Taxonomy & naming
Nothobranchius hassoni was described by Stefano Valdesalici and Robert Wildekamp in 2004, with the type locality given as the Lufwa River basin, Katanga Province, Democratic Republic of Congo. The species is placed within the large African annual killifish genus Nothobranchius, family Nothobranchiidae, an assemblage of some 90-plus species distributed across sub-Saharan Africa from Senegal and Senegal east to the Horn and south into Mozambique, Zimbabwe, and the DR Congo. Eschmeyer's Catalog of Fishes (Fricke, Eschmeyer & Fong) treats Nothobranchius hassoni Valdesalici & Wildekamp, 2004 as the valid name; the authority is not parenthesised because the species has not been transferred from its original genus.
The specific epithet hassoni honours Michel Hasson, who provided material assistance to the describing authors in obtaining the type specimens — an eponym acknowledging fieldwork support rather than describing any morphological or ecological trait. Such dedications are common in the Nothobranchius literature, where new species are frequently discovered through collaboration between professional taxonomists and dedicated amateur collectors.
Within Nothobranchius, hassoni belongs to the cluster of central and southern African species associated with the Lufira and upper Congo drainage systems — a biogeographically distinct group from the East African Rift-associated species (such as the N. rachovi and N. furzeri groups) most familiar in the hobby. Its isolated population, restricted to a single river system, is characteristic of a genus whose species often have extremely limited ranges, each tied to a specific catchment or even a single valley.
Morphology
Nothobranchius hassoni is a small, compact fish. FishBase records a maximum standard length of 1.5 in for males and 1.5 in SL for females, placing it among the smaller members of its genus. The body form is typical of the genus: laterally compressed, moderately deep-bodied, with a rounded head, an upturned mouth positioned for surface feeding, and relatively large scales. The caudal fin is rounded to slightly lyrate in adult males.
Males are substantially more colourful than females, as is universal in Nothobranchius. The precise colour pattern of hassoni in life has not been extensively documented in the aquarium literature, but Nothobranchius males from the central African groups typically display iridescent blue, turquoise, or green flanks with red, orange, or yellow pigment in the fins and caudal fin edge — colours that intensify during courtship. Females are drab olive-brown, a camouflage suited to life in turbid, vegetated, ephemeral pools. The turbid water typical of the Lufwa floodplain habitat is noted by FishBase, suggesting that the vivid male colouration serves as a close-range courtship signal rather than long-distance display.
Habitat
This species is known exclusively from ephemeral pools in the floodplain of the Lufwa River system, itself part of the lower Lufira drainage in Katanga Province, Democratic Republic of Congo. The Lufira is a major tributary of the upper Lualaba (upper Congo), and the Katanga floodplains are characterised by seasonally inundated grasslands and dambos — shallow, seasonally waterlogged depressions — that fill during the October-to-April rainy season and dry completely during the austral dry season.
FishBase records the water as turbid — a common feature of savanna floodplain pools where grazing ungulates, rain-disturbed soils, and the rapid decomposition of grass and leaf litter create silty, tannin-stained conditions. Water temperature in Katangan floodplain pools through the rainy season typically falls in the range of 70–81 °F, consistent with the general Nothobranchius water parameter guidance for this region. The substrate is soft mud and detritus — exactly the medium into which breeding pairs drive eggs and in which those eggs undergo their dry-season diapause. There is no record of N. hassoni from permanent watercourses.
Feeding
Like all Nothobranchius species, hassoni is a micro-predator of invertebrates. In the ephemeral pools it inhabits, the principal prey items are small aquatic insects and their larvae (chironomid midges are a staple for many annual killies), copepods, small cladocera, and any other invertebrates that colonise the pools at the start of the wet season. The upturned mouth is adapted for taking prey at or near the water surface as well as in the water column, and the short gut typical of carnivorous killies reflects a diet dominated by animal matter rather than plant material.
In the aquarium, N. hassoni and similar Nothobranchius species accept a wide range of small live and frozen foods. Brine shrimp nauplii and adults, white worms (Enchytraeus), grindal worms, small bloodworm (frozen Chironomus larvae), and micro-pellets are all taken. Live foods elicit the most vigorous feeding response and keep adults in optimal condition. Because the fish is small — under 2 in — prey items must be appropriately sized; large bloodworm can be fed to adults but should be used sparingly to avoid fatty-liver issues.
Mating
Nothobranchius hassoni is an annual killifish whose mating system is built entirely around the ephemeral nature of its habitat. Males are strongly territorial and display intensely to one another and to females from the moment pools form at the beginning of the rains, because the window for completing the entire reproductive cycle before the pool evaporates is measured in weeks to a few months. Competition between males is intense and courtship is direct and often forceful.
During courtship a male approaches a female with fins fully spread, displaying his colours. If she is receptive, the pair position themselves side by side at the substrate surface. The male wraps his body around the female — the typical Nothobranchius spawning embrace — and the pair dive together into the soft mud or detritus bottom, burying one egg per embrace just below the surface. The pair return to the water column, recover briefly, and may repeat the dive within seconds. Individual spawning sessions can involve dozens of dives. Because wild males frequently harass females to the point of physical damage, and because aquarium conditions concentrate this harassment without the natural spread of open floodplains, the sex ratio and availability of shelter in captivity are critical husbandry considerations.
Breeding
The breeding biology of N. hassoni follows the annual killifish pattern precisely. Eggs are deposited individually in the substrate during each spawning embrace; in a productive pair kept in a peat-substrate aquarium, production of ten to thirty or more eggs per session is typical for the genus. The eggs are spherical, relatively large for the fish's body size, and encased in a tough, water-resistant chorion that allows them to withstand the complete desiccation of the substrate during the dry season.
Once laid, the eggs enter diapause — a state of developmental arrest that can persist across multiple diapause phases (Diapause I, II, and III, as described for other Nothobranchius species), during which the embryo's metabolism drops to near-zero and it survives months of drought buried in dry soil. In the hobby, the standard technique is to spawn the pair over a layer of peat or coir, then remove and gently dry the peat until barely moist, seal it in a labelled plastic bag, and store it at room temperature for a minimum of three months (or closer to the natural dry-season length of four to five months for maximum hatching synchrony). Re-wetting the peat triggers hatching; fry are often large enough to take Artemia nauplii as a first food almost immediately. Development is extremely rapid: fish reach sexual maturity in six to eight weeks under warm conditions, consistent with the selective pressure of a life cycle compressed into a single rainy season. Wild fish rarely live beyond one year; aquarium individuals may survive somewhat longer under optimal low-temperature conditions.
In the aquarium
Nothobranchius hassoni is primarily of interest to specialist killifish hobbyists rather than the general community-tank keeper. Its small size (under 2 in), short lifespan, and requirement for substrate spawning with a dry-storage incubation phase set it apart from most aquarium fish. The American Killifish Association (AKA) recognises it, and the species circulates within the killifish club network, though it is rarer in the hobby than celebrated Nothobranchius species such as N. rachovii or N. furzeri.
A breeding setup is straightforward: a small aquarium of 10–15 US gal with a layer of peat or fine coir substrate several centimetres deep, a single male and one or two females, a gentle sponge filter, and subdued lighting. The Fishlore genus-level guide recommends soft, slightly acidic water — pH 6.0–6.5, temperature 70–79 °F — which aligns with conditions in Katangan floodplain pools. Hard alkaline water suppresses colour and reduces lifespan. Live or frozen small foods fed twice daily maintain peak condition. Males will periodically chase females; providing ample plant cover (floating plants work well) and monitoring for injury is important. Because the lifespan is inherently short, maintaining a stock of stored eggs is the practical way to keep the species long-term: a dedicated hobbyist typically runs overlapping cohorts so that a new batch is hatching as the current adults approach senescence.
Conservation
The IUCN Red List assessed Nothobranchius hassoni as Vulnerable (VU) under criterion B1ab(iii) in December 2018 — meaning the species has a restricted geographic range (extent of occurrence below the VU threshold), that its range is fragmented or represented by few locations, and that the quality of its habitat is in continuing decline. The threats identified in the assessment are housing and urban development and agricultural conversion (both annual and perennial non-timber crops), reflecting the pressure on Katangan floodplains from expanding settlement and smallholder farming that drains or fills seasonal pool habitats.
The species does occur within at least one protected area, according to the IUCN assessment, which provides some buffer, but the assessors noted that improved habitat protection at existing sites and further field surveys are needed — a familiar refrain for range-restricted African annual killies, many of which were last collected at their type localities decades ago and have uncertain current population sizes. The ephemeral pool habitat of N. hassoni is among the most vulnerable freshwater habitat types in tropical Africa: a pool that exists for only a few months per year leaves no permanent water body to conserve, and the loss of a single damba or floodplain meadow to maize cultivation or housing can eliminate an entire isolated population. Given the species' confirmed restriction to the Lufwa River system floodplain in Katanga — a single catchment in a country with extensive ongoing land-use change — its Vulnerable status reflects genuine risk rather than precautionary over-classification.