalis is a small fish; FishBase records a standard length of under 2 in for the species, consistent with the general size range of coastal Tanzanian Nothobranchius. The body form is the robust, slightly compressed, deep-chested shape typical of the genus — a build that reflects the metabolic demands of growing fast and breeding in a pool that may persist for only a few months before evaporating.
Male colouration is the species' most distinctive feature. The snout carries a red flush, and the pectoral and anal fins are vividly red — the latter trait distinguishing N. cardinalis from the superficially similar N. rubripinnis. The flanks show the iridescent scalation typical of Nothobranchius males, with blues, greens, and reds varying in intensity with condition and light. Females are cryptically coloured in olive-brown to buff, lacking the iridescent scaling of males; they are slightly smaller and noticeably deeper-bodied when gravid.
Sexual dimorphism is pronounced, as in virtually all Nothobranchius species, and makes sex assignment straightforward even in young adults.
Habitat
The species is endemic to the coastal floodplain of south-eastern Tanzania, restricted to temporary freshwater pools, seasonally flooded depressions, and residual pools of seasonal rivers within the Mbwemkuru River basin and adjacent areas. These habitats are emphatically ephemeral: they form when seasonal rains flood the flat coastal plain and are gone — often completely dry — before the next wet season arrives. The fish has no alternative refugia; when the pools go dry, the entire adult population perishes.
The water chemistry of these coastal-plain pools is typically soft and slightly acidic to neutral, warmed by the tropical sun to temperatures consistent with low-altitude coastal Tanzania. The substrate is fine mud, detritus, and organic material — the soft, oxygen-poor medium into which the fish spawns and in which the drought-resistant eggs overwinter. Aquatic vegetation may be sparse; the pools are nutrient-rich from the accumulation of terrestrial organic matter washed in with the rains, and the water is often turbid or tannin-stained.
This narrow habitat specificity — seasonal coastal floodplain pools in a single river basin — defines the conservation situation of the species: there is no refuge population elsewhere, and the pools themselves are disappearing.
Feeding
In common with other members of the genus, Nothobranchius cardinalis is a generalist micro-predator, foraging opportunistically on the invertebrates that colonise and breed in its seasonal pools during the wet season. The diet in the wild centres on aquatic insect larvae, small crustaceans, and zooplankton — the food resources that appear rapidly and abundantly when the pools fill and that allow the fish to grow and reproduce at the pace demanded by a habitat measured in weeks.
In captivity the species accepts live foods readily — Artemia nauplii, Daphnia, micro-worms, and small chironomid larvae — and most individuals adapt without difficulty to high-quality frozen equivalents. Prepared dry foods of appropriate size are accepted by many individuals. Feeding multiple small meals daily supports the fast growth rate and high reproductive output characteristic of annual killies.
Mating
Nothobranchius cardinalis is a substrate-spawning annual killifish. In the wild, mating is triggered by the arrival of seasonal rains that refill the coastal floodplain pools: males come into full colour rapidly once the pools fill and begin competing for access to females and to spawning territories on soft-substrate patches. Courtship is brief and intense — males display laterally to females and to rival males, and spawning follows quickly once a female is receptive.
The spawning act itself is the defining behaviour of the annual guild: the male drives the female into the soft substrate — peat, mud, or detritus — and the pair briefly dive into the sediment together, releasing eggs and sperm simultaneously before resurging to the surface. The fertilised eggs are left buried in the substrate, sealed from the surface world. In the aquarium this behaviour is reproduced using a container of dampened peat or coconut fibre placed on the aquarium floor; the pair will spawn into it repeatedly across the wet season.
Breeding
Once buried in the substrate, the eggs of Nothobranchius cardinalis enter diapause — a state of arrested embryonic development that allows them to survive the complete desiccation of the habitat during the long dry season. The eggs are resistant to drying and can remain viable in the dry soil for months, waiting for the cue — water — that signals the return of the wet season. When the pools refill, the eggs resume development and hatch within days or weeks of re-wetting, producing miniature, already-feeding fry.
In the hobby, the damp peat or coir spawning medium is removed from the tank after a month of spawning, squeezed lightly to remove excess water, sealed in a plastic bag or container with the eggs still inside, and stored in a warm, dark place at approximately 77–82 °F for a storage period that typically ranges from six weeks to four months depending on the population and incubation temperature. After storage, the medium is re-wetted with aged or RO water and the fry appear within 24–72 hours if development is complete. This dry-storage and re-wetting cycle replicates the ecological transition from dry to wet season. Fry are small but robust; infusoria or commercial micro-fry foods serve as first foods, followed rapidly by Artemia nauplii. Growth to sexual maturity is swift — N. cardinalis, like all annuals, evolved to breed fast.
Research on Nothobranchius diapause biology (including studies published via PMC) has confirmed that the eggs enter distinct developmental arrest phases during dry storage and that the timing of diapause release is sensitive to temperature, humidity, and the duration of dry storage.
In the aquarium
Nothobranchius cardinalis is a specialist fish primarily kept by killifish hobbyists rather than the general aquarium trade, but it is not a difficult species once its annual life history is understood. A tank of 10–15 US gal is sufficient for a pair or small group; the aquarium should be densely planted on the surface to diffuse light and reduce male aggression, with a spawning medium — a container of damp peat or coconut fibre — placed on the bottom for the fish to use. Dark substrate and subdued lighting show the males' colours to best advantage.
Water temperature of 72–79 °F suits the species well; higher temperatures accelerate the life cycle but may reduce lifespan. The fish is not demanding about water chemistry beyond a preference for soft, slightly acidic to neutral water consistent with its natural habitat. Filtration should be gentle — a small sponge filter is ideal, since powerful currents are foreign to fish from still, shallow seasonal pools.
The American Killifish Association maintains species profiles and care information for N. cardinalis and can connect new keepers with breeders holding established strains and dried eggs. Because the species is Vulnerable in the wild and its wild habitat is under threat, captive populations represent a genuine contribution to the species' long-term security, and hobbyist networks have maintained N. cardinalis continuously in culture for decades.
Conservation
Nothobranchius cardinalis is assessed as Vulnerable (VU) on the IUCN Red List, with the most recent assessment conducted by Nagy and Watters in October 2018. The assessment reflects the species' extreme range restriction — confined entirely to temporary pools on the coastal floodplain of the Mbwemkuru River basin in south-eastern Tanzania — and the inherent fragility of a population whose entire existence depends on seasonal pool habitats that are themselves threatened by land-use change.
The coastal floodplain of Tanzania is under development pressure: drainage of seasonal wetlands for agriculture and coastal infrastructure reduces or eliminates the pool habitats that N. cardinalis depends on. Because the species completes its entire life cycle within these ephemeral pools, the loss of even a small number of pool systems can represent a significant fraction of the global population. The seasonal pools also have no buffer: in a poor rainy season, pools that form too shallowly or dry too quickly before the fish can complete their spawning cycle may produce no viable eggs at all.
Tanzania hosts nearly half of all known Nothobranchius species, and many face analogous pressures — the seasonal pool fauna of the country's coastal and interior lowlands is among the most threatened freshwater assemblages in Africa. Conservation of N. cardinalis ultimately depends on the protection of its seasonal floodplain habitat; in the interim, the established captive population maintained by the killifish hobby provides an important insurance against local extinctions.