Taxonomy & naming
Nothobranchius boklundi was described by Stefano Valdesalici in 2010, placing it within one of the most species-rich genera of annual killifish. The valid name as recognised by Eschmeyer's Catalog of Fishes is Nothobranchius boklundi Valdesalici, 2010 — the authority is not in parentheses because the species was described directly in Nothobranchius and has not been transferred since. The species belongs to the subgenus Zononothobranchius and falls within the N. brieni species-group, a cluster of small annual killies distributed across the drainage systems of south-central and eastern Africa.
The specific epithet honours Jørn Boklund, a Danish aquarist who participated in the field expedition that first collected the species. This is a long-standing tradition in killi taxonomy: collectors who fund or undertake difficult expedition work in remote seasonal habitats are often commemorated in the names of the novelties they bring back. Boklund's notho is consequently its primary English vernacular name.
Morphology
Nothobranchius boklundi is a small, laterally compressed fish; the maximum size recorded is 1.5 in standard length in males (FishBase gives a round figure of 1.5 in SL for males, 1.5 in SL for females). Like most members of its genus, it shows pronounced sexual dichromatism. Males occur in at least two colour phenotypes — a red form and a blue form — distinguished primarily by caudal-fin colouration; such intraspecific variation in male colour morph is documented in several Nothobranchius species and may reflect multiple founding colonisation events or ongoing polymorphism maintained by sexual selection.
Females are far more subdued in colouration, as is typical for the genus, but can be identified by pale red elongate spots on the dorsal and anal fin rays — a character useful for distinguishing this species from sympatric or allopatric congeners. The meristic formula includes 16–17 dorsal soft rays and 17–18 anal soft rays. The chromosome complement is 2n = 36, in line with karyological data recorded across the genus.
Habitat
The species is endemic to the Luangwa River drainage in eastern Zambia, an area south of the centre of Nothobranchius diversity in Tanzania. It inhabits temporary pools and seasonal swamps that form during the rainy season and disappear completely during the dry months. The type locality has been precisely recorded at approximately 13°06'31.1" S, 31°47'35.2" E, at an elevation of 1716–1732 ft, and is described as a pond of roughly 328 ft diameter and 5 ft depth with a light clay-grey turbidity and aquatic vegetation including water lilies (Nymphaea).
Water chemistry at the type locality, measured during collection, recorded a temperature of 72 °F and a pH of 7.20 — modest parameters that reflect the buffering effect of clay-bottomed soils and the relatively seasonal, non-blackwater character of savanna drainage pools. The pools support the soft, detritus-rich substrate of mud and organic matter into which spawning pairs dive and deposit their eggs. There is no surface connectivity to permanent water during the dry season; the pools are hydrologically isolated, reinforcing the population structure one would expect of a species adapted to living and dying within a single pool each year.
Feeding
Like all Nothobranchius, N. boklundi is a generalist predator of small invertebrates. In the wild the seasonal pools it occupies are typically rich in zooplankton, aquatic insect larvae, small crustaceans, and worms immediately following the onset of rains, providing an abundant food supply that annual killies exploit with remarkable efficiency given their compressed lifespan. Feeding begins almost at the moment of hatching from the substrate and continues at a high rate throughout the brief adult phase.
In the aquarium, the species readily accepts a varied diet of small live and frozen invertebrates — artemia (brine shrimp), daphnia, chironomid larvae (bloodworm), and tubifex — as well as high-quality small pellets or flake foods as a supplement. Live and frozen foods are strongly preferred and are essential for bringing fish into optimal spawning condition.
Mating
Nothobranchius boklundi is an annual killifish with a substrate-spawning mating system built around the certainty that its pools will dry out. Males compete vigorously for access to females, displaying their colour polymorphism — the red and blue caudal variants — in lateral displays that serve both to advertise condition and to suppress rival males. Courtship is direct and brief: once a male has oriented to a receptive female and she has responded to his display, the pair engage in a spawning dive.
The spawning dive is the defining act of annual killi mating. The pair align side by side, the male wraps his dorsal and anal fins around the female, and together they press down into the substrate — the soft mud, peat, or detritus at the pool bottom. Eggs and sperm are released simultaneously within the substrate, burying the fertilised eggs several centimetres beneath the surface where they will remain through the dry season. The pair separate, resurface, and the sequence can be repeated many times across a spawning session.
Breeding
Nothobranchius boklundi breeds by the annual soil-spawning mode shared by all members of its genus: fertilised eggs are buried in the substrate, enter diapause, and overwinter in arrested development through the dry season, hatching when the next rains refill the pool. In the wild the incubation period through diapause corresponds to the months-long dry interval; WildNothos reports an embryonic development period of approximately four to six months in moist peat moss kept at room temperature in the hobby.
Hobbyist breeders replicate this cycle by keeping spawning pairs on a bed of peat or coconut coir, collecting the peat after a few weeks, pressing it lightly to remove excess moisture, and storing it in labelled plastic bags at room temperature (typically 72–77 °F) for four to six months before re-wetting to trigger hatch. Larvae are tiny at emergence and require appropriately sized live foods — infusoria, vinegar eels, or micro-worms — before graduating to newly hatched artemia nauplii within days. Growth is rapid; N. boklundi, like other annual Nothobranchius, reaches sexual maturity in weeks rather than months, reflecting an evolutionary premium on reproducing before the pool dries.
In the aquarium
Nothobranchius boklundi is a manageable aquarium fish well-suited to the specialist annual killi keeper. Its small size (under 2 in) means it can be housed comfortably in tanks as compact as 10 US gal for a breeding pair or small group; a dark substrate of fine peat or coconut coir, dim lighting, and floating vegetation create conditions that promote natural behaviour and reduce male aggression directed at females. The water chemistry at the type locality (72 °F, pH 7.20) provides a practical target, though the species is likely tolerant of slightly cooler temperatures and a modest pH range around neutral.
The male colour polymorphism — red and blue caudal variants — makes this species visually interesting even relative to the already colourful Nothobranchius genus. Housing one male with two or three females is a practical ratio that spreads male attention and allows females to recover between spawning bouts. Because the wild lifespan is a matter of months, hobbyists must maintain a breeding programme to continue the line; eggs stored in damp peat are the standard means of preserving the species between generations. The Killifish Association and specialist killi clubs circulate eggs of Nothobranchius species widely, and N. boklundi is well represented in collections among Zambian Nothobranchius enthusiasts.
Conservation
The IUCN Red List classifies Nothobranchius boklundi as Vulnerable under criterion B1ab(iii), assessed on 20 October 2018 by Brian Watters and Béla Nagy. The B1 criterion flags a restricted extent of occurrence combined with an observed, inferred, or projected decline; the sub-criterion iii identifies ongoing decline in area, extent, or quality of habitat as the qualifying pressure. For a species endemic to a handful of seasonal pools within a single river drainage, the margin between Vulnerable and a more critical category is narrow.
The Luangwa Valley is subject to land-use change, and the seasonal pools that N. boklundi depends on are vulnerable to drainage, agricultural conversion, and altered hydrology — threats that affect many single-locality annual killifish across sub-Saharan Africa. Unlike permanent-water species, annual killies have no refuge in deeper, perennial habitat during dry years or periods of drought; the entire local population exists as buried eggs, and if a pool is drained or degraded before eggs have had the chance to hatch, recruit, and re-spawn, local extirpation follows quickly. The captive-breeding community provides a genuine conservation buffer for species such as this — distributed egg storage among hobbyists means that genetic material can survive even if a wild locality is lost — but ex-situ efforts are not a substitute for protecting the pools themselves.