Taxonomy & naming
Nothobranchius interruptus was first described as a subspecies of Nothobranchius jubbi by Rudolf Wildekamp and Hans-Joachim Berkenkamp in 1979, based on material collected from a temporary swamp west of Kikambala, roughly 9 mi north of Mombasa, Kenya, in August 1978. The holotype, a male of 2 in standard length (BMNH 1979.6.12.1), was deposited at the Natural History Museum in London. Lothar Seegers subsequently elevated the taxon to full species status in 1981, and the combination Nothobranchius interruptus (Wildekamp & Berkenkamp, 1979) is accepted by Eschmeyer's Catalog of Fishes as the current valid name.
Within the genus, WildNothos (the Wildekamp, Nagy & Watters database) places N. interruptus in the subgenus Adiniops and within the N. melanospilus species group — a cluster of coastal East African annuals sharing broad distributional and morphological affinities. No syntopic congeners have been recorded at the type locality, giving the species an unusual degree of isolation from close relatives at the collection site.
The specific epithet interruptus is Latin, meaning 'interrupted' or 'broken', most likely referring to a pattern element in the male's colouration — a common naming convention within Nothobranchius for colour or banding features that distinguish species in the melanospilus group.
Morphology
Males reach approximately 2–2.5 in standard length (FishBase gives a maximum of 2.5 in SL/TL); females are somewhat smaller and considerably plainer. As in other Nothobranchius, the body is deep and somewhat laterally compressed, with a rounded head, a short snout, and a large, upturned mouth suited to surface-feeding on insects.
Male colouration follows the vivid patterning typical of the melanospilus group: the flanks display red to brick-red scales with blue iridescent edges arranged in a reticulate or interrupted banded pattern — the feature the name alludes to. The dorsal and caudal fins are pigmented in contrasting reds and blues, and the tail may carry a marginal band. Females are uniformly brownish-grey with little ornamentation, providing camouflage in the leaf-litter and sediment of their ephemeral pools.
The caudal fin is rounded to lanceolate, and the dorsal fin is set well back on the body — a configuration shared across Nothobranchius that positions the fin for display and mating at the substrate surface. Scales are cycloid and large relative to body size.
Habitat
The species is endemic to the lower Mwatsuma river drainage in Kilifi County on the Kenyan coast, at elevations between 17 and 246 ft above sea level. WildNothos records type-locality water temperatures of 81–89 °F, pH 7.04–7.41, and total dissolved solids of 322–442 ppm — conditions broadly typical of coastal East African seasonal pools: warm, slightly hard to moderately mineralised, and near-neutral. These pools are set in coastal bushland and scrub, filling with the long and short rains and drying out between seasons.
N. interruptus inhabits shallow temporary pools and seasonal swamps rather than permanent water bodies. The substrate in such pools is typically soft mud or detritus over clay, accumulating leaf litter and aquatic vegetation during the wet phase. The surrounding coastal lowland vegetation provides shade and organic input, and invertebrate production in the pools supports the compressed, high-energy annual life cycle.
The species falls in the FEOW ecoregion of the Tana, Athi and Coastal Drainages (FEOW 567), a geographically limited coastal strip between the Tana and Umba rivers that has a distinct killifish fauna largely isolated from the interior East African rift drainages.
Feeding
In the wild, N. interruptus feeds primarily on small invertebrates — aquatic insect larvae, copepods, mosquito larvae, and small worms — as well as surface-fallen insects. The upturned mouth is well adapted for taking prey at or just below the water surface, an important foraging zone in the shallow, warm pools the species inhabits. Prey size is limited by the relatively modest body size of this species compared with larger Nothobranchius from the Mozambique channel drainages.
In the aquarium, the fish accepts a wide range of small live and frozen foods: Drosophila, brine shrimp nauplii and adults, bloodworm, Cyclops, and white worm. A varied diet with emphasis on live or frozen invertebrates supports best colouration in males and reproductive condition in females. Dry foods are accepted by many individuals once acclimated, but live food accelerates conditioning for spawning.
Mating
Nothobranchius interruptus is a substrate-spawning annual killifish. Mating behaviour follows the pattern common to the genus: the male pursues the female with intense lateral displays, showcasing his flanks and finnage. When the female is receptive, the pair dive side-by-side into the soft substrate — mud or peat — and the eggs are fertilised and deposited within the sediment in a single brief embrace. This can be repeated many times over the course of hours or days.
Male dominance hierarchies form rapidly in groups; subordinate males are harassed and may be killed if no refuge is available. For breeding purposes, a single pair or a small group of one male and two to three females is typically more productive than a larger group. The male's display colouration is at its most vivid during breeding activity, making the spawning period one of the most striking visual experiences in annual killi keeping.
WildNothos notes the species has a semi-annual life cycle at its type locality, meaning pools may refill twice yearly with the two Kenyan rainy seasons, giving fish a second opportunity to breed if conditions allow — a trait relevant to egg incubation management in the hobby.
Breeding
Eggs are laid buried in the substrate and enter a state of arrested development (diapause) that allows them to survive the complete desiccation of the pool during the dry season. In the wild, the embryos remain viable in the dry mud for the duration of the dry period — often three to four months — and hatch when the pool refills with rain. The adult fish die as the pool dries, so the next generation overwinters entirely as buried eggs.
In the aquarium, breeding is straightforward using a layer of peat or coconut-coir fibre 2–3 in deep in the spawning container. The fish are given one to two weeks to spawn, after which the peat is removed and lightly squeezed to a moist — not saturated — consistency, placed in a sealed plastic bag labelled with the date and collection data, and stored in a dark, warm location (75–79 °F). WildNothos records successful hatching after three to four months of dry incubation at room temperature. After storage, the peat is flooded with a few centimetres of soft, slightly acidic water; fry typically begin emerging within 24–72 hours and must be fed immediately on micro foods (paramecia, vinegar eels, commercial fry powder) before moving on to newly hatched brine shrimp nauplii.
Fry grow rapidly — annual killies are under evolutionary pressure to mature before the pool dries — and can begin breeding at two to three months of age. Adults in captivity rarely exceed one year of life even under ideal conditions, reflecting the compressed life history of the annual lifestyle.
In the aquarium
Nothobranchius interruptus is considered a moderately demanding species, noted by FishBase as 'difficult to maintain in aquarium' — a rating that reflects its sensitivity to water quality, its relatively short lifespan, and the specialised management of diapausing eggs rather than extreme fragility per se. For hobbyists already familiar with annual killies, the husbandry is well understood.
A small, heavily planted or peat-floored aquarium of 10 US gal is sufficient for a breeding trio. Water conditions matching the type locality — temperature 79–86 °F, pH 7.0–7.4, moderate hardness — suit the species well. Gentle filtration (a small sponge filter) avoids strong currents that disturb the peat substrate and create surface turbulence unsuited to a surface-dwelling predator. Daily small water changes help maintain the water quality that annual killies with thin-skinned, fast metabolism demand. A tight-fitting lid is essential, as these fish jump readily.
The species is best kept as a dedicated breeding pair or small group in a species tank; it is too active and competitive for mixed community setups. Its restricted range, specialised biology, and EN conservation status make it a responsibility fish for the enthusiast — maintaining genetically clean, collection-data-labelled lines contributes to the conservation insurance population held in the hobby.
Conservation
The IUCN Red List assessed Nothobranchius interruptus as Endangered (EN B1ab(iii,iv)+2ab(iii,iv)) in 2018, published in 2019, with the evaluation carried out by Nagy and Watters. This represents an upgrade from the previous Vulnerable (VU D2) listing of 2006. The criteria reflect the species' extremely restricted range — a single drainage system on the Kenyan coast near Mombasa — and continuing decline in the extent and quality of its habitat.
The primary threats documented in the IUCN assessment are residential and commercial development (Mombasa's urban fringe has expanded significantly northward through Kilifi County), agricultural conversion, invasive species, and domestic and urban wastewater pollution. The temporary coastal pools that constitute the species' entire known range are precisely the kind of marginal wetlands most vulnerable to drainage for development or conversion to agriculture, and they receive very little formal protection. There are no protected areas recorded as encompassing the type locality.
The hobby killifish community maintains ex-situ lines of N. interruptus, and the American Killifish Association lists it among the species of conservation concern in its species-maintenance programme. Given that the wild population may be confined to a handful of seasonal pools in a rapidly urbanising coastal strip, the hobby's captive lines represent a meaningful supplement to whatever in-situ population persists — provided those lines retain accurate collection data, are not mixed with other Nothobranchius, and are refreshed from legitimate wild collections when possible.