Taxonomy & naming
Nothobranchius malaissei was described by the Dutch ichthyologist Rudolf Hans Wildekamp in 1978, based on material collected from the Luanza River floodplain in Katanga Province, DR Congo. The holotype — MRAC 73-24-P-952, a 1.5 in TL (1 in SL) male — was collected in 1969 and resides in the Musée Royal de l'Afrique Centrale in Tervuren. The species is placed in the subgenus Zononothobranchius and assigned to the N. brieni species group within the large African annual killifish genus Nothobranchius (family Nothobranchiidae). Eschmeyer's Catalog of Fishes records the valid name as Nothobranchius malaissei Wildekamp, 1978.
The karyotype of N. malaissei is notable: 2n=48 with a fundamental number of 62 (NF=62), a combination documented by Krysanov and Demidova (2018) as unique within the genus. No syntopic congeners have been recorded at the type locality, suggesting the species occupies its fragment of the Luapula plain without competition from close relatives at the same site.
Morphology
Adult males reach approximately 1.5 in SL (around 2 in TL) under aquarium conditions; wild fish may be somewhat smaller given the compressed growing season. The body is moderately elongate and laterally compressed in the typical Nothobranchius manner. Male ground colouration is a clear light blue that intensifies on the flanks, overlaid with a distinctive red-brown reticulation and a series of irregular crossbars. The dorsal and caudal fins are spotted; the caudal fin of males carries a posterior border that reflects the species' subgenus affiliation.
Females, as in all Nothobranchius, are far more cryptic — olive-brown to tan with no bold patterning — and are noticeably smaller than fully coloured males. This extreme sexual dimorphism is characteristic of the genus and reflects the intense male-male competition driven by the extremely short reproductive window that annual life cycles impose.
Habitat
Nothobranchius malaissei is confined to the lower Luapula plain in Katanga Province, south-eastern DR Congo, in the area downstream of Mambilima Falls. The type locality lies approximately 1 mi east of Kabiasha (10°16' S, 28°08' E) at an elevation of 1,019–1,128 ft. Water chemistry measurements taken at or near the type locality record a temperature of 75 °F, a pH of 6.20, TDS of 35 ppm, and a conductivity of 70 µS — soft, slightly acidic water typical of the well-leached floodplain soils of the Central African plateau.
The species occupies temporary pools, seasonal overflow pans, and shallow river-margin wetlands that fill with the rains and contract completely during the dry season. The substrate of these pools is soft — fine silt, mud, and decaying organic matter — and it is into this substrate that the eggs are deposited and in which they persist through the dry period. Vegetation cover is typically sparse to moderate savanna grasses and sedges at pool margins.
Feeding
Like all Nothobranchius, N. malaissei is a micro-predator in the wild, feeding primarily on small invertebrates available at the pool surface and in the water column — aquatic insect larvae, microcrustaceans, small worms, and similar prey that the species locates by sight. The relatively wide gape and upturned mouth characteristic of the genus are adaptations to surface and mid-water feeding on invertebrates.
In the aquarium, the species accepts a wide range of small live and frozen foods readily: Daphnia, Artemia (brine shrimp), bloodworm (chironomid larvae), Tubifex, and micro-worms. Dry foods are taken by many individuals but should be supplemented with live or frozen material to maintain condition and bring out the best colouration in males. Given the species' short wild lifespan, regular high-quality feeding has a more direct bearing on reproductive success than in longer-lived fish.
Mating
Nothobranchius malaissei is an annual, substrate-spawning killifish in which the pair dive together into the soft mud or peat at the pool bottom to deposit eggs. Mating in the wild is compressed into the weeks or months during which the pools hold water, imposing strong selective pressure for rapid mate acquisition. Males compete intensely for access to females through lateral displays that show off the reticulated blue flanks and spotted fins; direct male–male combat occurs when space is limited.
A mating pair aligns side by side and drives or curves downward into the substrate, depositing one or a small number of eggs per dive with simultaneous fertilisation. Each egg is individually buried a few millimetres into the sediment. This spawning behaviour is repeated many times throughout the season with the same or different partners. No pair bond is maintained between spawning drives; males resume competitive display immediately after each descent.
Breeding
The defining characteristic of N. malaissei is embryonic diapause: eggs deposited in the substrate enter a state of arrested development that enables them to survive the complete desiccation of the habitat through the dry season. When the pools refill with the return of the rains, the change in moisture, osmotic conditions, and temperature triggers resumption of development and hatching. Wild incubation periods vary with environmental conditions; hobbyist experience with closely related Nothobranchius species places typical diapause-and-hatch windows at three to five months, and WildNothos records a peat-moss incubation of 3–5 months at room temperature as appropriate for N. malaissei.
In the aquarium, the species is maintained on a simulated annual cycle. Breeders are kept in a small tank with a peat or coconut-fibre substrate layer into which the pair dives to spawn. After several weeks of spawning, the peat is removed, lightly squeezed, and stored in a sealed plastic bag at room temperature for three to five months — mimicking the dry season. The bag is then rewetted (re-watered with aged soft water at appropriate temperature) to trigger hatching. Fry emerge as fully formed miniatures and can accept newly hatched Artemia nauplii immediately; growth is rapid and fish may be ready to spawn themselves within two to three months. Breeding temperature of 79–82 °F is optimal for Nothobranchius (Tozzini et al. 2020); incubation temperature during diapause also significantly influences developmental timing.
In the aquarium
Nothobranchius malaissei is a specialist fish suited to killifish hobbyists rather than general community aquaria. A pair or a small group of one male and two or three females can be maintained in a modest aquarium of 10–15 US gal; larger is not necessary given the short lifespan and small size. A dark substrate of peat or coconut coir — both suitable spawning media — should be provided to a depth of several centimetres. Dim lighting and cover from floating plants or surface vegetation reduce male aggression and show off the blue flanks to best effect.
Water should be soft (conductivity 50–100 µS, TDS under 60 ppm) and slightly acidic to neutral, matching the type-locality parameters of pH 6.2 and 75 °F. Room temperature suffices in many climates; a small heater can maintain the 75–82 °F range. Filtration must be gentle — a small sponge filter is ideal — since the shallow, still conditions of the natural habitat mean N. malaissei is not adapted to current. Males should be watched for aggression; more than one male per tank risks persistent fighting and fin damage.
Because this species is Vulnerable with a restricted wild range, captive maintenance carries genuine conservation value. The AKA (American Killifish Association) and European killifish societies (KFN, BKA) maintain studbooks and strain-exchange programs for threatened Nothobranchius; acquiring fish through these channels supports responsible stewardship and avoids pressure on already fragile wild populations.
Conservation
The IUCN assessed Nothobranchius malaissei as Vulnerable (VU) in 2019 under criterion B1ab(iii) — a designation reflecting a restricted extent of occurrence (EOO) and ongoing decline in the quality and extent of its habitat. The assessors (Nagy & Watters 2019) upgraded the species from a previous Data Deficient listing (2009), reflecting improved knowledge of its range and the pressures bearing on it. The entire known distribution is confined to the lower Luapula plain in Katanga Province, an area subject to human encroachment, agricultural conversion of seasonal wetlands, and drainage modification associated with settlement and small-scale farming.
The temporary pools and seasonal floodplain wetlands on which the species depends are intrinsically vulnerable: they are shallow, seasonally predictable, and easily drained or converted. Any change in the hydrological regime — whether from altered rainfall patterns, diversion of seasonal inflow, or direct drainage for agriculture — can eliminate breeding sites permanently. The introduction of mosquitofish (Gambusia spp.) or other non-native fish species into such pools, a problem documented elsewhere in the genus's range, would pose a further acute threat given the species' complete dependence on a single isolated watershed fragment.
Captive populations held by specialist killifish hobbyists represent a practical insurance population for this and many other threatened annual killies. Coordination through national killifish associations helps maintain genetic diversity in captive strains and ensures that living material remains available if a targeted reintroduction or supplementation program were ever warranted.