Taxonomy & naming
Nothobranchius milvertzi was described by Béla Nagy in 2014 in the journal Ichthyological Exploration of Freshwaters (volume 24, issue 4, pp. 347–360), on the basis of material collected from an ephemeral pool in the Lake Mweru drainage of Luapula Province, Zambia. Eschmeyer's Catalog of Fishes (Fricke, Eschmeyer & Fong) treats the name as valid. The species is placed within the N. brieni species-group on the basis of shared coloration and morphometric characters; Nagy's diagnosis distinguished it from related species by the male's unique combination of a red caudal fin with an orange semicircular submarginal band and a wide dark red-brown distal margin, and an anal fin with an orange submarginal band and a similarly dark distal margin.
The holotype (MRAC B2-027-P-0001) is a male measuring 1.5 in standard length, collected from a single locality: an ephemeral pool north of Lake Mweru, approximately 6 mi north-west of Chienge village (8°35.85′ S, 29°06.55′ E), at an elevation of 3045–3051 ft. Diagnostic morphometric ratios include a head length of 28.5–31.5% SL, interorbital width of 39–46% head length, and snout length of 18–20% head length. Chromosome number is 2n = 38, NF = 54.
No congeners were found syntopically with N. milvertzi at its type locality, an unusual situation for a genus in which two or three Nothobranchius often co-occur in the same pool system. The species has not been synonymised or transferred since its description.
Morphology
Nothobranchius milvertzi is a small annual killifish with a maximum recorded standard length of approximately 1.5 in (FishBase; holotype 1.5 in SL). The body form is typical of the genus: robust and laterally compressed, with a slightly upturned mouth, large eyes positioned high on the head, and the compact, rounded fins characteristic of Nothobranchius.
Colour pattern in males is the primary means of identification within the N. brieni group. The caudal fin is red, bearing an orange semicircular submarginal band and a wide dark red-brown distal margin. The anal fin carries an orange submarginal band and a dark red-brown distal margin. Body colouration follows the iridescent scales-with-dark-edging pattern common to Nothobranchius, though detailed body-scale colouration data for this species are sparse in published sources. Eggs are slightly oval, measuring 0.5 in in diameter, with short filaments.
Females, as in all Nothobranchius, are cryptically coloured — olive-brown with little or no patterning — allowing them to remain inconspicuous during the brief wet-season window when both sexes are active. Sexual dimorphism is pronounced: males are conspicuously coloured and slightly smaller-bodied on average; females are drabber and become noticeably rounded when gravid.
Habitat
The only confirmed locality for N. milvertzi is a cluster of seasonal ephemeral pools in the Lushiba Marsh, Luapula Province, north-western Zambia, within the Lake Mweru drainage of the southern Congo River basin. The type-locality pool lies at approximately 3048 ft elevation in savanna-woodland terrain, where the water table seasonally supports shallow, often dense-vegetated pools at the marsh periphery.
Field measurements at the type locality recorded water temperatures of 80–83 °F, pH 6.80–6.88, and conductivity of 56–360 µS/cm (TDS 28–180 ppm) — notably soft and near-neutral water, slightly turbid, with dense emergent and marginal vegetation. The IUCN notes the habitat as seasonal freshwater ephemeral pools; the pools fill during the Zambian rainy season (roughly November–April) and dry completely in the dry season, which is the driver of the annual life cycle.
The extent of occurrence estimated by the 2019 IUCN assessment is approximately 497 mi², with an area of occupancy of 5–62 mi². The species is known from a single location, and no additional populations have been discovered since the original description. The surrounding landscape is Miombo woodland and seasonal floodplain typical of the Bangweulu–Mweru freshwater ecoregion.
Feeding
Detailed dietary data specific to N. milvertzi have not been published. Nothobranchius in general are opportunistic micro-predators: in the wild they consume small invertebrates, zooplankton, aquatic insect larvae, and any other invertebrate prey small enough to ingest, exploiting the rapid invertebrate bloom that follows pool filling at the start of the rainy season.
In captivity, N. milvertzi and other members of the genus accept a variety of small live and frozen foods — Artemia nauplii, Daphnia, Cyclops, bloodworm (Chironomid larvae), and Tubifex — as well as small-particle dried foods. High-protein foods maintain the vivid male colouration and support the fast growth rate that these short-lived fish require to reach breeding condition within the first weeks after hatching. Overfeeding with fatty live foods should be avoided, as obesity can shorten the already brief natural lifespan.
Mating
Nothobranchius milvertzi is an annual killifish, and its reproductive behaviour is entirely adapted to the boom-and-bust ecology of temporary pools. Males compete intensely for access to females as soon as the pool fills and both sexes reach sexual maturity — often within three to four weeks of hatching. Male N. brieni-group fish display their vivid caudal and anal fin coloration in lateral displays toward rival males and in courtship chases directed at females.
Spawning occurs by substrate-diving: the male and female approach the substrate together, then plunge head-first into the soft mud or peat layer at the pool bottom, releasing eggs and sperm in a brief embrace buried in the sediment. Each dive typically deposits one to a few eggs. The pair resurface and may dive again multiple times over the course of a spawning session. Because wild adults survive only as long as the pool persists — a matter of months at most — males spawn frequently and with urgency throughout the wet season. In the aquarium, courtship and spawning can be observed year-round if conditions are maintained, though the fish's natural lifespan remains short regardless.
Breeding
The eggs of N. milvertzi are deposited into soft substrate and undergo diapause — a period of arrested embryonic development that allows survival through the dry season. The IUCN notes that eggs remain buried in clay and mud through the dry season, hatching when the pools refill with the following year's rains. Field and laboratory data (WildNothos) indicate that embryonic development in peat moss takes approximately 3–4 months at room temperature before the eggs are ready to hatch on wetting.
In the hobby, annual Nothobranchius are bred on a layer of peat moss or coconut coir placed on the aquarium floor. After spawning, the medium is removed, gently squeezed to remove excess water, sealed in a labelled plastic bag with the date, and stored in a warm, dark place for the appropriate diapause period — typically 3–4 months for N. milvertzi. Wetting the peat by flooding it with soft, slightly acidic water triggers hatching, which often occurs within hours. Fry are large enough at hatching to accept Artemia nauplii immediately. Growth is rapid; the fish can reach sexual maturity within three to four weeks and, in keeping with the annual life history, rarely survive beyond 8–12 months even under optimal captive conditions. Separating the sexes early or housing them in a well-planted aquarium reduces male-on-male aggression during the rapid maturation phase.
In the aquarium
Nothobranchius milvertzi is an attractive specialist fish suited to experienced killifish keepers rather than general community aquarists. A 10–15 US gal species aquarium is adequate for a pair or small group; multiple males can be maintained together in larger tanks if line-of-sight breaks are provided by floating plants, Java moss clumps, and leaf litter. The fish is not a jumper in the manner of some killis, but a close-fitting lid is still advisable.
Water conditions should reflect the soft, near-neutral chemistry of the type locality: temperature 79–82 °F, pH 6.5–7.0, and low to moderate conductivity (soft to moderately hard water). A small sponge filter provides gentle circulation without disturbing the substrate spawning medium. A 1–1.5 in layer of peat moss or coconut coir on the aquarium floor serves as both spawning medium and natural decor; the dark substrate also reduces stress and enhances the male's colours. Regular partial water changes maintain water quality in a small, densely spawning tank.
Because the natural lifespan is short, the keeper's main task is maintaining a rolling breeding programme: harvesting peat every two to four weeks, dating and storing batches, and wetting batches on a rolling schedule so that young fish are always maturing as older fish age out. N. milvertzi is not a widely distributed hobby species given its restricted range and Endangered status, but it is maintained by specialist killifish societies in Europe and elsewhere as a conservation-breeding responsibility fish. Keepers are encouraged to maintain pure-locality stock and to share surplus with other hobbyists through killiefish associations rather than discarding spent fish.
Conservation
The IUCN Red List assessed Nothobranchius milvertzi as Endangered under criterion B1ab(iii)+2ab(iii) in January 2019 (Nagy & Watters). The assessment estimated an extent of occurrence of approximately 497 mi² and an area of occupancy of 5–62 mi², and identified the species as known from a single location — the Lushiba Marsh pool system in Luapula Province, Zambia. Both EOO and AOO thresholds fall well within the EN range for the B-criterion, and continuing decline in the quality of habitat (subcriterion iii) is confirmed.
The primary threats documented by the IUCN are conversion of the surrounding land to rice cultivation and other small-scale agriculture, general land-use change at the margins of the marsh, and small-scale fishing in and around the pools using mosquito nets and traps — the latter removing adult killifish and disrupting the spawning substrate. The species has no formal protected area coverage at its type locality. A secondary concern is the biological fragility inherent to a single-location endemic annual fish: any single drought more severe than normal, a disease event, or the introduction of mosquitofish or other non-native species to the pool could extirpate the entire species.
The ornamental trade in N. milvertzi is noted by the IUCN as not commercially significant. Hobbyist conservation breeding by killifish societies represents the only functioning ex situ safety net for the species. Given the single known location and the range of active threats, any formal in-situ protection of the Lushiba Marsh pool complex — even at the level of a local community reserve — would significantly reduce extinction risk.