Taxonomy & naming
Nothobranchius sylvaticus was formally described by Nagy, Bellstedt, van der Merwe, Cotterill, Luke, and Watters in 2025, in the journal Zootaxa (volume 5601, issue 1, pages 86–108; DOI 10.11646/zootaxa.5601.1.4). The holotype — a male of 1.5 in SL, catalogue number NMK ICH FW/6014/1 at the National Museum of Kenya — was collected on 4 July 2018 by D.U. Bellstedt from a coastal ephemeral swamp at approximately 85 ft elevation in Gongoni Forest. The specimens were obtained during Base Titanium mine biodiversity-monitoring surveys carried out in 2017–2018.
Eschmeyer's Catalog of Fishes (CAS) is the authority for the valid name Nothobranchius sylvaticus Nagy, Bellstedt & Luke, 2025. The species is placed within the family Nothobranchiidae, the African annual killifishes, alongside its close relatives in the large and diverse genus Nothobranchius. Molecular and karyotypic analysis published in the original description indicates that N. sylvaticus carries a unique karyotype within the genus, supporting its status as a distinct species and underlining its position as an evolutionary relict from the Coastal Forest refugium of south-eastern Kenya.
Morphology
Nothobranchius sylvaticus is a very small fish; FishBase records a maximum standard length of 1.5 in, and the holotype — a full-grown male — measured exactly 1.5 in SL. Dorsal fin soft-ray counts run 14–15; anal fin soft-ray counts 13–15. As is typical for male Nothobranchius, the coloration is vivid: the genus is notable for bright reds, blues, and oranges in males, which intensify during courtship. Females, as across the genus, are drabber — principally brownish or olive in tone, cryptically matching the leaf litter and dark substrates of their forest-pool habitat.
The body plan is compact and laterally compressed, the caudal fin rounded to slightly pointed. The mouth is moderately sized and upturned, suited for surface feeding in shallow, vegetation-choked pools. No detailed published morphometric data beyond the fin-ray counts and holotype length are reproduced here, as precise measurements are drawn directly from the Zootaxa original description.
Habitat
The species is known exclusively from ephemeral seasonal swamps within and immediately adjacent to Gongoni Forest, a coastal forest patch in Kwale County, south-eastern Kenya, within the Mkurumudzi River drainage. Gongoni Forest is part of the Eastern African Coastal Forests biodiversity hotspot — a globally significant but heavily fragmented refuge for endemic species. The pools that N. sylvaticus inhabits fill during the long rains (roughly April–June) and dry out completely during the dry season; the fish must complete its full life cycle within the wet phase.
Water chemistry data from a historical sampling of the area (de Rham, 1982) records pH 8.1, conductivity 465 µS/cm, KH 7.7°F (degrees German carbonate hardness), and GH 11.2°F — a moderately hard, near-neutral to slightly alkaline chemistry unusual among Nothobranchius species, many of which inhabit more acidic or blackwater conditions. Elevation at the type locality is 85 ft above sea level. The surrounding habitat is lowland coastal forest with dense leaf-litter accumulation in the pools, contributing humic material to the water despite the near-neutral pH.
Feeding
Like other members of the genus, N. sylvaticus is an opportunistic predator and micro-predator adapted to the invertebrate-rich environment of seasonal forest pools. Wild Nothobranchius in comparable habitats feed chiefly on small aquatic invertebrates — chironomid larvae, copepods, cladocerans, ostracods, and insect larvae — supplemented by any terrestrial invertebrates that fall into the water. The upturned mouth and surface-oriented posture are consistent with pursuit of small prey at or near the water surface and in the water column.
In the aquarium, Nothobranchius are readily conditioned on live and frozen foods: daphnia, cyclops, brine shrimp nauplii and adults, mosquito larvae, and finely chopped bloodworm. Given the very small size of N. sylvaticus (max ~1.5 in), appropriately small food items — micro-worms, vinegar eels, baby brine shrimp — should anchor the diet, with larger items reserved for conditioning adult fish. A varied live-food diet produces the best coloration in males and conditions females most effectively for spawning.
Mating
Nothobranchius sylvaticus is an annual killifish with a substrate-spawning reproductive strategy driven entirely by the seasonal availability of its pool habitat. As pool water levels rise with the rains, males display intensely to females using their bright body coloration, fin-spreading, and lateral threat postures. Rival males compete vigorously for access to females and prime spawning territories, which are typically areas of loose substrate — mud, decomposing leaf litter, or fine detritus — at the pool bottom.
Courtship ends with the pair diving together into the soft substrate and depositing eggs directly into the sediment, typically one to a few eggs per dive. The male wraps around the female during the spawning act, and the eggs are driven several millimetres into the substrate. This is the defining reproductive mode of the African annual killifishes: eggs are not attached to plants, not suspended in the water column, and not guarded — they are buried in the substrate and left.
Breeding
Once buried, the eggs of N. sylvaticus enter embryonic diapause — a state of arrested development that allows them to survive the complete desiccation of the pool through the dry season. The WildNothos resource documents an embryonic diapause period of approximately two to three months at room temperature in peat moss for this species. When the pool refills, the eggs resume development and hatch into fry within a matter of days to weeks, beginning the next generation's race against the drying season.
The hobby replicates this cycle using a spawning container filled with peat or coconut-coir substrate, into which the breeding pair spawns repeatedly over several weeks. The peat is then removed, gently dried to barely moist, and stored in a sealed bag or jar at room temperature — labelled with the species, collection code (if any), and date. After the diapause period (two to three months minimum for this species), the peat is placed in a shallow tray and rewetted with aged water; fry appear within 24–72 hours. Multiple wetting attempts, two to three weeks apart, often produce further fry from eggs that did not hatch on the first wetting. Fry are tiny at hatching and require micro-foods immediately.
In the aquarium
Nothobranchius sylvaticus is available only in specialist killifish circles and is not a trade fish in any conventional sense — it is held by a small number of dedicated hobbyists, and preserving captive-bred populations has genuine conservation importance given the dire status of the wild population. It is best kept as a single-species project in a small, well-planted aquarium of 10–15 US gal, with a tight-fitting lid (killifish jump readily) and a dark substrate. Gentle, filtered, clean water is important; although wild readings suggest moderately hard, near-neutral conditions, captive Nothobranchius generally tolerate slight variation. Temperature of 72–79 °F suits most East African Nothobranchius.
Given its small size and somewhat retiring nature, tankmates — if used at all — should be restricted to similarly sized, peaceful species or maintained as a species-only group. The priority for hobbyists keeping this fish should be successful breeding and the maintenance of multiple genetic lineages, ideally in coordination with other keepers. Clubs affiliated with the American Killifish Association (AKA) and the British Killifish Association (BKA) are the main channels through which eggs and fish are shared. Recording the source provenance of fish is important, since even very small geographic or genetic differences in wild Nothobranchius populations may matter to conservation efforts.
Conservation
Nothobranchius sylvaticus was assessed as Critically Endangered (CR B1ab(iii)+2ab(iii)) by the IUCN Red List in April 2025 — simultaneously with its formal description. The assessment reflects an extremely small extent of occurrence and area of occupancy, both under the CR thresholds, combined with continuing decline in the quality and extent of its habitat. All known populations lie within or immediately adjacent to Gongoni Forest in Kwale County, within the footprint of active mineral-extraction operations (the Base Titanium mining concession), where the fish was first discovered during environmental baseline surveys in 2017–2018.
The threats are multiple and immediate. Gongoni Forest itself is a highly fragmented remnant of the Eastern African Coastal Forest — one of the world's most threatened forest systems — and the ephemeral swamp pools that the species depends on are vulnerable to any change in local hydrology, including extraction of groundwater, alteration of drainage by road construction or mine infrastructure, and sedimentation. The fish has a generation time of under one year and no buffer of a long-lived adult population; if the pools fail to fill during any single rainy season, or if the egg bank in the substrate is disturbed or destroyed, local extinction can follow within a single dry season. The species may also face competition from introduced mosquitofish (Gambusia affinis), which have spread into coastal Kenyan drainage systems and adversely affect small annual killifish across sub-Saharan Africa. Captive breeding by specialist killifish hobbyists represents a meaningful ex-situ safety net for this species.