Taxonomy & naming
Nothobranchius nubaensis was described by Valdesalici, Bellemans, Kardashev and Golubtsov in 2009, with the holotype a male of 1.5 in standard length (MRAC 2008-05-P-1) collected on 5 December 2002 at Angarko, 13 mi south of Dilling and approximately 62 mi north of Kadugli in South Kordofan, Sudan (11°53'05.23"N, 29°42'31.82"E). The species name references the Nuba Mountains region in which the type locality lies.
Within the genus Nothobranchius, the species is placed in the subgenus Zononothobranchius and assigned to the ugandensis-group, an assemblage of morphologically similar East African annuals. Eschmeyer's Catalog of Fishes (Fricke, Eschmeyer & Fong) records Nothobranchius nubaensis Valdesalici, Bellemans, Kardashev & Golubtsov, 2009 as the valid name. The species was collected syntopically with the congener Nothobranchius virgatus, with which it shows approximately 40 % habitat overlap at known sites.
Morphology
Nothobranchius nubaensis is a small killifish reaching a recorded maximum of 2 in standard length (specimen ZMMGU P22009), with FishBase citing a maximum size of 2 in SL for the species. Meristic data from the type description give 17–19 dorsal rays, 17–19 anal rays, and 29–30 median lateral scales.
Males are vividly coloured. The head is orange-red and crossed by three oblique opercular bars. The dorsal and anal fins are light blue, densely marked with orange-red and yellow spots. Pelvic fins are yellow; pectoral fins orange-red with a blue margin; the caudal fin is orange-red. This polychromatic pattern is characteristic of the ugandensis-group and functions in mate recognition and rival assessment in the brief breeding season. Females, as in all Nothobranchius, are cryptically coloured — brownish or olivaceous — and noticeably smaller than fully grown males.
Habitat
The species is known from three drainage systems in the southern Sudan–western Ethiopia region: the Wadi Al Ghallah drainage in South Kordofan, the Khor Abu Habl in West Kordofan (White Nile basin), and the Sobat River system in the White Nile drainage of western Ethiopia. Localities span an elevation range of approximately 1450–2300 ft. The IUCN assigns it to Upper Nile Freshwater Ecoregion 522.
Habitat consists of freshwater inland wetlands in a seasonally arid savanna biome: seasonal and intermittent rivers, rainwater-filled depressions, and seasonal marshes and pools typically under 8 ha. The substrate is characteristically vertisol — swelling clays that crack when dry and retain water when wet, forming conditions suited to egg burial. Water chemistry at sampled sites was measured at approximately 73 °F, pH 8.73, conductivity 126 µS/cm, and TDS 63 ppm — moderately hard, slightly alkaline, and notably turbid. These pools form during the rainy season and evaporate completely during the dry season, with the fish surviving as buried eggs.
Feeding
Like other Nothobranchius, N. nubaensis is a micro-predator feeding primarily on small invertebrates — aquatic insects, crustaceans, and zooplankton — available in the ephemeral pools it inhabits during its brief active life. The species must feed, grow, and reproduce within the duration of a single rainy season, imposing a fast metabolic pace.
In the aquarium, the species accepts live and frozen small invertebrates readily: Daphnia, Artemia nauplii, chironomid larvae (bloodworm), small tubifex, and micro-worm are all suitable. High-quality micro-pellets are taken by acclimatised individuals. A varied diet of predominantly small live and frozen foods supports the best colouration and condition.
Mating
Nothobranchius nubaensis reproduces by the annual soil-spawning mode that defines the genus. Males establish and defend small territories within the pool, and courtship is direct and rapid — the compressed breeding season tolerates little delay. The male displays his full coloration to the female, aligning alongside her before initiating a spawning dive.
At the moment of spawning, the pair press close together and drive downward into the soft substrate — mud, peat, or detritus — where eggs and sperm are released simultaneously and the eggs are left buried. There is no parental care; adults do not guard or tend the eggs. In the wild, the male may spawn with multiple females over the course of the season; in captivity, pairs or trios are typically maintained.
Breeding
The buried eggs of N. nubaensis enter diapause as the pool dries — a state of arrested embryonic development that allows the eggs to survive the months-long dry season within the cracked vertisol clay. Wild Nothos research data for the species indicate an embryonic development period of approximately 6–8 months in moist peat moss at room temperature before hatching readiness is reached. Eggs hatch when the pool refills with the following year's rains, and the fry grow rapidly to sexual maturity within weeks, completing the generation in a single season.
In the aquarium, the annual cycle is replicated by maintaining spawning pairs over a peat or coconut-fibre substrate for four to six weeks, then removing and drying the medium at room temperature for three to six months before re-wetting it to trigger hatching. Fry hatch over several days; first foods are infusoria and freshly hatched Artemia nauplii, graduating to larger invertebrates as the juveniles grow. Because the parents are themselves short-lived — wild fish typically survive less than a year — maintaining a rested peat batch from each generation is the primary means of preserving the strain.
In the aquarium
Nothobranchius nubaensis is kept by specialist killifish hobbyists who maintain African annual species. A small, well-planted species tank of 10–15 US gal is sufficient for a pair or trio; males are combative toward one another and multiple males require adequate visual barriers or separate housing. A soft peat or coconut-fibre layer on the floor is essential for spawning. Dried leaf litter and floating plants provide cover and diffuse the lighting to encourage natural behaviour in males.
The water chemistry at the type locality — approximately pH 8.7, conductivity 126 µS/cm, 73 °F — is higher in pH than is conventional for many Nothobranchius, but the species is known to adapt across a range of moderately hard, clean water conditions. Temperatures in the range of 72–79 °F are appropriate for long-term maintenance; higher temperatures accelerate the fish's already brief lifespan. Partial water changes of 20–30 % weekly maintain quality. Because the species is Vulnerable with a fragmented wild distribution, hobbyist maintenance of captive strains linked to known collection localities contributes meaningfully to ex-situ conservation.
Conservation
The IUCN assessed Nothobranchius nubaensis as Vulnerable (criterion B2ab(iii)) in January 2019, with the assessment conducted by Nagy and Watters. The extent of occurrence is estimated at 44,400 mi², with an area of occupancy of only 24–2,0 mi² across 3–10 threat-based locations. The population trend is unknown and the distribution is fragmented.
The primary threat identified is drought, affecting an estimated 50–90 % of the species' range, compounded by water management and abstraction. Critically, conversion of seasonal wetlands into permanent reservoirs — infrastructure built for irrigation and water supply — eliminates the annual drying cycle that annual killies require: eggs that cannot undergo the full diapause period fail to develop normally, and permanently flooded habitats favour predatory introduced species such as mosquitofish over the native annuals. No formal conservation measures are currently in place specifically for the species, though some populations fall within the boundaries of Gambella National Park in Ethiopia. The combination of single-country range fragmentation, climate-driven drought pressure, and hydrology alteration makes this one of the more exposed Nothobranchius species in the White Nile basin.