Taxonomy & naming
Pterolebias phasianus was described by the Brazilian ichthyologist Wilson J.E.M. Costa in 1997. Eschmeyer's Catalog of Fishes (Fricke, Eschmeyer & Fong) records the valid combination as Pterolebias phasianus Costa, 1997, placing it within the family Rivulidae — the Neotropical annual killifishes — and specifically within the rachovine clade, a group of South American annuals known collectively to killifish specialists as the Rachovia group.
The genus name Pterolebias derives from the Greek pteron (wing or fin) and lebias (a small fish of classical authors, applied to small cyprinodont-like fishes). The compound name alludes to the distinctively elongated, wing-like fin extensions that characterise males of the genus — the dorsal, caudal, and anal fins develop exaggerated filamentous rays that give displaying males a striking, almost baroque appearance. The specific epithet phasianus is the Latinised form of the Greek phasianos, meaning pheasant, a direct reference to the richly patterned, iridescent colouration of the male that recalls the ornamental plumage of a pheasant cock.
Phylogenetically, Pterolebias sits within the seasonal South American annual killifish radiation and shows close affinity to the genera Rachovia and Austrofundulus based on shared morphological and molecular characters. The rachovine assemblage is united by annual life histories tied to the savanna wetlands and flooded grasslands of the Orinoco and Paraguay drainages.
Morphology
Pterolebias phasianus is a moderately sized annual killifish. FishBase records a maximum total length of 3 in, though the hobbyist literature — including the It Rains Fishes database — gives a working maximum of around 2 in for most captive individuals. As with many annual killies there is pronounced sexual dimorphism in size as well as colour, with males typically larger and more ornate than females.
Males are among the more spectacular of the Pterolebias species. The body is deep and laterally compressed, and the unpaired fins carry the elongated, filamentous rays characteristic of the genus: the dorsal and caudal fins in particular develop extended points that flutter during courtship displays. Colouration in breeding males is vivid — the body shows an iridescent pattern of metallic blue-green to turquoise scales overlaid with reddish-brown to chestnut reticulations and spots, with the fin extensions edged in contrasting colours. The overall effect evokes the iridescent eye-spots and barring of pheasant plumage, explaining the species epithet.
Females are considerably plainer: the body is more uniformly brownish or olive, the fins are rounded and unelaborated, and the iridescent body markings are reduced to a faint freckling. This extreme dimorphism is common across the rachovine annuals and reflects strong sexual selection operating over the brief adult life span, during which males must display and compete effectively to secure matings before the pool dries.
Habitat
Pterolebias phasianus is native to the Paraguay River basin of South America. This drainage encompasses seasonally inundated savannas, flooded grasslands (the Pantanal and surrounding lowlands), and gallery-forest pools across parts of Brazil, Paraguay, and Bolivia — one of the largest seasonal wetland systems on Earth. Within this landscape the species occupies the most ephemeral of habitats: shallow temporary pools and flooded depressions that form during the wet season rains, persist for weeks to months, and then evaporate completely in the dry season, leaving the substrate bare.
These pools are typically shallow (often less than 19.5 in at peak water level), with soft, organic-rich substrates — accumulated leaf litter, fine detritus, and mineral mud — into which spawning fish burrow to deposit eggs. Water chemistry in the Paraguay lowlands tends toward warm and moderately acidic to near-neutral: FishBase records a thermal range of 72–82 °F, consistent with the seasonally warm lowland climate. The pools accumulate dissolved organic material as they age through the wet season, producing soft, tannin-stained water typical of many South American seasonal wetland habitats.
Because the entire water body disappears in the dry season, P. phasianus cannot coexist with most other fish — its ephemeral habitat is defined by the absence of permanent fish communities. The species is therefore both specialised for conditions (low oxygen, high temperatures, seasonal extremes) that would exclude less tolerant fishes, and reliant on the precise annual regime of flood and drought that structures the savanna wetland cycle.
Feeding
Like most annual killifishes, Pterolebias phasianus is a carnivore whose wild diet is composed primarily of small invertebrates: aquatic insects and their larvae, microcrustaceans, zooplankton, and other small animals that colonise seasonal pools during the wet season. The brevity of the active life cycle places a premium on rapid growth, so feeding is essentially continuous while the pool persists.
In the aquarium, the It Rains Fishes database notes that P. phasianus thrives on live food offered in rich variety. Small live invertebrates — Daphnia, Artemia nauplii, bloodworm (chironomid larvae), tubifex, and small live blackworms — are ideal for conditioning adults and triggering spawning behaviour. Frozen equivalents are accepted by most individuals. The fish should be fed frequently and generously, consistent with the compressed natural life history: rapid growth, early maturity, and high reproductive output demand consistent energy intake.
Mating
Pterolebias phasianus is a substrate-spawning annual killifish. Male courtship is elaborate and conspicuous: displaying males erect the elongated dorsal and caudal fin extensions, intensify their body colouration, and swim alongside or below the female in lateral and S-curve displays that are thought to communicate male quality. Males compete with one another at close range, engaging in parallel displays and occasional brief physical encounters.
The It Rains Fishes account describes both sexes as peaceful with one another by the standards of annual killies, and multiple pairs can be maintained in the same aquarium without serious aggression, provided space and structure are adequate. Once a receptive female accepts a male, the pair move together to the substrate. Pterolebias males are notable among rivulids for the prominently elongated dorsal fin that arcs over the female during the spawning embrace — a behaviour described as a canopy display — before the pair descend together to the sediment surface.
Breeding
Spawning in Pterolebias phasianus is a substrate-diving event: the pair press close together and plunge briefly into the soft substrate — peat, mud, or detritus — releasing and burying eggs with each dive. Each dive deposits one to a few eggs; breeding sessions produce a modest number of eggs per encounter, spread across repeated dives over days or weeks. The buried eggs are the sole mechanism for surviving the dry season, and the embryos enter a state of arrested development — diapause — in which metabolic activity falls to a minimum, allowing survival in dry or nearly dry substrate for months.
In the hobby, breeding follows a well-established protocol. The aquarium is furnished with a deep layer of peat moss or coconut coir as spawning substrate. Pairs are conditioned on live foods and allowed to spawn for several weeks. The peat is then removed, partially dried (squeezed but not completely dried out), sealed in a plastic bag labelled with the date, and stored at room temperature for three to four months — the It Rains Fishes database records an incubation period of three to six months, while FishBase gives approximately four months. At the end of the storage period the peat is re-wetted with soft, slightly acidic water, and fry typically hatch within hours to days. Newly hatched fry are small but immediately free-swimming and accept Artemia nauplii as a first food. The It Rains Fishes account notes that the peat substrate should be changed every two weeks during the active spawning phase to prevent fouling, and that the aquarium requires a secure lid with some height above the water to accommodate the leaping behaviour of adult fish.
In the aquarium
Pterolebias phasianus is a rewarding fish for the annual killifish specialist, but it demands care practices quite different from conventional aquarium keeping. The It Rains Fishes database rates it as moderately difficult, appropriate for hobbyists with some experience of annual killifish management. The most important departure from standard practice is the spawning-and-storage cycle: success depends on correctly drying, storing, and re-wetting the peat-packed eggs on the appropriate schedule, and on providing correctly sized aquariums equipped with both a secure lid and adequate headroom to prevent escape.
The aquarium should be small to medium in size — a species-only or pair tank of 10–15 US gal is appropriate. Filtration should be gentle (annual killies are sensitive to strong currents, and the temporary-pool habitats they come from are still water). Temperature is maintained at 75–82 °F. The peat substrate layer should be at least 2 in deep to allow unobstructed spawning dives. Décor of driftwood and floating vegetation will be used for shelter and display. Water changes of around 20–30% weekly maintain quality.
The species is not suited to mixed community tanks: most companion fish are potential predators or competitors, and the specific water conditions and management regime required are best maintained in a dedicated setup. Within a species aquarium, multiple pairs can coexist if space allows. The spectacular male fin extensions and the active courtship displays make P. phasianus an especially attractive species for the patient hobbyist willing to engage with the full annual cycle.
Conservation
The IUCN Red List assessed Pterolebias phasianus as Least Concern (LC) in December 2020 (assessment ID 164535241). The assessment notes no immediate documented conservation threats specific to this species. Its distribution in the Paraguay River basin — which includes the vast Pantanal floodplain system — gives it association with one of the largest remaining intact tropical wetland complexes on Earth, and the scale and seasonal resilience of that system buffers many of its inhabitants from the most acute habitat losses affecting smaller-range specialists.
However, the broader threats facing South American seasonal wetlands are not trivial and affect P. phasianus along with many other annual killifishes. Agricultural conversion of savanna and seasonal floodplain habitats — principally for soy cultivation and cattle ranching — has accelerated across the Cerrado and Chaco regions that border the Pantanal. Temporary pools used by annual killies are often among the first features to be lost when land is levelled for cultivation, since they lack the permanence and connectivity that attracts legal protection. Drainage infrastructure built for agriculture or flood control directly eliminates the fill-and-dry cycle on which annual killies depend.
The species is maintained in the collections of annual killifish specialists worldwide, including American Killifish Association members, providing an ex-situ insurance population. This hobbyist network, which keeps and exchanges eggs through the peat-storage protocol, has historically served as an unplanned but effective conservation buffer for several annual killifish species whose wild populations have contracted. Continued monitoring of the Paraguay basin's seasonal wetland habitats and formal population surveys would improve confidence in the LC assessment for future review cycles.