Taxonomy & naming
Nothobranchius ocellatus was described by Lothar Seegers in 1985, placed at that time in a new monotypic genus, Paranothobranchius, which Seegers erected to accommodate morphological features he considered distinct from the core Nothobranchius assemblage. Subsequent phylogenetic work collapsed Paranothobranchius into synonymy with Nothobranchius, and the Catalog of Fishes (Eschmeyer, Fricke & van der Laan) now records the valid combination as Nothobranchius ocellatus Seegers, 1985. Because the species was the original author's name-bearing type, the authority is cited without parentheses.
The specific epithet ocellatus is Latin for 'having small eyes' or 'marked with eye-like spots' — from ocellus, diminutive of oculus (eye) — referring to the ocellated, ringed colour markings on the body flanks and unpaired fins that are diagnostic for the species. The common name 'ocellated notho' directly translates this same feature.
The type locality, as given in the original description and confirmed by subsequent field surveys, is a swamp between Mtanza and the northern entrance of the Selous Game Reserve, Tanzania. Collection records also document the species from pools in the Rufiji drainage near Kibiti and from localities in the Ruvu drainage, all within coastal lowland Tanzania.
Morphology
Nothobranchius ocellatus is a large notho by genus standards. FishBase records a maximum total length of 4 in, while field accounts from killifish specialist sources report males attaining up to 4 in standard length — placing it among the largest Nothobranchius species recorded, substantially exceeding the 1.5–2.5 in typical of most genus members. The divergence between sources may reflect genuine population variation, sex-linked size differences (females are smaller), or different measurement conventions (TL vs. SL).
Males are strikingly coloured, with a body pattern of ocellated rings or target-like spots set against a vivid base colouration; the unpaired fins — dorsal, caudal, and anal — carry similarly ringed markings that give the species its name. Females follow the cryptic pattern common to Nothobranchius: an overall brownish or olive ground colour with reduced or absent fin ornamentation. Sexual dimorphism is pronounced — in a live pair the male and female are barely recognisable as conspecifics without prior knowledge. The body is laterally compressed, moderately deep, with the upturned mouth and large eyes characteristic of a fish that hunts in shallow, turbid water near the surface.
Habitat
Nothobranchius ocellatus is endemic to the coastal lowlands of Tanzania, confined to seasonal pools and swamps on the floodplains of the lower Rufiji and Ruvu river drainage systems. The Rufiji is one of Tanzania's largest rivers, draining a vast inland watershed before crossing the coastal plain; the Ruvu runs parallel to the north. In both systems, the annual flood pulse inundates shallow floodplain depressions and roadside pools that persist through the wet season and then dry out completely, sometimes within weeks, as the rains end.
WildNothos field data collected at known localities give a mean water temperature of 82 °F, ranging from 77–88 °F across seasons and sites — warm, as expected for a lowland tropical floodplain. The water chemistry is near-neutral to slightly alkaline: mean pH 7.5 (range 7.0–7.9), total dissolved solids averaging 132 ppm (range 30–320 ppm), and conductivity averaging 264 μS/cm (range 60–640 μS/cm). The pools are characteristically highly turbid, with suspended sediment reducing visibility to centimetres. The presence of the species in floodplain depressions within or adjacent to the Selous Game Reserve — now the Nyerere National Park, one of Africa's largest protected areas — provides partial habitat security for at least some localities.
Feeding
Like most Nothobranchius species, N. ocellatus is an opportunistic carnivore in the wild, feeding on invertebrates available in the shallow turbid pools it inhabits: aquatic insect larvae, small crustaceans, zooplankton, worms, and any small organism the pool produces during its brief wet-season existence. The turbidity of its habitat and its large size for the genus suggest it is primarily a visual ambush hunter at the water surface and mid-column rather than a bottom-sifter.
In captivity the species accepts a wide range of meaty foods readily: live or frozen bloodworm, daphnia, brine shrimp (artemia), tubifex, and mosquito larvae are all standard fare. Its large body size means it can take bigger prey items than many Nothobranchius species — small earthworms and appropriately sized frozen foods are suitable for adult males. A varied diet high in protein maintains colour and condition and supports the rapid gonad development that annual killies require to reproduce before their seasonal pools disappear.
Mating
Nothobranchius ocellatus reproduces by substrate-spawning with diapausing eggs — the defining reproductive mode of annual and semi-annual killifish. In the wild, pairs and small groups court and spawn in soft, muddy or organic-rich substrate at the bottom of seasonal pools. The male pursues the female with lateral displays, his body curved to maximise the visual impact of his ocellated pattern and extended fins, then drives alongside her and the pair dive together into the soft sediment, depositing one to a few eggs per embrace. Spawning is intermittent across the pool's wet-season lifespan rather than a single mass event.
Field and hobby observations characterise N. ocellatus as semi-annual rather than strictly annual: its embryos can complete development and hatch in three to four months in moist peat or coir at room temperature, meaning the eggs may hatch within the same wet season in years when rains are prolonged, or carry over through the dry season when conditions are short. This distinguishes it from strictly annual species whose eggs require an obligate diapause of many months. In the aquarium the male is best kept separate from females except for brief supervised spawning sessions, as males of this size are capable of stressing and injuring females with constant pursuit.
Breeding
The standard hobbyist protocol for N. ocellatus follows the annual killifish peat-spawning method. A male and female (or one male and two females) are placed together in a species tank with a small container of soft peat or coconut coir covering the base or placed as a separate spawning dish. The pair spawns into this medium over days to weeks. The peat is then removed, lightly pressed to remove excess water but not allowed to dry completely, placed in a labelled sealed bag or container, and stored at room temperature (approximately 72–79 °F) in darkness for three to four months. After the incubation period the peat is re-wetted with room-temperature or slightly cooled soft water to trigger hatching; fry appear within hours to days.
Fry are large by killifish standards, reflecting the species' overall size, and accept newly hatched brine shrimp (artemia nauplii) and microworm immediately upon hatching. Growth is rapid: at warm temperatures juveniles can reach sexable size in six to eight weeks, and breeding condition in males develops quickly. The tank should be tightly covered — all Nothobranchius are excellent jumpers — and kept at 75–82 °F. Soft to moderately hard water at neutral to slightly alkaline pH (7.0–7.6) matches the documented wild chemistry. Water quality must be maintained carefully, as the turbid pools the species inhabits in nature, while appearing dirty, turn over their water continuously during the rainy season; a stagnant aquarium is not a replica of that environment.
In the aquarium
Nothobranchius ocellatus is an impressive display fish for the specialist killifish keeper: the males are large, boldly patterned, and active, and the species presents the full annual killifish lifecycle in captivity. Its size — up to 4–6 in — means it requires more space than the typical dwarf Nothobranchius; a 60-litre or larger dedicated species tank per pair or trio is appropriate, with tight-fitting lid, subdued lighting, and a clump of floating or fine-leaved vegetation to break line of sight and reduce male aggression toward females.
Water chemistry should replicate the wild measurements: temperature 77–82 °F, pH 7.0–7.6, soft to moderately hard. The killis.org.uk husbandry notes for the genus recommend neutral to slightly alkaline water and temperatures in the 75–79 °F range for long-term maintenance, with breeding setups at the warmer end to accelerate development. Tankmates are not recommended in a breeding setup; if kept as a display-only fish, similarly sized, robust species that occupy different water layers and cannot fit in the notho's mouth are the only viable companions.
Because the species is effectively short-lived in nature and in captivity (wild fish seldom survive beyond one to two years), maintaining a rolling breeding programme — always incubating the next generation of eggs — is the primary husbandry commitment. The species is not widely available in the mainstream trade and is typically obtained through killifish society members and specialised breeders; the American Killifish Association and equivalent national societies are the primary sources of both fish and detailed care guidance.
Conservation
The IUCN Red List, assessed in 2020 by Nagy and Watters, places Nothobranchius ocellatus in the Near Threatened (NT) category. The assessment reflects the species' restricted endemic range — it is known only from the coastal floodplain lowlands of the Rufiji and Ruvu drainage systems in Tanzania, an area of significant and ongoing land-use pressure. Conversion of floodplain habitats to agriculture, drainage of seasonal pools, and disturbance of the coastal wetland belt through infrastructure development and human settlement all threaten the temporary water bodies this species depends upon.
The presence of some confirmed localities within or immediately adjacent to the Nyerere National Park (formerly Selous Game Reserve) provides a degree of habitat protection for the northern Rufiji populations, but localities in the Ruvu drainage and outside protected-area boundaries remain vulnerable. The species does not appear to be targeted by any specific collection pressure for the aquarium trade, but its appeal to specialist killiphiles means that over-collection of wild individuals from accessible sites is a potential risk if access to captive-bred stock becomes limited.
As with many Nothobranchius species, the semi-annual lifestyle makes the species exquisitely sensitive to changes in the timing and duration of the rainy season — a shift of even a few weeks in rainfall patterns can determine whether a pool persists long enough for the adults to spawn and the fry to develop sufficiently before desiccation. Climate-driven shifts in East African rainfall seasonality therefore represent a background threat even to populations within protected areas. Maintaining captive populations through killifish societies serves an insurance function for a species with a range narrow enough that a single severe drought or land-use event could eliminate multiple wild localities simultaneously.