Taxonomy & naming
Simpsonichthys parallelus was described by Wilson J. E. M. Costa in 2000. Eschmeyer's Catalog of Fishes (CAS) records the valid name as Simpsonichthys parallelus (Costa, 2000) within the family Rivulidae, the New World rivulines. The genus Simpsonichthys honours the British naturalist and killifish enthusiast Keith Simpson, and its members are collectively known as Brazilian annual killies; S. parallelus belongs to a cluster of small Simpsonichthys species that also includes S. cholopteryx and S. nigromaculatus, from which it is diagnosed by a combination of characters described below.
The species epithet parallelus is Latin for 'parallel', referring to the striking parallel distal outline shared by the dorsal and anal fins of breeding males — an immediately visible field mark that separates this species from relatives with more rounded or pointed fin margins.
Morphology
Simpsonichthys parallelus is a dwarf annual killifish. FishBase records maximum standard length at 1 in for males and 1 in for females; iTrainsFishes (It Rains Fishes) notes a maximum of approximately 1 in under aquarium conditions. The species is sexually dimorphic, as is typical of Simpsonichthys: males carry vivid coloration while females are cryptically patterned in the beige and brown tones common to rivuline hens.
The most diagnostic feature of males is the parallel distal margin of the dorsal and anal fins — unlike the rounded or pointed fins of related species — combined with an inner caudal fin that is pure orange with no dark markings. A further distinguishing character is the complete absence of ventral fins, which are present (if small) in several close relatives. Body coloration in males features the iridescent blues, greens, and reds typical of the genus, expressed over a compact, deep-bodied silhouette. Females show the brownish countershading usual for female rivulines, providing camouflage against leaf litter and peat while depositing eggs.
Habitat
The type locality of S. parallelus is a seasonal swamp in the floodplains of the rio Formoso, a tributary of the rio Correntes within the rio Paranaíba drainage (itself part of the Paraná basin), located in the Parque Nacional da Emas, Goiás, Brazil, at approximately 22°45′S, 42°00′W. All documented collection localities — including the American Killifish Association codes for Formosa (Goiás), Emas, Rio Formoso (Parque Nacional da Emas), and Rio Jacuba II — fall within or immediately adjacent to this protected area in the Brazilian Cerrado.
The habitat is the classic South American annual-killi pool: small, shallow, highly seasonal depressions in the Cerrado savanna that fill with the summer rains and contract to muddy patches or dry out entirely during the dry season (roughly April–September). Water is typically soft and acidic, derived from rainfall accumulating in seasonally flooded grassland rather than from perennial streams. Submersed macrophytes and leaf litter provide cover, and the substrate is a soft mix of peat, mud, and organic detritus — the medium into which eggs are deposited.
The species is sensitive to hard, calcium-rich water; hobbyists working with it recommend using rainwater or reverse-osmosis water for conditioning and hatching at temperatures around 64–68 °F.
Feeding
Like all annual rivulines of the Brazilian Cerrado, S. parallelus is an invertivore in the wild, feeding on whatever small aquatic invertebrates — microcrustaceans, insect larvae, worms — its pools produce during their brief productive season. Given the fish's tiny size and short lifespan, rapid energy intake supports both fast growth and the production of large numbers of eggs before the pool dries.
In the aquarium, newly hatched and juvenile fish readily consume Artemia nauplii from the start, an unusually early readiness that reflects the fast developmental tempo of annual killies. Daphnia and other small crustaceans are added as the fish grow. Adults can be maintained on a varied diet of live and frozen invertebrates; frozen bloodworm, Daphnia, and cyclops are all accepted.
Mating
Simpsonichthys parallelus is a substrate-diving annual killi. Courtship is initiated by the male, who displays his vivid dorsal-anal-fin pattern and orange caudal-fin markings to the female. When a female is receptive, the pair swim side by side and press close together above a soft substrate patch, before driving downward to bury briefly in the peat or mud. In nature this dive may carry the pair a centimetre or more into the substrate before they disengage and return to open water.
Spawning is a protracted, multiple-dive affair spread across days or weeks as the pool season permits. Males are active and persistent suitors; in confined aquaria a single male may harass a female relentlessly, and the usual management strategy is a ratio of two or more females per male, with adequate cover to allow females to rest between spawning bouts.
Breeding
Eggs are deposited singly, piece by piece, deep in the soft substrate. In the wild the pools then dry out, leaving the eggs buried in moist peat through the dry season in a state of arrested development — diapause — that can last many months, until rainfall reopens the pool and triggers mass hatching. This resilient egg stage is the cornerstone of the annual life-history strategy: the adults perish, but the buried clutch carries the population through to the next season.
In captivity, spawning is accomplished on a layer of softened peat or coir fibre in the tank bottom. After two to four weeks of spawning, the medium is collected, gently squeezed to reduce excess water, sealed in a plastic bag or container, and stored at 72–77 °F in darkness for an incubation period of approximately 2.5–3 months (around 10–12 weeks), though some eggs may hatch as early as 5–6 weeks. When hatching time approaches, the peat is placed in shallow, cool water (64–68 °F) — the temperature drop plus re-wetting mimics the return of Cerrado rains. Fry hatch large relative to body size and can consume Artemia nauplii from their first day. They grow quickly: sexual maturity is reached at approximately six weeks of age. Wild adults typically live seven to eight months before their pool dries; captive individuals kept in permanent water have survived up to about 1.5 years, but the fish are short-lived by any measure.
In the aquarium
Simpsonichthys parallelus is a demanding species recommended for experienced killifish keepers rather than general hobbyists. Its requirements are specific: very soft, ion-poor water (rainwater or RO water strongly preferred, as calcium sensitivity has been documented), temperatures in the 72–79 °F range during the season and cooler for conditioning and hatching, and a peat or coir substrate medium managed through a proper dry-storage incubation cycle. The fish are also rarely available in the trade, appearing sporadically through the specialist killifish keeper network (AKA and affiliated clubs) rather than through commercial channels.
A small aquarium of 10 US gal is adequate for a breeding trio. Dense surface planting with floating vegetation or fine-leaved plants diffuses light and reduces stress; the addition of dried leaves (Indian almond, beech) helps acidify and stain the water toward the tea-brown conditions of a Cerrado seasonal pool. Multiple females per male and visual breaks in the tank reduce aggression from persistent males. Despite its small size and habitat specialisation, the species rewards the investment of careful husbandry with spectacular male coloration and the remarkable spectacle of the egg-dive behaviour — one of the more theatrical acts in the freshwater hobby.
Conservation
The IUCN Red List assesses Simpsonichthys parallelus as Endangered (EN, criterion B1ab(iii)), assessed 2018, coordinated with the Instituto Chico Mendes de Conservação da Biodiversidade (ICMBio). The B1 criterion reflects that the species' extent of occurrence is below 5,0 mi², the ab(iii) qualifier that it is known from very few locations and that the quality of its Cerrado swamp habitat is in continuing decline.
The species is almost entirely confined to the Parque Nacional da Emas and its immediate surroundings, making the park's ecological integrity the primary determinant of the species' survival. The Cerrado biome — the world's most species-rich tropical savanna — has lost more than half of its original extent to soybean and cattle ranching, and remaining fragments face continuing conversion, altered hydrology from drainage works, and invasion by introduced species including tilapia and mosquitofish. Within the park boundary the immediate threats are more subtle — altered fire regimes, groundwater drawdown from surrounding agriculture, and the progressive drying of small seasonal pools as rainfall patterns shift — but the park is an island in a largely hostile agricultural matrix.
The species appears occasionally in specialist hobbyist collections coordinated through the American Killifish Association; maintaining captive stocks provides an insurance population, but the primary conservation priority is the protection and hydrological integrity of the Parque Nacional da Emas and the Cerrado seasonal swamps it contains.