Taxonomy & naming
Spectrolebias costai was described by Wilson J. E. M. Costa and Uwe K. Nielsen in 1997. Eschmeyer's Catalog of Fishes recognises the name as valid within the genus Spectrolebias, placed in the family Rivulidae (South American annual killifishes). The genus Spectrolebias is characterised by the brilliant iridescent lateral scales visible in breeding males — the 'spectro-' prefix (from Latin spectrum, meaning image or appearance) refers to this shimmering optical quality, while 'lebias' is a traditional killi suffix derived from the Greek lepis (scale). The species epithet costai honours the Brazilian ichthyologist Wilson Costa, whose extensive work on Neotropical annual killifishes underpins the taxonomy of a large fraction of the Rivulidae.
Spectrolebias belongs to the South American annual killifish radiation, a clade that has independently evolved annual life-history traits — diapausing eggs that survive substrate desiccation — in response to the highly seasonal savanna and wetland landscapes of central and interior Brazil, Paraguay, Bolivia, and the Orinoco llanos. Within that broad clade, Spectrolebias is most closely associated with the Tocantins–Araguaia and adjacent basins. The 1997 description placed costai alongside other Spectrolebias species known from nearby drainage systems, and subsequent revisions of the rivulid tree have not disturbed its generic placement.
Morphology
Spectrolebias costai is a compact killifish, reaching approximately 1.5 in standard length — typical for annual rivulids of the central Brazilian savanna. The body plan is that of a laterally compressed, moderately deep-bodied fish with the rounded caudal fin and upturned mouth characteristic of the genus. The lateral scales of breeding males display the iridescent quality that defines the genus: metallic blues and greens with a lampeye-like sheen that intensifies under natural or point-source lighting.
Sexual dimorphism is pronounced, as in most annual killifishes. Males are more intensely coloured throughout, with vivid body iridescence and distinct pigment patterns on the unpaired fins. Females are cryptically coloured — drab olive-brown to tan — with little ornamentation, an adaptation common to fish whose females make rapid spawning decisions in dense, short-lived populations. This contrast is not merely ornamental; it reflects the intense sexual selection operating in annual species where a male's entire reproductive career may span only weeks.
Habitat
Spectrolebias costai is endemic to the Araguaia–Tocantins river basin in central Brazil. Its specific habitats are the temporary seasonal pools, ponds, and shallow wetland margins that form across the cerrado (Brazilian savanna) landscape when the wet season brings heavy rains, typically from October through April. These pools are characteristically shallow — often only 4–15.5 in deep — warm, heavily vegetated with floating and emergent aquatic plants, and floored with soft organic-rich mud and detritus.
Water chemistry in cerrado seasonal pools reflects the soft, slightly acidic conditions of tropical lateritic soils: the FishBase record gives pH 6.0–6.8 with hardness around 8 dH and temperature 75–86 °F — the warmth of the dry-season sun and the chemistry of mineral-poor seasonal runoff. Dissolved oxygen can be low in these dense, stagnant pools, and surface-breathing and tolerant physiology are important. The IUCN assessment confirms that this species is recorded from Araguaia National Park on Bananal Island, the world's largest river island, situated at the confluence of the Araguaia and Tocantins rivers in Tocantins and Pará states. The extent of its range within the basin beyond this locality is not well documented.
Feeding
In the wild, Spectrolebias costai feeds on the small invertebrates abundant in productive seasonal pools: aquatic insect larvae, microcrustaceans, worms, and other zooplankton-scale prey. The upturned mouth is well adapted for surface and mid-water feeding, and the short lifespan of annual species puts a premium on efficient energy acquisition — these fish must grow from egg to reproductive adult in a single wet season, and a rich diet of live prey supports that rapid growth.
In the aquarium, the species is typically offered live or frozen small invertebrates: Artemia nauplii, Daphnia, Moina, small Tubifex, and bloodworm. Dry foods are accepted by some individuals once the fish is established, but live and frozen foods remain the preferred diet for conditioning breeding adults and growing fry. The relatively small adult size limits prey to appropriately small items.
Mating
Spectrolebias costai is a substrate-spawning annual killifish — the pair spawns not among plants but directly into the soft sediment of the pool floor. Males advertise with their iridescent lateral scales and extended fin displays, pursuing females actively in the shallow water column. Pairing is rapid and sequential; in a productive pool males may court multiple females during the wet season.
The spawning act itself is brief and repeated: the male curves alongside or beneath the female, and together they dive into or press down against the soft substrate — peat, mud, or detritus — releasing eggs and sperm simultaneously into the sediment matrix. Eggs are left buried and unguarded. In natural pools, spawning continues opportunistically throughout the wet season until water levels fall and the fish perish. The species belongs to the guild of annual annuals whose entire adult existence is spent racing the dry season.
Breeding
The defining trait of Spectrolebias costai's reproductive biology is the diapausing egg. Each egg, buried 0.5–2 in in the substrate, can survive complete desiccation of the pool through the dry season — months of drought — because the embryo enters a state of arrested development (diapause) that allows it to halt and resume development in response to environmental moisture cues. FishBase records an incubation period of approximately six months for this species under the peat-storage conditions used in the hobby, though the precise duration varies with storage temperature and humidity.
In the aquarium, hobbyists replicate this cycle using a peat-spawning protocol: the breeding pair is placed over a layer of peat moss or coconut coir on the tank floor. After several weeks of spawning, the peat is removed, gently squeezed to remove excess moisture, and sealed in a labelled plastic bag or container. The bag is stored at room temperature (around 75–79 °F) for approximately four to six months, then the peat is re-wetted with soft, slightly acidic water in a shallow dish or small tank. Fry hatch within 24–72 hours of re-wetting and are immediately free-swimming and hungry. First foods must be very small — freshly hatched Artemia nauplii or micro-worms. Growth is rapid under warm conditions; the fish reach sexual maturity in weeks to a few months, reflecting their compressed natural life cycle.
Parental care is entirely absent — eggs and fry are on their own from the moment of spawning.
In the aquarium
Spectrolebias costai is regarded as a moderately challenging annual killifish — rewarding for the hobbyist willing to maintain the peat-storage cycle but not a species for beginners expecting typical aquarium fish behaviour. Adults are short-lived; even under ideal conditions most individuals will not survive much beyond a year, and the hobbyist's attention must be directed at maintaining a robust stock of incubating eggs.
The breeding aquarium is typically small — a 5.5–10 US gal tank with a dark substrate or a peat layer in a separate spawning container, dim lighting to match the murky seasonal pools, and soft, slightly acidic water matching the wild chemistry (pH 6.0–6.8, temperature 75–82 °F, low hardness). Filtration should be gentle — a small sponge filter is appropriate — as strong flow is not found in the pools this species inhabits. A tight-fitting lid is essential; killies are accomplished jumpers. The species is best kept as pairs or small groups of one male to one or two females; males may spar but rarely cause serious injury at the scale typical of annual rivulids. Tankmates, if any, should be similarly sized and non-aggressive.
This species is maintained primarily by killifish specialist societies such as the American Killifish Association and the British Killifish Association, whose members exchange eggs through club postal auctions and spawning-peat distribution networks — the natural distribution format for a fish whose young are most practically shipped as buried eggs in moist peat rather than as live fish.
Conservation
The IUCN Red List assessed Spectrolebias costai as Least Concern in November 2018, citing the absence of identified major threats and the occurrence of the species within the Araguaia National Park — a substantial protected area on Bananal Island at the confluence of the Araguaia and Tocantins rivers. The protected-area overlap provides meaningful security for at least part of the population.
However, the broader context for annual killifish in the Brazilian cerrado is one of significant long-term pressure. The cerrado is one of the world's most biodiverse savanna biomes and one of the most heavily converted: large-scale soy and cattle agriculture has removed or degraded vast areas of cerrado habitat, and the seasonal wetlands and temporary pools that annual killifishes depend on are disproportionately affected by drainage for agriculture and the alteration of seasonal hydrology through deforestation of catchments. Species with restricted ranges and highly specific habitat requirements — small seasonal pools in a single basin — are inherently vulnerable to such landscape-scale change. The IUCN noted the population trend as unknown, reflecting limited monitoring data. No active conservation programme specifically targets this species.