Taxonomy & naming
Nothobranchius hengstleri was described by Stefano Valdesalici in 2007 in Zootaxa (1587: 61–68). The holotype is a male measuring 1.5 in standard length (ZSM 34483, Zoologische Staatssammlung München), collected from temporary pools approximately 3 mi north of Nassoro village, Cabo Delgado province, northeastern Mozambique (10°53.222'S, 40°22.094'E). Eschmeyer's Catalog of Fishes records the valid combination as Nothobranchius hengstleri Valdesalici, 2007.
The species is placed within the subgenus Adiniops and belongs to the N. melanospilus species group — a clade of East African annual killifishes sharing broadly similar coloration patterns but distinguishable by meristics, body depth, and fin markings. N. hengstleri is distinguished from its two nearest relatives, N. melanospilus and N. makondorum, principally by its deeper red body colouration, the distinctive spotted anal fin, and the caudal-fin pattern (deep red with a well-defined black margin). It also bears a unique karyotype within the genus: 2n = 38 chromosomes arranged as 7 ft + 2sm + 34st/a — a chromosome structure not recorded in any other Nothobranchius at the time of description.
No syntopic congeners are known from the type locality. The species was described from a single collection made by Holger Hengstler, the German amateur ichthyologist and Nothobranchius specialist for whom it is named.
Morphology
Males reach 1.5 in standard length (holotype), making this a small to mid-sized Nothobranchius. The body is moderately deep and laterally compressed in the typical killifish profile. The most immediately striking character is the colouration: males carry a deep, saturated red across the flanks and dorsum, an intensity that exceeds what is seen in N. melanospilus and N. makondorum. The caudal fin is deep red with a sharply defined black distal margin, giving a bicolour or banded appearance. The anal fin is distinctively spotted rather than broadly patterned, a feature that Valdesalici identified as a reliable diagnostic.
Females, as in all Nothobranchius, are substantially plainer — brownish or olive-grey, lacking the vivid male pigmentation. The sexual dimorphism in this genus is among the most pronounced in freshwater fish; distinguishing the sexes is never difficult in adults. The unique karyotype (2n = 38, with an unusually high proportion of subtelocentric and acrocentric chromosomes) is not visible externally but distinguishes N. hengstleri at the cytogenetic level from congeners in the same species group.
Habitat
Nothobranchius hengstleri occupies seasonal freshwater habitats in the coastal lowlands of Cabo Delgado province, northeastern Mozambique — specifically ephemeral pools on and near the Afungi peninsula that are not connected to any permanent river system. The type locality sits at low elevation (FishBase records the species from between 6 and 236 ft above sea level), in pools that fill during the seasonal rains and dry out completely during the annual dry season.
Water chemistry recorded at the type locality is near-neutral (pH 7.5 — FishBase), somewhat atypical for the genus since many Nothobranchius inhabit more acidic or even alkaline seasonal pools depending on the underlying geology; the coastal Mozambique lowlands produce moderately buffered, relatively soft freshwater. The surrounding vegetation is coastal East African savanna and bushland. The pools form on flat or gently undulating terrain and may cover varying areas depending on seasonal rainfall, but share the defining characteristic of all Nothobranchius habitat: they are entirely temporary, drying to bare substrate or dry mud during the dry season every year.
The species is placed within the Coastal East Africa freshwater ecoregion (FEOW ecoregion 564), a biogeographically distinct zone of short, isolated coastal drainages between the Ruvuma and Save river systems.
Feeding
No published feeding studies specific to Nothobranchius hengstleri are available. Based on the biology of Nothobranchius broadly, wild individuals forage primarily on aquatic invertebrates — zooplankton, small crustaceans, aquatic insect larvae, and worms — that proliferate rapidly in the newly flooded pools at the start of the wet season. The short-season life cycle places intense pressure on rapid growth, so an energy-rich, protein-heavy diet of invertebrate prey is the expected mode.
In the aquarium, N. hengstleri accepts live and frozen invertebrate foods readily: bloodworm, Daphnia, Artemia, Tubifex, and similar items. High-quality small pellets and granules may be accepted by settled fish. Varied, protein-rich feeding accelerates development and promotes vivid male colouration, which is important for breeders attempting to assess the quality of a strain.
Mating
Nothobranchius hengstleri is a substrate-spawning annual killifish. Males establish loose territories around soft-substrate patches in the seasonal pool and compete vigorously for access to females through lateral displays, fin extension, and colour intensification. The deep red body and patterned caudal fin of the male serve as the principal visual signals in courtship; female choice in Nothobranchius is thought to be influenced by the intensity and contrast of male colouration.
No syntopic congeners are present at the known localities, so the elaborate species-specific colour patterns that in sympatric Nothobranchius assemblages serve as reproductive isolating mechanisms may here operate primarily as intraspecific quality signals. The courtship itself follows the typical Nothobranchius pattern: the male drives the female toward a soft substrate patch and positions himself alongside her before the pair execute a spawning dive.
Breeding
Breeding in Nothobranchius hengstleri is annual and obligately tied to the seasonal drying of its pools. During courtship the pair dive together into soft substrate — mud, peat, or decomposing organic matter — and a single egg is deposited and fertilised at depth per dive. Spawning continues across many events throughout the wet season, with eggs accumulating in the substrate. When the pool dries out, the adults die; the buried eggs enter diapause, a state of arrested embryonic development that allows them to survive desiccation for the full length of the dry season.
Hatching is triggered when rains return and the substrate is re-wetted, with embryos capable of responding to physical contact with water after an appropriate diapause period. FishBase records embryonic development times of 3–4 months in peat moss at room temperature for this species. In the hobby, the standard management technique mirrors this natural cycle: the spawn medium (peat moss or coconut coir) is removed from the tank, gently dried to slightly moist condition, sealed in a labelled plastic bag, and stored at room temperature for 2–4 months before being re-wetted over a shallow layer of soft water to trigger hatching. Fry are large relative to many killifish and typically accept newly hatched Artemia nauplii as a first food.
Because the species has a short adult lifespan — wild fish may live only a few months as adults — aquarium strains must be bred regularly to maintain viable populations. Hobbyists working with this species are therefore maintaining not just live fish but also batches of eggs at various stages of diapause.
In the aquarium
Nothobranchius hengstleri is a specialist species kept principally by killifish enthusiasts with experience maintaining annual Nothobranchius. A small species tank of 10 US gal is adequate for a breeding pair or small group; the tank should be heavily planted or decorated with floating cover and dim lighting to reduce male aggression toward females, which can be intense in confined spaces. The substrate should include a layer of soft peat moss or coconut coir at least 2–3 in deep to serve as the spawning medium — without it the fish cannot complete their natural reproductive behaviour.
Water conditions based on the type locality data are near-neutral (pH around 7.5) and soft to moderately hard, at temperatures consistent with the coastal East African lowlands (roughly 72–79 °F in the aquarium). These are undemanding parameters by killifish standards. The key husbandry challenge with annual Nothobranchius is not water chemistry but the egg management cycle: keepers must harvest spawn medium regularly, store eggs through diapause, and hatch them on schedule to maintain the line. The lifespan of adults in captivity is typically 6–18 months, though wild individuals likely live considerably less.
N. hengstleri is rarely encountered in the general trade and circulates mainly within specialist killifish societies. Its distinctive deep-red colouration and diagnostic fin patterns make it a sought-after species among collectors of East African annual killies.
Conservation
The IUCN Red List assessed Nothobranchius hengstleri as Endangered (EN) in 2019 under criteria B1ab(iii)+2ab(iii), assessed by Nagy and Watters. These criteria reflect a species with a severely restricted range — the known localities are confined to ephemeral coastal pools on and near the Afungi peninsula in Cabo Delgado province — combined with continuing decline in the extent and quality of its habitat.
The Afungi peninsula has been the focus of significant industrial development linked to natural gas extraction operations in Cabo Delgado, a region that also experienced prolonged conflict from 2017 onward. Both industrial land clearance and the generalised habitat degradation associated with conflict and population displacement pose direct threats to the fragile seasonal pools that the species depends on. More broadly, the ephemeral wetlands of coastal Mozambique face pressure from agricultural conversion, drainage, and extraction of water for irrigating smallholder and plantation crops.
The species' entire known range is small enough that a single severe disruption — drainage of the type-locality pool complex, a prolonged drought amplified by climate change, or the introduction of invasive fish such as mosquitofish (Gambusia) into its pools — could extirpate wild populations entirely. Because the eggs can survive dry storage for months, ex-situ conservation in specialist killifish collections provides a meaningful buffer against extinction, and the killifish hobby community has historically played a role in maintaining insurance populations of threatened Nothobranchius species. Formal survey work to confirm the current status of the Afungi and Nassoro populations, and to search for additional localities in Cabo Delgado, is an identified conservation priority.