Taxonomy & naming
Spectrolebias reticulatus was first described by Wilson J. E. M. Costa and Gert Nielsen in 2003 as Simpsonichthys reticulatus, published in the journal Aqua (volume 7, number 3, page 120), with the type locality recorded as Altamira, Pará state, Brazil — floodplain pools of the Rio Xingu drainage. The authority is given in parentheses because the species has since been moved: subsequent molecular and morphological work placed it in the genus Spectrolebias Costa & Nielsen, 1997, a small clade of annual rivulids distributed across the Araguaia, Xingu, Mamoré, and Chaco basins of central and western South America. Eschmeyer's Catalog of Fishes treats Spectrolebias reticulatus (Costa & Nielsen, 2003) as the valid combination.
Spectrolebias comprises roughly ten recognised species, all annual killifish of seasonal temporary pools. The genus name is a compound of Latin spectr- (appearance, image) and the Greek lebias, a diminutive applied historically to small carp-like fishes, alluding to the small size and distinctive patterning of the clade. The specific epithet reticulatus is Latin for 'net-like' or 'reticulate', an explicit reference to the male colour pattern in which dark pigment concentrated on the exposed margins of the flank scales produces a fine mesh or net across the body — the character most useful for distinguishing this species from its congeners in the field and in hobbyist collections.
Morphology
Spectrolebias reticulatus is an extremely small fish: FishBase records a maximum standard length of 1 in for males and 0.5 in SL for females, placing it among the smallest annual killifish in South America. The body is laterally compressed with the rounded profile typical of rivulid annuals and the relatively large eyes characteristic of a fish that hunts by sight in shallow, dimly lit seasonal pools.
Male coloration is the species' most conspicuous feature. The body sides carry a greenish-blue ground colour with dark red to crimson pigmentation concentrated on the posterior exposed margins of each flank scale, producing the distinctive reticulate net pattern for which the species was named. Finnage is similarly ornate, with males displaying colours in the unpaired fins consistent with the body pattern. Females are notably drabber — pale to brownish with reduced or absent patterning — as is typical across annual rivulids where sexual selection drives extreme male ornamentation. Sexual dimorphism is therefore strongly marked: males and females look little alike, and field identification of females requires association with a male or comparative reference material.
Habitat
Spectrolebias reticulatus is known from seasonal temporary pools on the floodplains of the Rio Xingu in Pará state, Brazil, with collection material from the vicinity of Altamira. The iNaturalist record confirms the species also occurs in Paraguay, suggesting the range may extend into the Chaco drainage, consistent with the broader distribution of Spectrolebias across multiple central South American basins.
Like all annual rivulids, it inhabits pools that are present only during the wet season. These are typically shallow, warm, humus-rich waterbodies over soft peat and mud substrates; the killifische.info record notes a substrate of peat and humus with water depths of 0–3 ft. Water chemistry in seasonal Xingu pools is soft and only mildly acidic to near-neutral: combined sources give a pH range of approximately 6.5–7.0, temperature 68–77 °F, and hardness of 2.5–5 dH. These are relatively moderate parameters for a South American annual killi, distinctly less acidic than the blackwater habitat of some Amazonian congeners. The pools are ephemeral — filling with the rainy season, then contracting and drying completely, often over a period of only a few months, which imposes the defining constraint of the annual life strategy.
Feeding
Spectrolebias reticulatus is a small-bodied carnivore that feeds primarily on tiny invertebrates within the water column and at the surface of shallow seasonal pools. At less than 1 in standard length, prey items are necessarily minute: zooplankton, small insect larvae, aquatic microcrustaceans, and other invertebrates small enough to be engulfed by a fish of this size. FishBase classifies the species as omnivorous, though for a rivulid of this scale the dietary base is effectively invertebrate protein.
In the aquarium, Spectrolebias reticulatus should be offered appropriately sized live and frozen foods: newly hatched brine shrimp nauplii, micro-worms (Panagrellus redivivus), small Daphnia, Moina, and similar zooplankton-scale items. Dry foods will be accepted by some individuals, particularly if offered in small granule or powder form, but live foods are important for conditioning breeding pairs and maintaining the full expression of male colour.
Mating
Spectrolebias reticulatus is a substrate-spawning annual killifish in which the male courts the female through fin-display and lateral presentation of his reticulate flank pattern. The reticulate coloration is a direct product of sexual selection: in the compressed, shallow environment of a seasonal pool it serves as a species-recognition signal and quality indicator, allowing females to assess mate quality before committing to spawning. Courtship is energetic and males are persistent — in a confined aquarium without adequate space or cover females can be harassed, making plant or peat moss refugia important.
Once a female accepts the male's display, the pair adopt a side-by-side spawning position and dive together into the soft peat or mud substrate. Eggs are deposited individually, buried in the substrate during each brief dive. Unlike the non-annual killifish (which scatter adhesive eggs among plants or in fine-grained substrate without burying them), annual rivulids actively press eggs into the substrate, placing them in a location where they will remain protected during the dry period.
Breeding
The reproductive biology of Spectrolebias reticulatus is organised entirely around the annual drying cycle of its seasonal pools. After spawning, the eggs are left buried in the peat or mud substrate; the adults continue feeding and spawning over the remaining weeks or months of the wet season, accumulating eggs in the substrate, and then perish when the pool dries — the generation investing all reproductive effort into the buried egg bank rather than in parental care or survival through the dry season.
The buried eggs enter a state of developmental arrest (diapause) and remain viable for weeks to months in dry or damp substrate while the pool is absent. When the rains return and the pool refills, the eggs hatch and the next generation begins. In the aquarium, breeders replicate this cycle by spawning fish over a deep layer of soft peat moss or coconut coir, then periodically removing and lightly drying the medium for a storage period of six to twelve weeks before re-wetting to trigger hatching. The spawning medium should be kept just damp — not soaking wet — during storage. Fry are small and require correspondingly tiny first foods (infusoria, paramecia) before progressing to nauplii. Growth is rapid and fish can reach reproductive maturity within a few months, consistent with the short natural season.
In the aquarium
Spectrolebias reticulatus is a dwarf annual killi suited to nano and species-only aquaria. A pair or small group can be maintained comfortably in a tank of 5.5–10 US gal or even less, provided water quality is stable. The tank should be densely planted (Java moss, floating plants) or at minimum furnished with peat or dried leaf litter on the base to provide visual cover and reduce male-on-female aggression. A lid is advisable as killies are capable jumpers.
Water conditions should reflect the moderate soft-water environment of the Xingu floodplains: temperature 72–77 °F, pH 6.5–7.0, very soft to soft water (under 5 dH). Regular partial water changes are important — a small tank housing actively spawning fish accumulates waste quickly, and the tannin-stained, humus-rich conditions of a seasonal pool are very different from a neglected and fouled aquarium. Because the species is so small, tankmates are limited to similarly miniature, peaceful fish; many keepers maintain them alone to focus on breeding and ensure fry are not predated. At under 1 in the fish cannot cohabit safely with most other aquarium species.
Spectrolebias reticulatus is an uncommon species in the hobby, encountered mainly through specialist killifish societies and breeders. Its tiny size, vivid male coloration, and annual breeding cycle make it an appealing project fish for dedicated killifish keepers; the relatively moderate water parameters (compared to highly acidic blackwater annuals) make it somewhat more accessible for hobbyists without access to very soft or acidic water.
Conservation
The IUCN Red List assessed Spectrolebias reticulatus as Near Threatened (NT) in 2022. The designation reflects the species' restricted range — it is an endemic of the Rio Xingu floodplain system in Pará state, Brazil, with possible occurrence in Paraguay — and the ongoing transformation of Xingu floodplain habitats. The Belo Monte hydroelectric complex on the lower Xingu, one of the largest dam projects in Amazonian history, has substantially altered the hydrology and seasonal flood pulse of the river; seasonal floodplain pools of exactly the kind that annual killies depend on are among the habitat types most vulnerable to changes in flood timing and magnitude caused by dam operation and upstream deforestation.
Like many single-basin annual killifish, Spectrolebias reticulatus has no buffer of redundant populations across a broad range — loss of key Xingu floodplain pools through hydrological change, deforestation-driven sedimentation, or direct conversion for agriculture represents a potentially irreversible threat to wild populations. The species' extremely short life cycle and the dependence on the buried egg bank in the substrate mean that events which prevent the wet-season pool from forming even one year (such as severe drought exacerbated by deforestation) can locally extirpate a population with no possibility of recolonisation until conditions return. Monitoring the status of Xingu floodplain pools and the annual killi communities they support is a meaningful conservation priority for the Xingu basin's freshwater biodiversity.