Taxonomy & naming
Hypsolebias radiosus was originally described as Simpsonichthys radiosus by Wilson J. E. M. Costa and Geovane Brasil in 2004, based on material collected from seasonal floodplain pools near Formosa, Goiás, in the upper Tocantins drainage. The type locality is specified as the floodplains of the Rio Crixás, approximately 4 mi from its confluence with the Rio Paranã, at an elevation of about 2625 ft. The species was transferred to the genus Hypsolebias by Costa in 2006 and the genus was subsequently elevated from a subgenus of Simpsonichthys to full generic rank in 2011. Eschmeyer's Catalog of Fishes recognises the valid combination as Hypsolebias radiosus (Costa & Brasil, 2004); the parenthetical authority reflects the fact that the original genus has since changed.
Hypsolebias is a large genus of South American annual killies in the family Rivulidae, concentrated in the seasonal river basins of central and northeastern Brazil. Hypsolebias radiosus belongs to a superspecies complex that includes H. auratus, H. gibberatus, H. nielseni, H. notatus, H. rufus, H. similis, H. stellatus, H. trilineatus, and H. virgulatus — a cluster of closely related forms occupying adjacent river drainages across the central Brazilian plateau. Population-level designations used by hobbyists and the killifish association, such as GO 484 HVP 2017-15, encode the collection locality and expedition year and are important for maintaining strain integrity in captive breeding programmes.
Morphology
Hypsolebias radiosus is a small fish, reaching a maximum standard length of approximately 1.5 in (FishBase; It Rains Fishes). It exhibits the typical compressed, deep-bodied profile characteristic of the Hypsolebias genus. Meristic counts recorded for the species are: dorsal fin rays 24.5 (mean), anal fin rays 22.0 (mean), and 26 lateral-line scales — figures consistent with the broader superspecies complex.
Sexual dichromatism is pronounced. Males display the diagnostic colour pattern that defines the species: three to five vivid light-blue flame-shaped stripes on the caudal fin, alternating with subdistal light-blue spots — a combination not found in any of the closely related superspecies members and the primary character used to separate H. radiosus in the original description. The body ground colour in males is typically reddish to brownish with blue or greenish iridescence on the flanks and unpaired fins. Females are, as in all rivulids, considerably plainer — generally brownish and cryptic, with no significant fin ornamentation. The size and pattern difference between sexes is the most reliable field identification character.
Habitat
The species is known from seasonal floodplain pools on the Rio Crixás drainage near Formosa, Goiás, in the upper Tocantins basin, at an altitude of roughly 2625 ft above sea level. This portion of Brazil falls within the Cerrado biome — the vast tropical savanna of central Brazil characterised by a pronounced dry season during which shallow ephemeral pools and floodplain wetlands completely desiccate. The entire life history of H. radiosus is structured around this seasonal cycle.
The pools that H. radiosus inhabits are warm, shallow, and typically have soft substrates of mud, peat, and accumulated organic detritus into which the fish spawn and in which their eggs overwinter through the dry season. Water chemistry in Cerrado seasonal pools tends toward soft and moderately acidic conditions, though the specific parameters recorded for this species' type locality are not well documented in the available literature. Temperatures in the pool environment during the wet season range from warm to hot, consistent with the tropical central Brazilian climate. The restricted and highly specialised nature of this habitat makes the species inherently sensitive to any hydrological or land-use change affecting its known localities.
Feeding
Like all rivulid killies, Hypsolebias radiosus is a micro-predator in the wild, feeding opportunistically on small invertebrates — aquatic insect larvae, microcrustaceans, zooplankton, and any other invertebrate prey available in the ephemeral pool habitat. The diet is broadly typical of annual killifish of the Cerrado region, where food availability is tightly coupled to the flooding cycle: newly inundated pools produce blooms of invertebrate prey that fuel the rapid growth and early reproduction that annual fish must achieve before the pool dries.
In the aquarium, the species accepts a range of small live and frozen foods without difficulty. Artemia nauplii, micro-worms, daphnia, and small chironomid larvae are all suitable and preferred. Dried or prepared foods can be accepted once fish are accustomed to captivity, but live and frozen foods promote the best condition and coloration in males. Feeding frequency should match the fish's fast pace of life — generous daily feeding supports rapid maturation, which mirrors the natural environment where fish must reproduce quickly within a short wet season window.
Mating
Hypsolebias radiosus is an annual killifish with a substrate-diving spawning mode. Males are highly active and persistently court females, displaying their brilliant caudal-fin pattern laterally to demonstrate fitness. Courtship involves the male swimming alongside the female, quivering and flaring his fins to exhibit the blue flame pattern on the caudal. A receptive female signals acceptance, and the pair align side by side before executing the characteristic rivulid spawning dive.
Spawning occurs by the pair pressing together and driving down into the soft substrate — peat, mud, or a equivalent organic medium — where one or a few eggs are deposited and fertilised with each dive. The act is repeated multiple times per session. Each dive buries the eggs just below the substrate surface, where they will remain through the dry season in a state of arrested development called diapause. Because male annual killies of this group are strongly territorial and competitive, females benefit from visual cover and the presence of only one male per small tank to reduce harassment. The brilliant male caudal pattern is primarily a female-choice signal; males in good condition with fully developed blue flames are preferentially selected.
Breeding
The reproductive biology of H. radiosus follows the annual killifish pattern precisely. Eggs are deposited into the substrate over an extended period during the wet season; in the wild, the pool then desiccates and the eggs enter diapause — suspended development — inside the dried mud, resisting months of complete dehydration. Eggs resume development when the rains return and the pool refills, and hatch within 2–6 hours of being re-wetted, producing larvae ready to feed almost immediately.
In captivity, the standard technique is to maintain the spawning pair over a container of peat moss or coir fibre, which the fish treat as they would the natural substrate. After several weeks of spawning, the peat is removed, gently squeezed to remove excess water, placed in a labelled plastic bag with a small amount of residual moisture, and stored at 77–84 °F for 8–10 weeks — or at 70–75 °F for a slower, extended diapause. After the storage period, the peat is tipped into a shallow tray of soft, warm water, and fry hatch within hours. Hatchlings are large enough to accept baby brine shrimp nauplii and micro-worms from the first day. Sexual maturity is reached in approximately 5–6 weeks post-hatch, consistent with the rapid life cycle of an annual that must reproduce before its pool dries. Wild fish typically live 6–8 months; captive individuals under good conditions may reach 10–12 months.
In the aquarium
Hypsolebias radiosus is a suitable killifish for hobbyists with experience keeping South American annuals, and it is maintained by specialist killifish keepers affiliated with organisations such as the American Killifish Association. It is classified as a Class 3 fish (South American Annuals) in the AKA system. The short lifespan and requirement for periodic egg storage means the maintenance approach differs fundamentally from keeping non-annual fish — keepers must commit to managing dry-stored peat batches and scheduling regular re-wettings.
A small, well-planted species tank is ideal: a 5.5–10 US gal aquarium with a dark substrate, floating plants or surface cover for diffused lighting, and a tight-fitting lid (annual killies are capable jumpers). Water should be soft and slightly acidic to neutral, kept in the range of approximately 72–82 °F; the species does not require extreme chemistry but thrives in conditions that broadly mimic the soft, warm, slightly acidic Cerrado pool environment. One male per tank, or one male with one or two females, is the safest social arrangement; male–male aggression can be intense in confined quarters. Because of the small maximum size and the specific requirements of annual maintenance, H. radiosus is not recommended as a community fish and is better appreciated as a specialist's subject.
Conservation
The IUCN Red List assessed Hypsolebias radiosus as Data Deficient (DD) in 2018, reflecting the limited information available on the species' distribution, population size, and trends rather than confirmed security. The assessment acknowledges that the species may be rare and that its restricted type locality — seasonal floodplain pools of the Rio Crixás near Formosa, Goiás — makes it inherently vulnerable to localised threats.
The principal conservation concern for H. radiosus, as for most Brazilian Cerrado annual killies, is the progressive degradation and loss of the seasonal wetland habitats on which the species depends. Cerrado deforestation has accelerated dramatically over recent decades, driven by conversion to soybean agriculture, sugarcane production, and cattle ranching; this land-use change alters watershed hydrology, reduces the size and duration of ephemeral pools, and increases sedimentation. Introduced species — particularly Poecilia reticulata (the guppy) and other livebearers released by mosquito-control programmes — can establish in pools and compete with or predate on annual killifish fry and eggs. Climate change projections for central Brazil point toward longer, more intense dry seasons, which would further shorten the wet-season window available for the fish's entire reproductive cycle. The captive population maintained by specialist killifish keepers represents an important insurance collection for this and related Hypsolebias species, many of which are similarly restricted in range and similarly under-surveyed.