Taxonomy & naming
Simpsonichthys nigromaculatus was described by Wilson J. E. M. Costa in 2007 as a new miniature seasonal killifish from the upper rio Paraná basin of central Brazil. GBIF records the full citation as Costa, W.J.E.M. (2007) and places the species within Teleostei, Cyprinodontiformes, Rivulidae — the neotropical rivulids, the largest family of annual killifishes. The authority appears in parentheses in the standard Catalog of Fishes convention because the genus combination used today differs from the original publication context.
Within Simpsonichthys, the species is regarded as closely related to S. parallelus on the basis of shared morphological characters. The name nigromaculatus is Latin: niger (black) + maculatus (spotted), referring to the 2–4 black spots distinctive on the dorsal fin of males — a character that helps separate this species from allied taxa. Catalog of Fishes (Eschmeyer, Fricke & van der Laan) treats Simpsonichthys nigromaculatus (Costa, 2007) as the valid name.
Morphology
This is one of the smallest members of the genus: males reach 1 in SL, females 1 in SL (FishBase). Body depth in males is 33.5–36.0% SL. A diagnostic peculiarity of the species — shared with some other Simpsonichthys — is the complete absence of pelvic fins and the pelvic girdle; meristic counts include 24–25 vertebrae and 17–20 anal-fin rays in females.
Male coloration is striking for such a small fish. The flanks carry bright blue bars; the head displays five alternating bright blue and pink-to-red bars. The dorsal, anal, and caudal fins are edged with a dark grey band, and the caudal fin is scattered with yellow dots. The dorsal fin bears 2–4 black spots — the character encoded in the species name. Females are, as is typical for annual killies, considerably more subdued in coloration, presenting a plain brown or olive body without the iridescent barring of the male.
Habitat
The type locality and known range of S. nigromaculatus lie in temporary shallow canals and floodplain pools in the drainage of the rio da Prata, a tributary system within the upper rio Paraná basin of central Brazil. These are seasonal wetlands: sun-lit, open, and only roughly 19.5 in deep at their maximum. Water chemistry recorded at the type locality is light yellow (slightly stained) with a pH of approximately 5.5 — mildly acidic but not strongly blackwater. Temperature data are consistent with lowland tropical seasonal pools in the Brazilian cerrado-pantanal transition zone, with warm dry-season air and cooler, wetter conditions during flooding.
The temporary nature of these habitats is the defining ecological fact for this species. The canals fill during the wet season rains, sustaining a brief growing season in which the fish hatch, mature, spawn, and die before the water disappears. The bare floodplain soils and shallow depths leave the eggs — buried in the substrate — to wait out the dry season in diapause.
Feeding
No detailed dietary study has been published for S. nigromaculatus specifically. By analogy with other miniature Simpsonichthys and annual rivulids of similar size and habitat, the wild diet is presumed to consist primarily of small invertebrates — microcrustaceans, aquatic insect larvae, and zooplankton — that colonise seasonal pools rapidly after refilling. The short growing season and small mouth of this species are consistent with a diet of fine-particle live prey rather than larger invertebrates or plant material.
In captivity, the species responds well to small live and frozen foods: baby brine shrimp (Artemia nauplii), micro-worms, daphnia, and similar items of appropriate size. Prepared micro-pellets formulated for small killifishes are accepted by conditioned fish. A varied diet with a strong live- or frozen-food component promotes the brightest male coloration and best reproductive condition.
Mating
Simpsonichthys nigromaculatus is an annual killifish, and its reproductive strategy is built entirely around the seasonal flood-drought cycle of its floodplain habitat. Courtship is initiated by the male, who displays his brilliant blue barring and yellow-dotted caudal fin to the female in a characteristic lateral posture. The display both signals fitness and positions the pair for spawning.
The defining feature of annual killifish mating is the substrate dive: rather than depositing eggs on plants or in open water, the pair sink together into the soft bottom material — peat, mud, or detritus — and release eggs and sperm simultaneously while partially buried. Each spawning dive deposits one or a small number of eggs. Males typically drive spawning activity repeatedly across the brief reproductive season, taking multiple mates as opportunity allows; female choice and male-male competition over display intensity are the principal selection axes.
Breeding
The eggs of S. nigromaculatus are left in the substrate after spawning. As the pools dry out, the eggs enter diapause — a state of arrested embryonic development that allows them to survive the complete desiccation of the pool, sometimes for many months. The eggs are resilient to drying, and successful incubation depends on the embryo passing through one or more obligate diapause phases (as described for Simpsonichthys and related genera) before completing development. Hatching is triggered by re-wetting when the rains return and the pool refills.
In the hobby, annual killifishes are bred by providing a peat moss or coco-peat spawning medium on the floor of the aquarium. After a period of spawning, the peat is removed, pressed lightly to remove excess water, sealed in a plastic bag, and stored at room temperature for a species-appropriate incubation period — typically two to four months for Simpsonichthys. The peat is then re-wetted with aged soft water and fry appear within hours to days. Newly hatched fry of miniature annual killies are very small and require infusoria or commercial fry powder as first food before graduating to Artemia nauplii. The fish grow rapidly and can reach sexual maturity within a few weeks under warm conditions, consistent with the compressed wild lifecycle.
In the aquarium
Simpsonichthys nigromaculatus is a specialist killifish for experienced keepers, not a mainstream community fish. Its small size (under 1 in), delicate water requirements, and the need for a dry-stored egg incubation routine place it firmly in the domain of killifish hobbyist societies such as the American Killifish Association. The rewards are considerable: the male's electric-blue and red-barred coloration is remarkable for a fish of this size, and the annual lifecycle — breeding, egg storage, and the dramatic emergence of fry from dried peat — is one of the more engaging experiences in the freshwater hobby.
Conditions should reflect the type locality: soft, slightly acidic water (pH around 5.5–6.5, low hardness), warm temperatures in the range of 72–79 °F, and subdued lighting over a dark substrate. A small, well-planted or peat-floored tank of 5.5–10 US gal is adequate for a pair or small group. Tankmates, if any, should be small, peaceful, and tolerant of soft acidic water; the safest approach is a species-only setup. Regular small water changes and the avoidance of large pH or temperature swings help maintain the fish in peak condition during their short lives.
Conservation
The IUCN lists Simpsonichthys nigromaculatus as Vulnerable (VU), reflecting the species' restricted and fragmented range within a landscape under significant human pressure. Its entire known distribution lies in the seasonal floodplain canals of the rio da Prata drainage, upper rio Paraná basin — a region of central Brazil where agricultural expansion, particularly large-scale soybean and cattle operations in the cerrado and pantanal margins, has driven extensive drainage and modification of the shallow seasonal wetlands that annual killies depend on.
Annual killifishes are among the most inherently vulnerable of freshwater fish because their populations consist entirely of eggs in the soil during the dry season: a single episode of soil disturbance, drainage for irrigation, or conversion of floodplain to pasture at the wrong moment can eliminate a local population with no adult survivors. The species' restricted range means that the loss of even a small number of floodplain canal systems could result in significant range contraction or extinction. No captive-conservation programme is currently documented for this species, though hobbyist collections through killifish societies represent a de facto insurance population. Surveying the full extent of its range and quantifying habitat loss remain priority needs.