Taxonomy & naming
Simpsonichthys rosaceus was described by Wilson J. E. M. Costa and Gisélia Brasil in 1990, giving the authority (Costa & Brasil, 1990). Under the Catalog of Fishes (Eschmeyer, Fricke & van der Laan), the name Simpsonichthys rosaceus remains the currently accepted valid combination, though the species has also circulated in the hobby and literature as Ophthalmolebias rosaceus following a 2006 subgeneric proposal by Costa that was subsequently elevated to full genus rank in 2010. Hobbyist databases and the American Killifish Association (AKA) primarily list it under Ophthalmolebias rosaceus, and much of the conservation literature uses that name as well.
The genus Ophthalmolebias sensu Costa (2010) contains six species distributed across coastal Atlantic drainages of eastern Brazil, with O. rosaceus at the northernmost part of the group's range in Bahia. Within the broader family Rivulidae (South American annual and non-annual killifishes), these species represent the small-eyed, soil-spawning annual clade characteristic of seasonal savanna pools. The specific epithet rosaceus is Latin for 'rosy' or 'rose-coloured', alluding to the pinkish or rose hue of the male's flanks.
Morphology
Simpsonichthys rosaceus is a compact killifish reaching approximately 2 in total length (FishBase), typical for the Ophthalmolebias group. The body form follows the standard rivulid plan: a laterally compressed, moderately deep trunk tapering to a rounded caudal fin, with the dorsal fin set well back — positioned posterior to the midpoint of the body, as in most annual New World killies.
Males are the more vividly coloured sex. The flanks carry the rosy to pinkish colouration implied by the specific epithet, often overlain with iridescent blue-green or turquoise scales that produce a jewelled, reticulate pattern. The fins, particularly the dorsal and caudal, typically display marginal banding or spotting in contrasting colours. Females are cryptically coloured in brownish or olive tones with little obvious patterning, and are somewhat more rounded in the belly region. Sexual dimorphism in colour and fin development is pronounced, as is typical for annual rivulids in which male display drives reproductive success in the compressed breeding window available each rainy season.
Habitat
Simpsonichthys rosaceus is endemic to the Rio Pardo basin in Bahia state, south-eastern Brazil — a drainage in the transition zone between Atlantic Forest and cerrado savanna that reaches the Atlantic coast below Salvador. Its habitat consists of small, temporary pools, flooded grassland depressions, and seasonal floodplain puddles that form during the rainy season and dry completely in the dry season, typically within months. The substrate in these pools is soft — fine mud, detritus, and accumulated peat-like organic material into which the fish spawn.
Water conditions in the pools of this region tend toward near-neutral pH: the AKA records pH 7 and a hardness of approximately 5 dGH, indicating soft but not markedly acidic water — distinct from the blackwater acid habitats occupied by some Amazonian annuals. Temperatures in Bahian seasonal pools fluctuate with the environment but remain in the warm tropical range for most of the fish's brief adult life. The pools are often shallow, sun-warmed, and highly productive in invertebrates during the wet season.
Feeding
Like other small annual rivulids, Simpsonichthys rosaceus is a microcarnivore in the wild, feeding opportunistically on small invertebrates — aquatic insect larvae, microcrustaceans, worms, and zooplankton that proliferate in the temporary pools following inundation. The brief adult life span of an annual killifish means feeding must be efficient: fish grow rapidly from hatching and must reach sexual maturity, spawn, and die within a single wet season, often spanning only a few months.
In captivity the species accepts a range of small live and frozen invertebrate foods readily. Daphnia, Artemia nauplii and adults, bloodworm (chironomid larvae), and small tubifex are all appropriate. Dried and pelleted foods may be taken but are less reliably accepted, and a predominantly live-food diet produces better colour, condition, and spawning behaviour. Overfeeding or poor water quality from uneaten food is the most common captive management error in annual killies.
Mating
Simpsonichthys rosaceus is an annual substrate-spawning killifish: the pair dive together into the soft mud or peat substrate at the bottom of the pool and deposit eggs below the surface. This spawning mode, shared by all Ophthalmolebias species and the broader rivulid annual clade, is the key adaptation to life in ephemeral habitat — it places the eggs beyond the surface desiccation layer, where they can survive the dry season in diapause.
Male display is elaborate. The vivid rosy and iridescent colouration of the male functions as a signal in pair formation; males court females with fin-spreading and lateral displays, and where multiple males are present, intense competition occurs. The compressed time window of the adult phase — typically only a few months before the pool dries and adults die — places a strong selective premium on rapid and successful mating. The fish do not form lasting pair bonds; both males and females may spawn with multiple partners across the season.
Breeding
Egg deposition occurs in the substrate: the male and female press together and dive into or press against the soft bottom, releasing eggs and sperm into the sediment. Eggs are deposited singly or in small numbers per dive, with repeated spawning bouts over days or weeks while conditions allow. The eggs are large relative to body size, with a tough chorion and resistance to desiccation — they enter diapause I or diapause II (arrested developmental stages) in the substrate and remain viable through the dry season, which may last several months.
In the aquarium, breeding Simpsonichthys rosaceus follows the standard annual killifish protocol. Pairs are placed over a container of soft peat or coco-peat spawning medium; eggs are collected by removing and gently wringing the peat, then placing it in a sealed bag or container with moist air for a dry-storage period of approximately three to four months at around 77 °F. After the storage period, the peat is flooded with soft, slightly acid water. Fry emerge within hours to days and must be fed immediately with infusoria or commercial fry powder, progressing quickly to freshly hatched Artemia. Growth is fast; the species can reach maturity in eight to twelve weeks under good conditions.
In the aquarium
Simpsonichthys rosaceus is kept by annual killifish specialists worldwide and is reasonably available in AKA and other killifish club circles, where it is exchanged as eggs in peat. It is not a fish for the general community aquarium — its small size, short adult life span, and specific care requirements place it firmly in the specialist niche. A pair or a small group (one male, two or three females) can be maintained in a tank as small as 10 US gal, kept dimly lit and furnished with a dark substrate and floating plants to reduce stress and bring out male colouration.
Water should be soft (around 5 dGH) and near neutral (pH 7.0), matching the Bahian pool parameters recorded in the field. Temperature in the range of 72–79 °F is appropriate. The fish are short-lived — adults rarely survive beyond eight to twelve months even under optimal conditions — so maintaining a breeding line requires regular egg management. The species is generally peaceful but males will display to one another; multiple males in a small tank can cause fin damage, so the ratio of females to males matters.
A reliable peat-spawning container (a small plastic tub of moist coco-peat placed on the bottom) is the essential piece of equipment. Eggs are harvested every two to four weeks, stored dry, and hatched in rotation to maintain a continuous population.
Conservation
The IUCN Red List assessed Simpsonichthys rosaceus (as Ophthalmolebias rosaceus) as Vulnerable (VU) under criterion D2 in 2018, reflecting its narrow, single-basin endemic range in the Rio Pardo drainage, Bahia, Brazil. The Rio Pardo basin sits in a highly threatened landscape: the Atlantic Forest and cerrado transition zone of Bahia has experienced severe deforestation and conversion to pasture, sugarcane, eucalyptus plantations, and — more recently — urban expansion associated with the growing towns of the region.
Seasonal pools and temporary floodplain habitats, which are ecologically marginal and unprotected by most Brazilian conservation frameworks, are among the most heavily impacted freshwater environments in this region. Drainage for agriculture, infilling for construction, and water extraction for irrigation all eliminate the ephemeral pool habitat on which O. rosaceus depends. The SciELO freshwater fishes of Bahia survey confirms the species as a Bahian endemic (specimen MZUSP 62572), and its restricted distribution means any further reduction in suitable habitat could push it toward Endangered or Critical status.
The genus Ophthalmolebias as a whole is under pressure from the same forces between the Rio Cachoeira (Bahia) and Rio São João (Rio de Janeiro), according to the It Rains Fishes genus overview. Hobbyist breeding programmes contribute to ex-situ conservation of this and related species, but the maintenance of wild habitat remains the only long-term safeguard for populations adapted to their local pool environments.