Taxonomy & naming
Hypsolebias fulminantis was originally described by Costa and Brasil in 1993 under the name Cynolebias fulminantes, drawing on the broad circumscription of Cynolebias that was then standard for South American annual killifish. The species was subsequently transferred to Simpsonichthys, a genus erected to contain a radiation of brightly coloured annual killies from the São Francisco drainage and adjacent drainages of eastern Brazil. Huber formalised the combination in 1995, providing the authorship as it is now cited.
More recently, several authors have advocated splitting Simpsonichthys further, elevating a number of constituent groups to genus level and placing this species in Hypsolebias — a name that appears in current FishBase records. The Catalog of Fishes, which governs valid nomenclature for this atlas, continues to carry the species under Simpsonichthys, and that treatment is followed here; the alternative placement in Hypsolebias should be understood as reflecting an ongoing taxonomic debate rather than a settled rearrangement. The specific epithet fulminantis is Latin, the genitive of fulminans, meaning 'lightning' or 'striking like lightning' — a reference to the intense, electric patterning of the male's finnage.
The type localities cited in the original description are in the vicinity of Pindaí (5 mi south) and Guanambi, both in the state of Bahia, central-eastern Brazil, within the middle São Francisco basin.
This fish was long listed as Simpsonichthys fulminantis; Eschmeyer's Catalog of Fishes places the species in Hypsolebias, and the current combination Hypsolebias fulminantis is followed here.
Morphology
Males of Hypsolebias fulminantis reach approximately 2–2.4 in total length; females are smaller and considerably plainer. The dorsal fin is notably large, carrying 21 or more rays — a count that has been used diagnostically in separating this and related species within the Simpsonichthys complex. The combination of this extended dorsal with a rounded caudal fin gives large males a fanlike silhouette in display that accounts for the informal hobbyist name 'Magnificent Fanfish' sometimes applied to Guanambi-locality fish.
Male colouration is intense. The body ground colour is deep blue to turquoise, overlaid with an intricate pattern of iridescent scales, dark edging, and red to orange markings, particularly in the unpaired fins. The precise pattern varies among individuals and is highly sensitive to condition, light angle, and mood. Females are cryptically coloured in drab olive-brown with faint darker mottling — a near-universal pattern among annual killies in which the female must remain inconspicuous while the male is spectacularly visible during courtship.
Habitat
This species inhabits shallow, temporary pools and seasonally flooded lowland areas within the middle São Francisco River basin of Bahia state, Brazil. These habitats are characteristic of the cerrado-caatinga transition zone of central-eastern Brazil — a semi-arid to subhumid region with a strongly seasonal rainfall regime in which surface waters fill rapidly after rains and contract or disappear entirely in the dry season. The pools that support annual killifish are often small, isolated, and surrounded by terrestrial habitat; connectivity between pools may be very limited.
Water chemistry at the type locality and at Guanambi is moderately soft and near neutral: the American Killifish Association records pH 7 and hardness of approximately 5 dGH, with a temperature around 77 °F. These figures are consistent with shallow seasonal pools in the cerrado, which can warm considerably in the afternoon sun and are fed by rainfall runoff rather than by groundwater springs. The soft, fine substrates of silt, mud, and decaying organic matter in which annual killies spawn are a defining feature of these habitats.
Feeding
Like all annual killifish in the Simpsonichthys lineage, H. fulminantis is a carnivore in the wild, feeding on small invertebrates — aquatic insect larvae, microcrustaceans, worms, and similar prey — that are abundant in the temporary pools it occupies. The seasonal nature of its habitat means that feeding opportunities are compressed into the wet season; growth rates are correspondingly fast.
In the aquarium, live foods are strongly preferred and produce the best colouration and breeding condition: Artemia nauplii, Daphnia, Cyclops, chironomid larvae, and small worms are all appropriate. Frozen alternatives of similar nutritional profile are accepted by established fish. Fry are too small for Artemia nauplii at hatching and require infusoria, Paramecium, or commercial micro-fry foods for the first week before graduating to nauplii.
Mating
Courtship in Hypsolebias fulminantis is an intense and visually striking affair driven by male competition and female choice. Males display laterally to one another and to females, fully extending the large dorsal fin and angling the body to maximise the apparent size of the fin — the 'fan' display from which the common name derives. Colouration intensifies during display; rival males may posture, jaw-lock briefly, and snap at one another, though serious injury is uncommon in a well-managed aquarium with multiple shelters.
Once a female signals receptivity, the pair position themselves side by side at or near the substrate surface before executing a brief downward dive together into the soft substrate to deposit eggs. This spawning dive is the behavioural hallmark of substrate-spawning annual killifish and distinguishes them sharply from plant-spawning killifish species that deposit eggs by pressing against vegetation. The act is rapid — a second or less below the substrate surface — and the pair immediately return to open water before repeating the sequence. A pair in good condition may execute many such spawning dives over the course of a day.
Breeding
Each spawning dive deposits one or a few eggs buried in the substrate, typically peat or mud in the wild, or peat fibre / coconut coir in aquarium setups. The eggs of H. fulminantis are large relative to adult body size, tough-shelled, and resistant to desiccation — they are adapted to survive months in moist substrate without free water during the dry season. Developmental arrest (diapause) allows the embryo to pause at specific stages while the pool is dry, resuming development when re-wetted.
In the hobby, the standard method is to collect spawned peat monthly, squeeze out excess moisture to a 'moist but not wet' condition, seal it in a labelled plastic bag, and store it in a warm, dark location for eight to twelve weeks — the approximate duration of the natural dry season. At the end of the drying period the peat is re-wetted with aged, slightly acidic water in a shallow tray. Fry hatch within 24–48 hours of re-wetting and are immediately free-swimming and feeding. Rearing fry requires very clean, shallow water and appropriately sized first foods; they grow quickly and can reach sexual maturity in as little as six to eight weeks under good conditions, consistent with their wild life expectancy of under one year.
In the aquarium
Hypsolebias fulminantis is kept primarily by specialist killifish hobbyists; it is not a fish for general community aquaria. A small, well-planted or peat-floored tank of 11–16 US gal is suitable for a pair or a trio (one male, two females). Water should be soft and slightly acidic to neutral: pH 6.0–7.0, hardness below 10 dGH, temperature 72–79 °F. The Garnelio blog records aquarium maintenance parameters of pH 6–7, temperature 72–75 °F, GH 6–9, and KH up to 3, which reflects the moderate soft-water requirements of the middle São Francisco region. Bright lighting is not necessary and subdued lighting with floating plants promotes natural behaviour.
Because wild adults live for under a year, maintaining a collection of this species requires consistent attention to spawning and egg management — the fish will not persist in the hobby without deliberate management of the peat storage cycle. Females may be stressed by a single persistent male; providing two or more females per male reduces harassment. The species is peaceful toward other fish of similar size and disposition but should not be combined with fin-nipping species that could damage the male's impressive dorsal.
The Guanambi locality strain is the form most commonly encountered in European and North American killifish club circles; British Killifish Association members have maintained and published on the species, with a cover feature in Killi-News (2002, issue 440) documenting the hobby experience of the time.
Conservation
The IUCN Red List assessed Hypsolebias fulminantis as Endangered in 2018. The primary driver of this status is the species' extremely restricted and fragmented range: it is known from only a small number of seasonal pool localities in the Guanambi and Pindaí areas of Bahia, and possibly a handful of nearby sites. Such pools are inherently vulnerable to drainage, infilling, and modification for agriculture and urban development in a semi-arid region where seasonal water is a scarce resource subject to competing demands.
The cerrado-caatinga biome of central-eastern Brazil has experienced significant habitat loss over recent decades driven by conversion to irrigated agriculture — particularly soybean cultivation and cattle pasture expansion — and by increased extraction of surface water for irrigation and domestic use. For an annual killifish dependent on seasonal flooding, even modest alterations to drainage patterns or the timing and depth of seasonal inundation can eliminate local populations. The introduction of tilapia and other exotic fishes into reservoirs and canals in the São Francisco basin adds predation pressure on the small pools that drain into these systems.
The Guanambi-locality population maintained in the killifish hobby represents a modest insurance population, but hobby stocks cannot substitute for habitat protection, and hobby populations of annual killies routinely experience genetic drift and loss of diversity over successive peat-storage generations. Formal surveys of the São Francisco seasonal-pool fauna and protection of key pool complexes in Bahia are the most effective conservation interventions available for this and other range-restricted annual killies endemic to the region.