Taxonomy & naming
Spectrolebias brousseaui was described by Nielsen (2013) in the journal Ichthyological Exploration of Freshwaters, based on material collected from a temporary pool in the upper río Mamoré basin, Departamento Santa Cruz, Bolivia. The valid name as listed in Eschmeyer's Catalog of Fishes is Spectrolebias brousseaui Nielsen, 2013, placed in the family Rivulidae (now often treated within the broader Cyprinodontiformes as subfamily Cynolebiatinae), which encompasses the South American annual killifishes.
The genus Spectrolebias was established by Costa and Nielsen in 1997 from material originating in the Araguaia basin of Brazil, and at the time of Nielsen's 2013 paper it contained roughly ten valid species. Phylogenetic studies place Spectrolebias within Cynolebiatinae, the clade of New World annual killifishes defined by their obligate soil-spawning reproduction and diapausing eggs. The American Killifish Association classifies the genus under Class 3 — South American Annuals — the standard organisational unit used in the killi hobby for this ecological guild.
The specific epithet honours Roger Brousseau, though the published description does not elaborate on which Roger Brousseau is commemorated. Patronyms of this kind in killifish taxonomy typically recognise field collectors, conservationists, or long-serving society members who contributed materially to the discovery or study of the group.
Morphology
Spectrolebias brousseaui is a small fish: FishBase records a maximum standard length of 1.5 in and a total length of 1.5 in, placing it among the smaller members of its genus. The body shape follows the Cynolebiatinae pattern — moderately deep and laterally compressed, with a rounded head, upturned mouth, and the broad, rounded caudal fin of a species that lives in still, shallow pools rather than flowing water.
Male colouration is the most distinctive feature of the species. The anterior third of the body is light brown, giving way abruptly to a dark blue ground colour that covers the posterior two-thirds of the flanks, and this blue field is overlaid with bright blue iridescent spots arranged in vertical alignment — a pattern not precisely replicated in any congener. The dorsal and anal fins carry long filamentous extensions at their tips, a character shared within the genus only with S. filamentosus. The pelvic fins are separated by a distinct gap — versus the contact position seen in most other Spectrolebias — and males bear contact organs (small, hardened projections used in male–male and male–female tactile encounters) on the scales of the flanks, a character absent in all other known members of the genus.
Females, as is typical across annual killifishes, are cryptically coloured — brownish and lacking the iridescent blue pattern — and do not develop the filamentous fin extensions or contact organs of the male.
Habitat
The type locality is a temporary pool in the upper río Mamoré basin, Departamento Santa Cruz, eastern Bolivia. The Mamoré is a major tributary of the Madeira River and drains a broad lowland region of Bolivia and Brazil; in the upper reaches where this species was collected, the landscape produces ephemeral pools that fill during the wet season and desiccate completely in the dry season — the defining habitat of South American annual killifishes.
FishBase records water parameters from collections associated with this species at approximately pH 6.8, a temperature of 88 °F, and a hardness of around 80 mg/L. These figures are consistent with warm, slightly soft, near-neutral seasonal pools on the Bolivian lowland plain, and differ markedly from the acid blackwater habitats occupied by some other Spectrolebias and Cynolebias species further east. The water is likely turbid during peak wet season and dwindles to warm, concentrated, shallow remnants before evaporating entirely.
Because the species is known from a single temporary-pool collection and the IUCN assessment flags data deficiency, the full extent of its range within the Mamoré basin is unresolved. The American Killifish Association database records eleven location points, suggesting it is not a pure single-site endemic but occupies a cluster of sites within the broader basin.
Feeding
No specific dietary studies exist for Spectrolebias brousseaui in the literature captured in the research record. By inference from the ecology of South American annual killifishes in similar temporary-pool habitats, the species is a generalised carnivore. Small invertebrates — aquatic insect larvae, ostracods, microcrustaceans, and worms — form the bulk of the diet during the brief wet season when the pools are populated. Because these pools receive pulses of invertebrate productivity as water levels rise, annual killies typically feed heavily and grow fast to reach sexual maturity before their habitat disappears.
In the aquarium, Spectrolebias brousseaui and its congeners readily take live and frozen invertebrate foods: Artemia nauplii and adults, Daphnia, Moina, chironomid larvae (bloodworm), tubifex, and similar items are all appropriate. Dry foods may be accepted but should be supplemented with live or frozen prey to maintain condition and encourage spawning behaviour. Overfeeding in the confined space of a species tank should be avoided, as uneaten food degrades water quality quickly in the small, soft-bottomed setups used for annual killies.
Mating
Spectrolebias brousseaui is an annual killifish — the most unambiguous mating classification in the family. In the wild, spawning takes place during the brief window when the temporary pools are full, and the pressure of that finite season drives a rapid, intense reproductive effort. Males are polygamous and compete for access to females; the contact organs on the flanks, diagnostic for this species, are tactile structures used during close-range male–male assessment and during the spawning embrace itself.
Courtship in Cynolebiatinae annuals typically involves lateral displays by the male, spreading the fins to reveal colour and assessing the female's receptivity. Once a pair is ready, they position side-by-side and dive in synchrony into the soft substrate, burying themselves shallowly while eggs and sperm are released into the mud. The act is rapid and repeated multiple times during a spawning session. Contact organs are thought to help the male maintain grip and positioning during the substrate dive, explaining their evolutionary importance in species with this mode.
Breeding
The reproductive biology of Spectrolebias brousseaui is that of a classic annual peat-spawner. Eggs are deposited directly into the soft substrate — mud, detritus, or peat in nature — during the spawning embrace, and are left buried when the pool dries. The embryos enter diapause, a state of developmental arrest that allows the eggs to survive months of desiccation in the dry substrate. Hatching is triggered by re-wetting when the rains return, completing the cycle in synchrony with the returning wet season.
In the hobby this life history is managed on a peat or coconut coir spawning medium placed at the bottom of the tank. After a productive spawning period, the medium is removed, lightly pressed to remove excess water, and stored in a sealed plastic bag at room temperature for a diapause period — typically three to five months for Spectrolebias species, though the exact duration for brousseaui is not documented in the sources examined. Re-wetting the stored medium triggers hatching, and fry are small but robust enough to take Artemia nauplii from the first day.
Generation time in annual killies is under twelve months: wild fish hatch, grow, spawn, and die within a single wet season. This extreme pace means that in captivity, populations must be actively maintained through successive spawning and hatching cycles, and hobbyists typically overlap generations to avoid losing a line entirely.
In the aquarium
Spectrolebias brousseaui is an advanced specialist's fish. The typical setup is a small species tank — 10–15 US gal is ample for a pair or trio — kept bare-bottomed or with a thin layer of soft substrate and a deep peat or coir pot for spawning. Subdued lighting suits the species; floating plants or a dark background reduce stress and bring out the male's iridescent blue colouring. Filtration should be gentle, as the still, shallow-pool habitat is not a high-flow environment, and the soft-bottomed breeding substrate must not be disturbed by strong currents.
Water should be soft to moderately hard and slightly acidic to neutral — targeting the parameters recorded at collection (around pH 6.8, 88 °F, 80 mg/L hardness) provides a reasonable starting point. Because the species is from warm Bolivian lowlands, temperatures above 82 °F are well tolerated, and a maintained aquarium at 79–88 °F suits the species year-round. Tankmates are impractical for productive breeding and unnecessary for species this small and specialised; a species-only or single-genus setup is standard practice.
Within the American Killifish Association classification system, S. brousseaui is a Class 3 annual, and members maintain it through regular spawning rounds, sharing eggs and fry through club fish auctions and the Annual Fish / Catfish Study Group networks. The species is not commonly found in mainstream aquarium retail and is primarily available through specialist killi societies.
Conservation
The IUCN Red List assessed Spectrolebias brousseaui as Data Deficient (DD) in 2016, with the assessment carried out by Jaime Sarmiento. The DD listing reflects the limited information available on the species' precise range, population size, and trend — not a conclusion that it is safe. Annual killifishes inhabiting temporary pools in the Bolivian lowlands face a suite of threats that are well documented for the habitat type: agricultural drainage and conversion of seasonal wetlands, changes to flood-pulse hydrology driven by land clearance, cattle grazing directly into the pools that form their habitat, and the introduction of mosquitofish (Gambusia spp.) that prey on eggs and fry and compete aggressively with small killifishes.
The upper río Mamoré basin has seen significant deforestation and expansion of cattle ranching and agriculture in Departamento Santa Cruz, one of Bolivia's most rapidly developing agricultural frontiers. Seasonal pools in this landscape are among the most poorly protected freshwater habitats in South America — they lack permanence and are rarely mapped or considered in environmental-impact assessments, making them invisible to regulatory protection even when the species that depend on them are of conservation concern.
Because S. brousseaui is maintained by killifish hobbyists through the AKA and affiliated European killi societies, an ex-situ insurance population exists, but its genetic diversity relative to wild populations is unknown. A field survey to clarify the species' range, abundance, and habitat condition across the Mamoré basin would be the minimum needed to move it out of Data Deficient toward a meaningful threat category.