Taxonomy & naming
Simpsonichthys marginatus was described by Wilson J. E. M. Costa and Gleise B. Brasil in 1990. Eschmeyer's Catalog of Fishes (Fricke, Eschmeyer & Fong) records the valid name as Simpsonichthys marginatus (Costa & Brasil, 1990), placing it within the family Rivulidae — the South American annual killifishes — a diverse assemblage of annual and semi-annual killifishes found throughout tropical and subtropical South America.
The species has also been treated under the combination Hypsolebias marginatus, reflecting a period when the subgenus Hypsolebias was elevated to full generic rank by some authors. A comprehensive revision by Costa (2007) examined 50 species across five subgenera within Simpsonichthys, confirming the generic arrangement and affirming the placement of this taxon. The American Killifish Association (AKA) lists both Simpsonichthys marginatus and Hypsolebias marginatus as names in use for the same fish, with the latter appearing on some older stock labels in the hobby.
The specific epithet marginatus is Latin for 'bordered' or 'margined', most likely a reference to the contrasting edge markings on the fins or flanks of the male in breeding colour.
Morphology
Simpsonichthys marginatus is a small fish reaching approximately 2–2.5 in total length; FishBase records a maximum of 2.5 in TL, while standard-length measurements of around 1.5 in SL have also been reported, which corresponds to roughly 2 in total. The body form is typical of the South American annual killifish guild: somewhat deep and laterally compressed, with a terminal mouth, large eyes, and rounded unpaired fins.
Sexual dimorphism is pronounced, as in most Simpsonichthys. Males are more vividly coloured, displaying a reddish-brown to brownish body overlaid with greenish-blue iridescent bars along the flanks. Females are considerably plainer — brownish in ground colour with grey bars — and are rounded in body profile when carrying eggs. This strong colour contrast between sexes is the rule in the genus and is used in both species recognition and mate-choice displays.
Habitat
The only confirmed locality for Simpsonichthys marginatus is a temporary pool near the Rio dos Patos, a tributary of the Rio Maranhão in the upper Tocantins basin, situated in Barro Alto municipality, Goiás state, central Brazil. This region falls within the Cerrado biome, the vast neotropical savanna that covers much of the Brazilian interior and is characterised by a sharply seasonal climate with a pronounced dry season during which small isolated pools and depressions evaporate entirely.
The pools this species inhabits are benthopelagic freshwater environments — shallow, seasonally isolated bodies with accumulated leaf litter, peat-like organic sediment, and soft substrate in which eggs are buried. Water conditions inferred for this habitat are soft and mildly acidic: temperatures in the range of 68–81 °F and pH approximately 5.5–7.0 are consistent with Cerrado seasonal pools, which accumulate organic acids from decomposing vegetation but can shift toward neutral as water chemistry fluctuates through the wet season.
The species' known range is extremely restricted — the total occupied area has been estimated at fewer than 62 mi² — making it one of the most geographically constrained killifishes in the upper Tocantins drainage.
Feeding
Like other members of the genus, Simpsonichthys marginatus is a benthic omnivore. FishBase assigns it a trophic level of 3.2, consistent with a diet that includes small invertebrates, aquatic insects and their larvae, crustaceans, and organic detritus — the food resources typically available at the sediment surface in shallow tropical pools.
In captivity, annual killies of this type are most commonly offered live or frozen small invertebrates: Artemia nauplii and adults, Daphnia, Cyclops, chironomid larvae (bloodworm), and small tubificid worms. The species is unlikely to accept dry pelleted foods readily, and a live-food-centred diet is generally recommended by killifish specialists to maintain condition and colouration.
Mating
Simpsonichthys marginatus is an annual killifish and breeds by diving into soft substrate to deposit eggs — a spawning strategy called peat- or substrate-spawning. The male courts the female with lateral displays that show off his iridescent barring before the pair turn nose-down and plunge together into the peat or organic sediment layer at the bottom of the pool, releasing and fertilising eggs within the substrate in a brief buried embrace.
Male competition in the wild is likely intense given the short adult lifespan and the brief period during which sexually mature adults overlap in temporary pools. Colouration is the primary signal in male–male rivalry and female choice: the vivid greenish-blue bars that overlay the reddish-brown flanks are characteristic of the genus and function in the close-range displays typical of annual killifish in confined pool habitats.
Breeding
The eggs of Simpsonichthys marginatus are buried in the substrate and undergo diapause — a state of arrested embryonic development — during the dry season, when the pool evaporates and the fish themselves perish. The eggs can remain viable in dry or semi-dry peat or mud for approximately six to nine months before the return of seasonal rains rewets the substrate and triggers hatching. This diapausing egg strategy is the defining biological adaptation of the South American annual killifishes and allows the species to persist entirely within the egg stage across the dry season.
In the aquarium, hobbyists replicate this cycle using a peat or coconut coir spawning medium. After a conditioning period during which adults spawn freely into the medium, the peat is removed, gently squeezed to remove excess water, and sealed in a labelled plastic bag at room temperature for a storage period of six to nine months. Re-wetting the peat by placing it in soft, slightly acidic water triggers hatching, typically within hours to a few days. Fry are relatively large at hatching compared with many tropical fishes and can accept freshly hatched Artemia nauplii from the outset. Growth is rapid; adults may be reached in under three months. Because wild fish live less than a year and even aquarium-kept individuals often survive only one to two years, consistent cycling of eggs through the peat-storage method is the only way to maintain a population.
In the aquarium
Simpsonichthys marginatus is a rare species in the hobby, seldom encountered outside dedicated killifish societies. Where it does circulate, it is maintained almost exclusively by specialist killifish keepers following the standard peat-spawning protocol described under Breeding. A small, covered aquarium with a dark substrate, soft and slightly acidic water (temperature 72–79 °F, pH 5.5–7.0), and gentle filtration suits the species. Floating plants or emergent stem plants help diffuse light and provide a sense of cover that encourages natural behaviour.
Because the fish is small and short-lived, it is best maintained as a dedicated breeding group with surplus peat batches rotating through storage. Given its Critically Endangered status and the possibility that it is already extinct in the wild, any captive stock held by hobbyists potentially represents the only living individuals of the species. The American Killifish Association and international killifish societies have emphasised the importance of maintaining pure, pedigree-tracked lines of rare annuals like this species rather than mixing collection localities or crossing with similar taxa.
Conservation
The IUCN Red List assessed Simpsonichthys marginatus as Critically Endangered (CR) under criterion B2ab(ii,iii,iv,v) in November 2018, reflecting an extremely small area of occupancy (estimated below 62 mi²), severely fragmented and declining habitat, and ongoing threats to the Cerrado pool systems in which the species occurs. The Cerrado is one of the world's most threatened biodiversity hotspots: large-scale conversion to soybean and cattle agriculture has destroyed or degraded the majority of its original extent, and seasonal pools in Goiás state have been lost to drainage, groundwater extraction, and land clearance at rates that have accelerated significantly since the 1980s.
No confirmed field records of Simpsonichthys marginatus are known from after 1996, a gap of three decades that has led some assessors to suggest the species may already be functionally or completely extinct in the wild. The single known type locality near the Rio dos Patos in Barro Alto municipality has not been surveyed adequately in recent years, and the status of the pool itself — whether it persists, has been drained, or converted — is not documented in the available literature.
Conservation options are severely constrained by the species' extreme range restriction and the likelihood that wild populations are absent or undetectable. Ex-situ maintenance by killifish hobbyist societies represents the principal remaining safeguard for the taxon. Targeted surveys of the Barro Alto region and adjacent Tocantins headwater drainages to locate surviving populations, combined with formal ex-situ breeding programmes, are the recommended next steps. Without intervention, Simpsonichthys marginatus faces imminent global extinction.