Taxonomy & naming
Simpsonichthys boitonei was described by J. C. Carvalho in 1959 in the Boletim do Museu Nacional, Rio de Janeiro (no. 201, p. 5), based on material collected from a temporary pool in the area of the Zoological Garden near Brasília, at the time still recorded under the state of Goiás and corresponding to what is now the Distrito Federal (approximately 15°45′S 48°00′W). It is the type species of the genus Simpsonichthys — the species around which the whole genus is anchored — and its authority is presented in parentheses because the species has been moved from the original genus since its description. Eschmeyer's Catalog of Fishes (Fricke, Eschmeyer & Fong) records the valid name as Simpsonichthys boitonei (Carvalho, 1959) in the family Rivulidae, the South American annual killifishes.
The genus Simpsonichthys was erected to accommodate a group of brilliantly coloured Brazilian annual killifishes from the cerrado and adjacent savanna biomes. These fishes share the annual life-history strategy that defines the subfamily: their habitat pools disappear on a predictable seasonal cycle, and the entire above-ground population dies with the dry season while a cohort of diapausing eggs buried in the substrate bridges the gap to the next rains. S. boitonei is thus a flagship for both the genus and for the conservation plight of cerrado-associated freshwater fauna.
FishBase records dorsal fin ray counts of approximately 22 and anal fin rays of approximately 18, confirming the relatively high meristic counts typical of Simpsonichthys compared with allied genera. The species has also been recorded from the São Bartolomeu River basin and Lake Paranoá, expanding the known range marginally beyond the type locality but keeping it firmly within the footprint of the modern Federal District.
Morphology
Simpsonichthys boitonei is a compact killifish with FishBase recording a maximum total length of 2 in, though many sources place typical adult males closer to 1.5 in TL. The body plan is laterally compressed and moderately deep — characteristic of the genus — with large, rounded fins on the male that create a striking display profile. The dorsal and anal fins are set far back on the body, nearly opposite each other, and together with the broad caudal fin give the male a flag-like silhouette in display.
Sexual dichromatism is pronounced. Males are brilliantly coloured: the flanks display vivid iridescent blues, greens, and sometimes red or orange tones arranged in scale-by-scale dots that give rise to the common name 'pearlfish'. Unpaired fins are similarly patterned with coloured rays and marginal bands. Females, as in most annual killifish, are cryptically coloured — pale olive to tan with inconspicuous speckling — and are noticeably smaller and less deep-bodied than males. This degree of dimorphism is among the most extreme found in the Rivulidae.
Habitat
Boiton's pearlfish is native to the Upper Paraná River basin in central Brazil, specifically the tributaries, seasonal pools, and associated drainages of the São Bartolomeu River basin and Lake Paranoá catchment within the Distrito Federal. The type locality is a temporary pool adjacent to the Brasília Zoological Garden — a site that now sits within one of Brazil's most intensively urbanised landscapes.
The species occupies shallow, ephemeral pools embedded in cerrado (Brazilian savanna) vegetation. These pools typically form with the onset of the wet season (October–March in this part of central Brazil) and dry down completely during the dry months. Water chemistry at such sites tends to be soft and slightly acidic: FishBase records conditions of pH 6.6 and hardness of approximately 6 °dH, consistent with the leached, lateritic soils of the cerrado plateau. Temperatures in the wet season average around 75 °F. The pools support aquatic insects, zooplankton, and microinvertebrates — the food base that fuels the rapid growth and early maturation annual killies require to complete their life cycle before pool desiccation.
Feeding
In the wild, S. boitonei feeds on the small invertebrates available in its shallow, productive pool habitats: zooplankton (copepods, cladocerans), small aquatic insects and their larvae, and any other similarly sized organisms. Given the seasonal nature of these pools, feeding is necessarily intensive — the fish must grow, reach sexual maturity, and spawn all within the months the pool persists, leaving no margin for poor condition.
In the aquarium the species accepts a wide range of suitably sized live and frozen foods without difficulty. Small live foods — Artemia nauplii (which newly hatched fry can tackle immediately), Daphnia, Moina, micro-worms, and tubifex — are all taken. Frozen bloodworm and similar midge-larva products are accepted by adults. High-quality, varied feeding is important for maintaining colour and conditioning breeders before a spawning period.
Mating
Simpsonichthys boitonei is an annual killifish with a substrate-spawning, soil-diving mating system. When a male encounters a female in breeding condition, he displays laterally — fanning his large, brilliantly coloured dorsal and anal fins — before driving her toward the substrate. The pair then dive together into the soft mud, peat, or detritus that forms the pool floor, pressing their bodies together and releasing eggs and sperm directly into the substrate. This 'diving' or 'plunging' spawning act deposits eggs one or a few at a time, buried where they will be protected from desiccation as the pool shrinks.
Male competition for females in close-quartered pool habitat can be vigorous; males use lateral display and sometimes physical chasing to establish breeding priority. Because the wild pools are small and can support high densities during the early wet season, male-male interactions are frequent. In captivity, experienced keepers typically maintain one male with two or more females to distribute male attention and reduce injury risk to individual females.
Breeding
The eggs of S. boitonei enter diapause after deposition — a state of developmental arrest that allows them to survive the complete drying of the pool, which may last several months. In the wild, rains eventually re-wet the substrate and break diapause, triggering synchronised hatching of a new generation. FishBase notes that eggs can hatch as quickly as five to six weeks post-spawning under optimal conditions, but the standard incubation period cited by specialist sources is approximately 2.5 to 3 months at 77–82 °F — a period during which the eggs are kept moist but not submerged in damp peat or coconut fibre.
In the aquarium, spawning is carried out on a layer of peat moss or fine coir fibre at least 2–3 in deep placed on the tank floor. After a spawning period of several weeks, the substrate and eggs are removed, lightly pressed to remove excess water, sealed in a labelled plastic bag or container, and stored in a warm (77–82 °F) location for the incubation period. Sensitivity to calcium hardness is noted: water used to hatch and rear fry should be very soft, ideally rainwater or RO water. Newly hatched fry emerge at a relatively large size for a rivulid and can immediately accept freshly hatched Artemia nauplii. Growth is rapid; sexual maturity is reached at approximately six weeks of age. Captive lifespan may reach 1.5 years under optimal care, though wild fish complete their full life cycle in approximately seven to eight months.
In the aquarium
Boiton's pearlfish is rated an advanced species in the hobby, demanding experience with annual killifish management. The species requires a dedicated, species-specific setup: a small, well-covered aquarium of 10–15 US gal with a substantial layer of fine peat or coir on the floor for spawning. A tight-fitting lid is essential, as killifish are accomplished jumpers. Water should be very soft and slightly acidic (matching the recorded wild parameters: pH around 6.6, hardness around 6 °dH, temperature near 75–79 °F), and rainwater or RO water is strongly preferred — particularly for raising fry, which are reported to be sensitive to calcium.
Because adults are short-lived, maintaining a rolling incubation schedule — spawning, removing and dating peat bags, hatching in staggered batches — is central to keeping the species continuously. Males should be housed with two or more females; a single female under constant male attention can be harassed into poor condition quickly. The American Killifish Association maintains an active network of members who exchange egg-peat bags, making this species more accessible than its Endangered status might suggest, though wild-caught fish are rarely offered in trade.
The vibrant colouration of males makes S. boitonei one of the most visually rewarding South American annuals for the dedicated specialist, and its status as the type species of Simpsonichthys gives it additional prestige among killifish enthusiasts. It is not a beginner fish and does not belong in a community aquarium; however, for keepers willing to learn the annual-killi rhythm of spawn, dry, wait, and hatch, it is a deeply satisfying species to maintain.
Conservation
The IUCN Red List assessed Simpsonichthys boitonei as Endangered (EN) in May 2022, under criteria B1ab(iii,iv), reflecting a severely restricted and declining extent of occurrence centred on the Distrito Federal and the São Bartolomeu River basin tributaries. The core threat is straightforward: Brasília, which did not exist when the species was described in 1959, is now one of Brazil's largest cities, and the cerrado pools that constitute the entire known range of this fish have been eliminated, fragmented, and degraded at pace with urban and residential expansion across the Federal District. Drainage of seasonal wetlands for construction, road building, and agricultural conversion directly destroys habitat; downstream water quality degradation and alteration of the hydrological regime of tributaries further impairs what habitat remains.
The 2022 assessment also identified biological resource use — including collection pressure for the ornamental fish trade — as a secondary threat. Given the species' small total range and the degree to which its type locality and surrounding pools have already been urbanised, even moderate collection from remaining wild populations carries meaningful risk. There are no protected areas that specifically encompass the known population sites.
Conservation of S. boitonei will ultimately depend on the protection and restoration of cerrado seasonal wetlands within and around the Federal District — a broader challenge shared by a suite of endemic freshwater fauna in this biome. The species is maintained in captivity by killifish enthusiasts worldwide, providing a demographic buffer, but captive populations do not substitute for wild habitat. The American Killifish Association and affiliated national clubs maintain studbook-style exchange networks for species such as this one, which represents one of the more achievable forms of ex-situ contribution to Endangered annual killifishes.