Taxonomy & naming
Pronothobranchius chirioi was described by Valdesalici and Bengoa in 2019, with a type locality in Guinea-Bissau. The species has not yet been assessed by the IUCN, having been described too recently for a formal Red List evaluation. The valid name as governed by Eschmeyer's Catalog of Fishes (Fricke, Eschmeyer & Fong) is Pronothobranchius chirioi Valdesalici & Bengoa, 2019.
The genus Pronothobranchius belongs to the family Nothobranchiidae, the African rivuline killifishes, a radiation of predominantly annual species tied to seasonal pool systems across sub-Saharan Africa. Pronothobranchius is a small, tightly defined genus whose members are concentrated in the Upper Guinea forest belt and its savanna margins — Guinea, Sierra Leone, Liberia, and Guinea-Bissau. Known congeners include P. kiyawensis, P. niger, P. loati, P. homalonotus, and P. viridis, all occupying similarly ephemeral habitats in the same broad region.
The genus name Pronothobranchius references its relationship to Nothobranchius, the large East and Southern African annual genus, with the prefix 'Pro-' suggesting an ancestral or sister-group relationship in the nomenclatural sense. The specific epithet chirioi honours an individual associated with the field collection or research, following the convention common in killifish taxonomy of commemorating collectors and collaborators in species names.
Morphology
Pronothobranchius chirioi is a small killifish, with a standard length estimated at around 1.5–2 in for adults based on the typical size range for the genus. Like other Pronothobranchius species it has the characteristic compact, slightly deep body of a West African annual killi — robust relative to its length, with a rounded caudal fin and a terminal mouth well adapted for surface and mid-water feeding on small invertebrates.
Male colouration in Pronothobranchius species typically features iridescent blue-green or blue-grey flanks with red to orange spotting or banding, fin ornamentation that is more vivid than the body base colour, and a generally jewel-like appearance that makes the group attractive to specialist keepers. Females are cryptic — drabber brown or olive tones with little of the male's iridescence — a pronounced sexual dichromatism that serves camouflage for the egg-laying sex during spawning dives into the substrate. Detailed morphometric data for P. chirioi specifically are sparse, reflecting the recency of its description.
Habitat
Pronothobranchius chirioi occupies temporary and seasonal pools in Guinea-Bissau, in the lowland forest and forest-savanna transition zone at elevations below 1640 ft. These pools form during the wet season as rainfall saturates the landscape, creating shallow, soft-bottomed bodies of water in forest clearings, gallery forest edges, and open savanna depressions. By the dry season the pools evaporate completely, leaving only cracked mud — and buried killifish eggs.
The water chemistry of these forest pools is typically soft and acidic. Pronothobranchius habitats share the character of much of the Upper Guinea forest zone's standing water: low conductivity, pH in the range of approximately 5.0–6.5, tannin-stained from leaf litter, warm (around 68–75 °F during the wet season), and very soft (general hardness around 4–8 °dH). These are not pristine wilderness pools but often human-influenced seasonal wetlands — roadside depressions, rice paddy margins, and forest clearings — subject to the same land-use pressures as the broader landscape.
Feeding
As with other annual killies in the Nothobranchiidae, Pronothobranchius chirioi is a carnivore that feeds primarily on small aquatic invertebrates. In its temporary-pool habitat the available prey during the wet season includes mosquito larvae, small crustaceans such as copepods and cladocerans, chironomid midge larvae, and other micro-invertebrates that colonise and bloom in seasonal pools alongside the killifish. The fish takes prey at the surface and in the water column.
In the aquarium, small live and frozen foods are the staple: Artemia nauplii, adult brine shrimp, daphnia, cyclops, micro-worms, and bloodworm (chironomid larvae) are all accepted readily. The compressed feeding season of an annual killi — adults must grow, mature, spawn, and die within a single wet season of a few months — means these fish feed opportunistically and often voraciously when food is available. High-quality, varied live and frozen foods help maintain the vivid colouration and reproductive readiness of males.
Mating
Pronothobranchius chirioi reproduces by the annual substrate-spawning strategy that defines the nothobranchiid annuals. Pairs engage in courtship behaviour in which the male displays his colouration and drives the female toward the substrate — typically soft mud or peat in the wild. The spawning act involves the pair diving together into the substrate or pressing against it, releasing a small number of eggs that are immediately buried in the sediment. The eggs are large, durable, and covered in a tough, adhesive chorion that anchors them in the substrate.
This is not plant-spawning or open-water egg scattering. The entire reproductive biology is orientated around getting fertilised eggs as deep into the substrate as possible before the pool dries. In the aquarium the standard practice is to provide a deep layer of peat or coconut coir as the spawning medium; eggs are collected by draining the medium, pressing out excess water, and storing it in a sealed bag or container at around 68–72 °F for the diapause period. Multiple spawning dives occur over the course of days to weeks.
Breeding
The buried eggs of Pronothobranchius chirioi enter a state of developmental arrest — diapause — that corresponds to the dry season in Guinea-Bissau. During diapause the embryo within the egg pauses at a defined developmental stage, its metabolism reduced to near-zero, surviving months of desiccation that would be fatal to the embryo of any non-annual fish. When the rains return and the pool refills, the eggs detect the moisture and resume development, hatching within days of re-wetting.
In the hobby the keeper replicates this cycle by storing the peat medium containing eggs in a warm, dark place for a species-appropriate diapause period — typically several months for West African annuals — before re-wetting the medium with soft, slightly acidic water. Fry emerge rapidly and must be fed immediately with appropriately sized live food (infusoria, freshly hatched Artemia nauplii, or commercial fry foods). Growth is fast, reflecting the evolutionary pressure of a short season: juveniles reach sexual maturity in weeks, and adults in a wild pool may live only a few months before the dry season ends their brief lives. In the aquarium, with stable water quality, adults can survive longer, but the species' natural lifespan is measured in one seasonal cycle.
In the aquarium
Pronothobranchius chirioi is a specialist killifish for experienced keepers. It is rarely encountered in the trade compared to more popular nothobranchiids such as Nothobranchius rachowi or N. furzeri, but is sought by members of killifish associations who exchange eggs through the post. The annual lifecycle and the diapause-storage requirement are not difficult once understood, but they demand a keeper who can commit to the egg-storage protocol and the fry-rearing work that follows re-wetting.
A small aquarium (10–15 US gal) with a tight-fitting lid, a substrate layer of peat or coconut coir for spawning, and a few floating plants for cover is the typical setup. Water should be soft and acidic (pH 5.5–6.5, hardness below 8 °dH) and temperature kept in the 68–75 °F range. Males are territorial toward one another and should be housed in a ratio of one male to one or two females unless the tank is large enough to reduce visual contact. Because the adults are short-lived, keeping a colony going requires regular egg collection and a rolling diapause-hatch schedule.
The species presents little compatibility challenge since its small size and specialist housing requirements effectively dictate a species-only or genus-only setup. Males in breeding colour are genuinely attractive fish, and for killifish keepers the West African annuals hold a particular appeal as compact, biologically fascinating subjects that reward careful husbandry.
Conservation
Pronothobranchius chirioi has not been formally assessed by the IUCN Red List; described only in 2019, the species falls into a queue of recently named taxa awaiting evaluation. However, the conservation picture for the genus Pronothobranchius as a whole is concerning. These fish are endemic to the Upper Guinea forest zone and its seasonal wetlands — a biodiversity hotspot that has suffered extensive deforestation over the past several decades. Guinea-Bissau, the type locality of P. chirioi, retains significant forest cover relative to some of its neighbours, but conversion of seasonal wetlands and forest margins for agriculture, including rice cultivation, directly removes or degrades the temporary pools that annual killies depend on.
Because Pronothobranchius species are tied to specific seasonal pool systems and cannot disperse between catchments when populations are extirpated, even localised habitat loss can eliminate entire local populations without possibility of recolonisation. The introduction of exotic mosquitofish (Gambusia species) to West African water bodies for malaria control has also been documented as a threat to small killifish in seasonal pools elsewhere in the region, through predation on fry and competition for micro-invertebrate prey. For P. chirioi specifically, the rarity of the species in collections and the paucity of distributional records beyond the type locality suggest it may be genuinely uncommon or restricted to a small range — making a precautionary approach to its habitat protection appropriate even without a formal IUCN assessment.