Taxonomy & naming
Notholebias cruzi was described by Tulipano in 1973 and is placed, under the current treatment in Eschmeyer's Catalog of Fishes (Fricke, Eschmeyer & Fong), in the genus Notholebias — a South American annual killifish genus in the family Rivulidae. The authority is presented in parentheses, indicating that the species was originally described under a different generic combination before being transferred to Notholebias.
The genus Notholebias groups a small number of diminutive annual killies from southeastern Brazil, most of them extremely range-restricted endemics tied to the precarious temporary pool systems of the Atlantic Forest biome. These species were historically confused or lumped with adjacent genera, and the exact boundaries and species content of Notholebias have been revised repeatedly as molecular and morphological studies have clarified relationships among South American rivulids. The species epithet cruzi honours a personal or institutional name rather than a geographical feature, though the precise etymology is not documented in readily accessible literature.
Morphology
Notholebias cruzi is a small, slender-bodied annual killifish typical of the miniaturised rivulid guild. FishBase places the species in an epibenthic habitat context consistent with the bottom-dwelling behaviour expected of a substrate-spawning annual. Like other Notholebias, the fish is compact in build with a relatively flattened ventral profile suited to life near the mud surface of shallow seasonal pools.
Detailed meristic and morphometric data from the original 1973 description are not accessible in summarised secondary sources. In common with related annual rivulids of southeastern Brazil, males are presumed to show more vivid colouration than females — the typical pattern among sexually dimorphic annual killies — but specific colour descriptions are not available from the sources reviewed. Given the species' likely near-extinction, colour and pattern are very poorly documented in modern literature.
Habitat
Notholebias cruzi is native to the Saquarema basin in the Southeastern Atlantic coastal drainages of Rio de Janeiro state, Brazil — a highly endemic region for freshwater fish that sits within one of the world's most heavily urbanised and deforested coastal landscapes. Its habitat consists of temporary seasonal wetlands: shallow, organically rich pools that form during the wet season and desiccate completely during the dry months. Research published in Natureza & Investigação recorded water conditions in these pools as distinctly acidic (pH 6.4 ± 0.2) with low dissolved oxygen (2.0 ± 0.9 mg/L), consistent with the warm, oxygen-poor, tannin-stained ephemeral waters typical of Atlantic Forest lowland pools.
The biotopes supporting N. cruzi lie outside formal conservation units — a critical vulnerability. The species' entire known range falls within an area subject to rapid and ongoing urban expansion, and researchers documented the destruction of the wetlands where it was collected. The species has not been recorded from any wild locality since 2002, a gap of more than two decades that strongly suggests severe population decline or local extinction rather than mere under-sampling.
Feeding
Specific dietary data for Notholebias cruzi are not available in the literature reviewed. Annual rivulids from the southeastern Brazilian Atlantic Forest are broadly microinvertivore-insectivore in their feeding ecology, taking zooplankton, small aquatic invertebrates, insect larvae, and surface-fallen terrestrial invertebrates from the water column and substrate of their shallow pools. The epibenthic foraging style noted in FishBase data for this species is consistent with bottom-gleaning of benthic invertebrates and organic detritus in the mud surface layer.
In aquarium settings, small annual rivulids of this type accept live and frozen invertebrate foods readily — Artemia nauplii, Daphnia, micro-worms, and finely chopped chironomid larvae are typical fare. Given the species' precarious conservation status, any aquarium specimens held in managed-care programmes would be fed in accordance with standard protocols for miniature annual killies.
Mating
Notholebias cruzi is an annual killifish that reproduces by substrate-spawning with diapausing eggs — the defining reproductive mode of the South American annual killi guild. In nature, pairs spawn by diving briefly into the soft mud, peat, or detritus at the pool bottom, depositing eggs directly into the substrate. The eggs are buried and left without parental attention; the adults continue to feed and spawn episodically until the pool dries. When the water disappears, the adults perish — their entire above-ground life cycle compressed into a single wet season — while the eggs enter diapause, a state of arrested embryonic development that allows them to survive weeks to months in dry substrate.
FishBase records an incubation period of approximately four months for this species, consistent with a diapause-dependent annual life history. Sexual dimorphism, with males typically brighter-coloured, is the pattern in related annual rivulids and is presumed present here, though detailed field observations of this species are extremely limited.
Breeding
In the hobby and in ex-situ conservation programmes, annual rivulids like Notholebias cruzi are maintained on the standard peat-moss or coconut coir spawning medium protocol. A layer of soft, slightly moistened peat is provided on the aquarium floor; the pair spawns by pressing into and diving through this medium, depositing eggs in the substrate over multiple sessions. After a few weeks the peat is removed, gently squeezed to remove excess moisture, and stored in a sealed plastic bag or container at room temperature for the diapause period — for this species, FishBase indicates approximately four months.
After the storage period, the peat is placed in a shallow container and rehydrated with soft, slightly acidic water (pH around 6.0–6.5, consistent with the wild pH of 6.4 recorded for its biotopes). Eggs hatch within 24–72 hours in favourable conditions. Fry are relatively large at hatching compared to non-annual rivulids and can take Artemia nauplii and micro-worms from first feeding. Given the species' reported difficulty in aquarium maintenance — noted in FishBase reference data — stable soft acid water and careful attention to incubation timing are particularly important. FishBase notes it is difficult to maintain, a likely reflection of its sensitivity to water chemistry and the short active-season window that characterises extreme annual species.
In the aquarium
Notholebias cruzi is documented as difficult to maintain in aquarium conditions (FishBase, citing reference 27139). This is not unusual for highly locality-specific annual killies from the southeastern Brazilian Atlantic Forest, which are adapted to a narrow set of soft, acidic, warm conditions and short active seasons, and which do poorly when water parameters deviate significantly from their natural range. A dedicated killifish specialist setup — small species-only tank, soft acidic water around pH 6.0–6.5, a temperature matching the warm subtropical climate of Rio de Janeiro state (approximately 72–79 °F), peat substrate, dim lighting, and minimal water movement — is the appropriate environment.
The species has no common English name and is essentially unknown outside specialist annual killifish circles. It does not appear in mainstream ornamental fish trade channels. Any individuals in captivity are almost certainly held by conservation-minded killifish hobbyists or in institutional managed-care programmes aimed at preserving the species' genetics pending habitat restoration or formal conservation assessment. The American Killifish Association has a species profile for N. cruzi and represents the primary community through which captive individuals might be located.
Conservation
Notholebias cruzi is one of the most conservation-critical freshwater fishes in South America, though it has not received a formal IUCN Red List assessment. Research published in peer-reviewed journals documents that its type locality and all other known collection sites in the Saquarema basin fall outside any formal protected area, and that the temporary wetlands constituting its habitat have been destroyed by urban expansion in the Rio de Janeiro metropolitan region. Field surveys have failed to locate the species since 2002 — more than two decades of absence from its known range. The literature characterises it as on the edge of survival or potentially extinct in the wild.
The Saquarema basin sits within the Southeastern Atlantic coastal drainages, one of the world's most imperilled freshwater ecoregions: a narrow coastal strip of the Atlantic Forest biome facing intense and decades-long pressure from the growth of greater Rio de Janeiro. Temporary pools — already marginal and isolated habitats — are particularly vulnerable to drainage for housing and infrastructure, and species with no other known sites have no refuge when their sole wetland is filled. A study of diversity and endemism patterns in this region (MDPI, 2025) notes the concentration of IUCN-assessed critically endangered killifishes in this ecoregion, in which N. cruzi sits as an unassessed but likely equally threatened taxon.
In the absence of a formal assessment, targeted field surveys to determine whether any population persists are an urgent priority. Parallel ex-situ conservation through specialist killifish breeders offers the only realistic insurance against total extinction if wild populations cannot be confirmed.