Taxonomy & naming
Nothobranchius lourensi was formally described by Rudolf Wildekamp in 1977 on the basis of material collected the previous year by Jan Lourens, a UNDP biologist then working in Tanzania. The holotype — a 1.5 in SL male registered as MRAC 77-16-P-28 under field code T 76-8 — was taken in July 1976 from a pool along the Morogoro–Dar es Salaam road, between Ruvu bridge and Kwaraza village, at approximately 06°41.8'S 38°42'E on the eastern Ruvu river floodplain. Eschmeyer's Catalog of Fishes (Fricke, Eschmeyer & Fong) treats Nothobranchius lourensi Wildekamp, 1977 as the valid name.
The species is placed in the subgenus Aphyobranchius and forms the nominal member of the N. lourensi species group within the genus. The genus Nothobranchius belongs to the family Nothobranchiidae (African rivulines), a lineage of annual and semi-annual killifishes distributed across sub-Saharan Africa. Karyotype studies have revealed a chromosomally polymorphic situation: the diploid number is reported as 2n = 32 in some populations and shows karyotypic variation between males and females (males 2n = 27, females 2n = 28) in others, a level of intraspecific chromosomal diversity unusual even within Nothobranchius.
Morphology
Nothobranchius lourensi is a compact killifish reaching a maximum of 2 in total length (FishBase). Sexual dimorphism is pronounced, as is typical of the genus. Males display blue-green scales with dark brown posterior margins that produce a reticulated or net-like pattern over the flanks; the posterior trunk carries a chevron pattern, and both the anal and caudal fins are striped. The caudal fin is additionally adorned with a light blue-yellow subdistal band and a black distal margin, giving it a layered, colourful terminal edge. Females are comparatively plain: grey-brown overall with a dark chevron pattern restricted to the posterior half of the body, their colouration serving as camouflage in the turbid, detritus-rich pools the species inhabits.
Like all members of the genus, the body form is moderately deep and laterally compressed, the head blunt, and the dorsal fin positioned well toward the rear of the body over the anal fin — a placement shared across the Nothobranchiidae and reflecting the bottom-directed spawning posture used during egg deposition.
Habitat
The species is endemic to seasonal freshwater habitats in coastal Tanzania, with confirmed populations from the Ruvu, Kilombero, Mbezi, and lower Rufiji river drainages at elevations between 20 and 994 ft. These are shallow, temporary floodplain pools and roadside ditches that fill with the rains and dry completely during the dry season — the defining habitat of annual Nothobranchius species worldwide.
Water parameters across the range span 75–88 °F and pH 6.5–8.0, with conductivity typically 20–160 µS, and the water is frequently turbid with suspended fine sediment and organic matter. The type description records 81 °F and pH 6.5–7.0 at the type locality. Populations near the coast exhibit a semi-annual lifecycle in some years when partial pool refilling allows a second cohort, while inland Kilombero populations follow a strict annual cycle tied to a single wet season. The species is syntopic with as many as eight Nothobranchius congeners in suitable habitat, including N. geminus, N. kilomberoensis, N. janpapi, N. melanospilus, N. fuscotaeniatus, N. luekei, N. rubripinnis, and N. ruudwildekampi.
Feeding
Like other small Nothobranchius species, N. lourensi is an opportunistic micro-predator in the wild, taking the invertebrate fauna available in its temporary pools: aquatic insect larvae, small crustaceans, worms, and zooplankton. The richness of this invertebrate community fluctuates dramatically with pool age and water level, and the fish adjusts accordingly.
In the aquarium the species accepts a wide variety of small live and frozen foods — Artemia nauplii and adults, Daphnia, Cyclops, bloodworm, and tubifex — as well as high-quality micro-pellets. Variety in the diet supports colouration and conditioning for breeding. Because adults are short-lived and invest heavily in reproduction, regular feeding with nutritious, protein-rich items is important to maintain condition across the brief adult life span.
Mating
Nothobranchius lourensi is an annual, substrate-spawning killifish whose mating system is shaped entirely by the impermanence of its pools. With a wild life span measured in weeks to a few months, males compete intensely for access to females from the moment they reach sexual maturity. Male–male interactions involve conspicuous lateral displays showing off the reticulated flanks and banded fins; prolonged or escalating encounters can result in fin damage in confined aquarium settings.
Spawning is initiated by the male driving the female toward the pool substrate — soft mud, peat, and organic detritus. The pair briefly dive together into the bottom material in a characteristic embrace, depositing eggs a few at a time directly into the substrate. This dive-spawning behaviour, repeated across many sessions, distributes the eggs through the top few centimetres of sediment where they will survive the dry season.
Breeding
The eggs of N. lourensi are deposited into soft substrate and undergo embryonic development in diapause — a state of arrested development that allows them to survive complete desiccation of the pool through the dry season. Incubation in captivity takes approximately three to four months at room temperature on moist peat moss, after which the peat is re-wetted to trigger hatching, mimicking the return of the rains. This storage-and-rewet method is the standard hobbyist technique for all annual Nothobranchius and is essential for N. lourensi: the eggs do not hatch on the same schedule as non-annual plant-spawning killies and must not be kept permanently submerged.
Fry hatch at a relatively large size and grow quickly on Artemia nauplii and micro-worms. The chromosome polymorphism documented in this species does not appear to impair captive reproduction. Adults are short-lived — wild fish typically complete their life cycle within a single wet season — so breeders maintain multiple age cohorts to ensure continuation of the strain. The species is described as difficult to maintain in the aquarium relative to more forgiving Nothobranchius, requiring careful management of water quality and the peat incubation period.
In the aquarium
Nothobranchius lourensi is considered a species for experienced killifish keepers rather than beginners, and FishBase notes it as difficult to maintain in the aquarium. The species demands clean, warm, slightly acidic to neutral water in the range of 75–82 °F and pH 6.5–7.5, with low conductivity reflecting its naturally soft pool habitat. A small, well-filtered species tank of 10–15 US gal suits a pair or a small group of two females to one male; intense male competition in a confined space can stress fish rapidly.
A dark substrate of fine sand or coconut fibre provides both camouflage and a spawning medium. Peat or coconut coir should be provided as a spawning mop equivalent: a layer of moist peat several centimetres deep into which the pair can dive. Spawning medium is removed every two to four weeks and stored slightly moist in sealed bags or containers at room temperature for the diapause period, then re-wetted to hatch a new cohort. Dense planting or floating cover reduces aggression and stress. Because adults age rapidly and die, the keeper's priority is a continuous supply of eggs in incubation; experienced keepers maintain two or three peat batches at staggered ages to ensure a rolling supply of fry.
The American Killifish Association lists N. lourensi among species held by specialist members, though it is not among the most widely kept Nothobranchius. Its relative scarcity in the hobby partly reflects the difficulty of maintaining it and partly the restricted distribution of wild-caught founding stock.
Conservation
The IUCN Red List assessed Nothobranchius lourensi as Near Threatened (NT) in December 2018, applying criteria B2ab(iii) — meaning the extent of occupancy is estimated below 2,0 mi², the range is severely fragmented or exists as fewer than ten locations, and there is a continuing decline in the area, extent, or quality of habitat. This represents an improvement over the previous Vulnerable D2 assessment from 2006, attributed primarily to refined understanding of the species' actual range extent, but the underlying threats have not diminished.
The primary threat is habitat loss: the seasonal floodplain pools and roadside ditches of coastal Tanzania that N. lourensi depends on are vulnerable to wetland drainage for agriculture, water abstraction for irrigation and domestic use, and road improvement projects that alter hydrology along the Morogoro–Dar es Salaam corridor where the type locality sits. The species' endemic range across the Ruvu, Kilombero, Mbezi, and lower Rufiji drainages is not large, and any reduction in the extent or duration of seasonal flooding directly reduces the number of pools available for a complete annual breeding cycle. Conservation of N. lourensi ultimately depends on the integrity of coastal Tanzanian seasonal wetlands, and specialist aquarium breeding populations maintained by killifish societies represent a potential insurance against further wild population decline.