Taxonomy & naming
Nothobranchius brieni was described by the Belgian ichthyologist Max Poll in 1938 from material collected in the upper Lualaba River drainage of what was then the Belgian Congo. Eschmeyer's Catalog of Fishes (Fricke, Eschmeyer & Fong) records the valid combination as Nothobranchius brieni Poll, 1938. The genus Nothobranchius belongs to the family Nothobranchiidae, one of the two major families of African annual killifishes, and encompasses the bulk of the annual killies found across sub-Saharan Africa east of the Congo Basin.
Within Nothobranchius, brieni occupies the western fringe of the genus' distribution. Most species are found in East and southern Africa — Tanzania, Mozambique, Zambia, Kenya, and surrounding countries — while brieni anchors the genus' presence in the Congo Basin. Its closest relatives are other Congolese and Zambian Nothobranchius, though phylogenetic placement within the genus remains under active research as more species are formally described and sequenced.
Morphology
Brien's notho is a small fish, reaching a maximum of approximately 2 in standard length (2 in SL), which is modest even by killifish standards. The body form is typical of the genus: moderately deep, laterally compressed, with a rounded head, an upturned mouth suited to surface feeding, and a rounded caudal fin. Males are strongly dichromatic, carrying the vivid body colours characteristic of the genus — though specific colour descriptions for brieni are less thoroughly documented in the literature than for more commonly kept Nothobranchius.
As in other Nothobranchius, sexual dimorphism is pronounced. Males are considerably more colourful than females, which are typically drab grey-brown with faint mottling — a common pattern in annual killies, where female cryptsis reduces predation risk during the breeding season. The male's finnage is larger and more elaborate.
Habitat
Nothobranchius brieni is endemic to the upper Lualaba River drainage in south-eastern Democratic Republic of Congo, specifically in Haut-Lomami province near the town of Bukama and in the vicinity of Lake Upemba. Its habitat consists of temporary pools and swamps — seasonal wetlands fed by the rains that expand across the Miombo woodland and savanna landscape and then contract and disappear entirely during the dry season.
Water chemistry data from field collections gives a pH of approximately 6.54 and a temperature of around 78 °F — mildly acid, warm water typical of Central African seasonal pools. These habitats are often shallow, sun-warmed, and rich in decomposing organic matter from surrounding vegetation. The fish shares these pools with a community of invertebrates, amphibians, and other seasonally adapted species. The entire annual cycle — from hatching through growth, maturation, spawning, and death — takes place within the months that the pool holds water.
Feeding
In the wild, Brien's notho feeds opportunistically on the invertebrate fauna of its seasonal pools: small insects (particularly dipteran larvae), crustaceans, and other small aquatic invertebrates form the core of the diet. Like other Nothobranchius, it is a surface-oriented predator, and its upturned mouth is adapted for intercepting prey at or near the water surface.
In the aquarium, the species should be offered a varied diet of live and frozen invertebrates — daphnia, artemia, bloodworm, tubifex, and similar items — to meet its protein requirements and maintain the males' colouration. Dry foods may be accepted by some individuals but live and frozen foods are preferable for condition.
Mating
Brien's notho is an annual killifish and follows the typical Nothobranchius mating system. Males establish territories over patches of soft substrate within the seasonal pool and actively display to females, spreading fins and intensifying colour to compete with rival males. Courtship is brief and direct: a male manoeuvres alongside or beneath a receptive female, and the pair dive together into the substrate to spawn.
The mating season is constrained by the hydrology of the habitat. As the pool fills following the onset of rains and hatchlings grow rapidly into breeding adults, the window for reproduction is weeks rather than months. The selective pressure to maximise reproductive output before the pool dries favours continuous, high-frequency spawning across the available season rather than discrete spawning events.
Breeding
The reproductive biology of Nothobranchius brieni is that of a canonical African annual killifish: the pair dive together into the soft substrate — mud, peat, or detritus — and deposit a single fertilised egg per dive, burying it just below the surface. The eggs are drought-resistant and enter diapause, a period of arrested embryonic development that allows them to survive the complete desiccation of the pool through the dry season. When the rains return and the pool refills, the eggs hatch and the next generation grows rapidly to sexual maturity, completing the cycle.
In the hobby, the species is spawned on a peat moss or coconut coir substrate several centimetres deep. After several weeks of spawning, the medium is removed, lightly squeezed, and stored in a sealed bag or container at room temperature for a storage period of several months — simulating the dry season. At the end of the storage period the medium is re-wetted with soft, slightly acid water, triggering hatching. Fry are tiny and require appropriately sized first foods (infusoria, commercially prepared fry powder, or freshly hatched artemia nauplii). The species is rated as very difficult in the aquarium trade, requiring careful management of both the spawning medium and the hatching process.
In the aquarium
Brien's notho is a species for the experienced killifish specialist rather than the general aquarist. Its care requirements are manageable in technical terms — a small, well-filtered aquarium with soft, slightly acid water, a dark substrate, and live or frozen foods — but the annual life cycle and the management of diapausing eggs demand both knowledge and commitment. The species is rated as very difficult, and successful breeding requires careful attention to the spawning medium, the dry-storage period, and the re-wetting process.
Water parameters for the aquarium should approximate field conditions: temperature around 75–79 °F, pH in the mildly acid range (around 6.5), and soft water. Males can be aggressive toward each other and should not be kept in large numbers in a small tank. A species tank or a tank shared only with similarly sized, non-competitive companions is preferable. Because adult fish are short-lived — rarely surviving beyond a year even in ideal conditions — the hobbyist's focus necessarily shifts to maintaining the egg stock between generations.
The species appears in specialist killifish society collections but is not commonly available in the general trade. Access typically comes through killifish clubs and hobbyist networks, where eggs are exchanged or sold between members.
Conservation
The IUCN Red List assessed Nothobranchius brieni as Vulnerable (VU) in 2018. The grounds for this assessment lie in the species' extremely limited distribution: it is known only from a small number of localities in the upper Lualaba River drainage of Haut-Lomami province, DRC, near Bukama and Lake Upemba — an area of endemism that constitutes its entire global range.
The seasonal wetlands that the species depends upon face multiple threats. Agricultural expansion, wetland drainage for cultivation, and human settlement in the Lualaba floodplain all reduce available habitat. Climate variability can alter the timing and duration of the rainy season, potentially disrupting the synchrony between egg hatching and pool filling that the annual life cycle depends on. The Lake Upemba area has also experienced significant human pressure over the decades since Poll's original description, including armed conflict that has limited scientific access and monitoring.
In common with many Nothobranchius species of restricted distribution, brieni exists as a series of geographically isolated seasonal pool populations — each one an independent lineage dependent on the persistence of its particular wetland. The loss of even a small number of these pools could represent a disproportionate reduction in the species' genetic diversity and long-term viability. Continued monitoring of the Haut-Lomami wetlands and formal protected-area coverage of the Lake Upemba region represent the most important conservation levers for the species.