Taxonomy & naming
Hypsolebias trilineatus was described by Wilson J. E. M. Costa and Gustavo C. Brasil in 1994, and their authorship is placed in parentheses in the current combination, indicating that the species has been moved from the genus in which it was originally placed. Eschmeyer's Catalog of Fishes (Fricke, Eschmeyer & Van der Laan) recognises Hypsolebias trilineatus (Costa & Brasil, 1994) as the valid name for this taxon. The synonym Hypsolebias trilineatus appears in older literature and some hobbyist sources; Killi-Data, Jean Huber's authoritative killifish database, lists it as subsumed under the current valid combination.
The genus Simpsonichthys was erected to contain a radiation of annual killifishes found in the seasonal drainage systems of central and eastern Brazil. The placement of this and related genera has been revised multiple times as molecular and morphological phylogenies have refined our understanding of the South American aplocheiloid killies, a lineage that diversified independently of their Old World counterparts. The specific epithet trilineatus is Latin for 'three-lined', referring to the pattern of longitudinal stripes or rows visible on males of the species.
This fish was long listed as Simpsonichthys trilineatus; Eschmeyer's Catalog of Fishes places the species in Hypsolebias, and the current combination Hypsolebias trilineatus is followed here.
Morphology
Hypsolebias trilineatus is a small killifish reaching approximately 2.4 in total length (FishBase). The body is slender and laterally compressed, consistent with the body plan shared by most Simpsonichthys, with the dorsal fin set well back toward the caudal peduncle — a silhouette typical of the soil-spawning South American annuals. The caudal fin is rounded to slightly pointed.
Sexual dichromatism is pronounced, as is characteristic of the genus. Males display vibrant, iridescent colouration, with the three longitudinal lines or stripe-rows from which the species takes its name running along the flanks. Females are cryptically coloured in olive or straw tones with little pattern, an adaptation that reduces predation risk when the male is absent and the female guards the egg-deposition site. The degree of colour intensity in males depends strongly on condition, water quality, and the presence of rivals.
Habitat
This species is native to the São Francisco River basin in Brazil, a large and ecologically diverse drainage that descends from the central Brazilian plateau through the semi-arid Caatinga region before turning eastward to the Atlantic. Hypsolebias trilineatus inhabits seasonal pools and shallow temporary water bodies within this basin — small lentic or near-lentic habitats that fill during the rainy season and contract or disappear entirely during the dry months.
The soft, silty or peaty substrate of these pools is essential: it is into this material that spawning pairs dive to bury their eggs. Water conditions in the São Francisco basin vary by location, but seasonal pools in the Caatinga and cerrado transition zones tend toward warm, soft to moderately hard, near-neutral water; the cache records a pH range of 6.5–7.5 and temperatures of 73–82 °F as appropriate parameters for this species in the hobby context. The pools are typically shallow, with abundant aquatic and emergent vegetation during the wet season, and are surrounded by seasonally inundated savanna or dry-forest habitat.
Feeding
In the wild, Hypsolebias trilineatus feeds opportunistically on the small invertebrates — insect larvae, microcrustaceans, worms, and zooplankton — that proliferate in seasonal pools during the wet season. The short life cycle places a premium on rapid growth and reproduction, and the fish feeds aggressively to fuel both.
In captivity the species thrives on a varied diet of live and frozen foods: Daphnia, Artemia (brine shrimp) nauplii and adults, Chironomid (bloodworm) larvae, Tubifex, and similar items are all accepted with enthusiasm. High-quality dry foods can supplement the diet but should not form its basis for a fish as colour-sensitive as this one. The feeding response is strong and competitive; multiple males in a tank will display and compete at feeding time, which also serves as a useful indicator of overall health and condition.
Mating
The mating system of Hypsolebias trilineatus is that of a substrate-diving annual killifish. Males establish loose territories and display intensely to females and rivals, flaring fins and presenting their full lateral colouration. When a female accepts a courting male, the pair move together to the substrate and the male drives the female downward into the soft mud or peat. Both fish disappear briefly below the surface layer of the substrate, where eggs and sperm are released and the eggs are deposited in the sediment. The pair then separate; no pair bond persists between spawning bouts.
Females produce a small number of eggs per dive — typically one to a few — and the process is repeated across multiple dives spread over hours or days. This trickle-spawning strategy maximises the spatial distribution of eggs within the substrate and reduces the risk that a single desiccation event destroys an entire clutch at a single depth. The eggs are large relative to the fish's body size, with tough, oil-rich yolks that fuel long periods of arrested development.
Breeding
Embryonic diapause is the defining biological feature of Hypsolebias trilineatus and all other annual killifishes. After the eggs are buried in the substrate, development proceeds through an initial phase and then enters diapause — a genetically programmed cessation of development analogous in function to the dormancy of plant seeds. In this state the embryos can survive for months in damp substrate even as the pool above them evaporates completely. Development resumes when the substrate is re-wetted by the next season's rains, and hatching follows within days to weeks. The entire cycle — hatching, growth to sexual maturity, reproduction, and death — takes place within a single wet season, often less than six months.
In the aquarium, this mode is replicated using a peat or coconut-coir spawning medium in the tank bottom or in a separate spawning container. After a spawning period of several weeks the medium is removed, gently squeezed to reduce moisture, and stored in a sealed bag or container at room temperature for an incubation period of roughly three to seven months before re-wetting. The eggs hatch rapidly once re-wetted; fry are relatively large and can take Artemia nauplii almost immediately. The keeper must monitor development by examining a small sample of eggs under magnification to assess when embryos are ready to hatch. Because adults age rapidly and die, maintaining this species in the long term requires consistent cycling of spawning and incubation batches.
In the aquarium
Hypsolebias trilineatus is best suited to a dedicated killifish setup rather than a community aquarium. A small species tank of 11–16 US gal is sufficient for a pair or a trio (one male, two females); additional males can be kept if the tank is large enough to provide visual separation, but male-to-male aggression can be intense. The substrate should be deep enough for the fish to dive and spawn — at least 2–3.1 in of fine, soft peat, coconut fibre, or a peat-sand mix. Floating plants or surface vegetation are useful to diffuse light and create a dimmer, more natural environment that encourages colour expression and reduces stress.
Water should be kept warm (73–82 °F), soft to moderately hard, and slightly acidic to neutral (pH 6.5–7.5). Gentle filtration is preferable to strong flow, as these are fish of still or near-still pool environments. Regular partial water changes maintain water quality without introducing large temperature or chemistry swings. The species is not long-lived even under ideal conditions — wild fish rarely survive beyond a single season — and aquarium specimens typically live one to two years at most. This short lifespan is part of the appeal for dedicated killiphiles, who manage breeding and egg-storage rotations as a deliberate part of the hobby.
Conservation
Hypsolebias trilineatus has not been formally assessed by the IUCN Red List and carries a status of Not Evaluated (NE). This is common among the Brazilian annual killies: the group is species-rich, taxonomically dynamic, and concentrated in a region where habitat loss is ongoing, making comprehensive assessment a slow process.
The São Francisco River basin faces significant pressure from water extraction for irrigation, hydroelectric development, and the advance of large-scale agriculture across the cerrado and Caatinga biomes. Seasonal pools — the only habitat of annual killies — are among the first features lost when land is cleared, drained, or converted. The introduction of mosquitofish (Gambusia spp.) and other invasive species into Brazilian seasonal waters poses an additional direct threat through predation on eggs and fry. Several congeners in Simpsonichthys and related genera have been flagged as threatened or critically endangered by regional Brazilian assessments, and it is reasonable to assume that H. trilineatus, as a specialist of seasonal pools within a single river basin, faces analogous pressures even without a formal global assessment. Conservation of the remaining seasonal-pool habitats in the São Francisco basin is the most effective safeguard for this and many other endemic annual killifish species.