Taxonomy & naming
Spectrolebias filamentosus was described in 1997 by Wilson J. E. M. Costa, Barrera, and Sarmiento from material collected in the río Mamoré basin of Bolivia. The original combination placed the species in what was then understood as the broader aplocheiloid genus complex; subsequent phylogenetic work redistributed Neotropical annual killifishes and established Spectrolebias as a valid genus. Eschmeyer's Catalog of Fishes (CAS) accepts Spectrolebias filamentosus (Costa, Barrera & Sarmiento, 1997) as the valid name. The FishBase summary still occasionally references the former synonym Simpsonichthys filamentosus, which reflects an earlier lumping into the large South American annual genus Simpsonichthys before cladistic analysis resolved the finer groupings.
The genus Spectrolebias comprises nine valid species, all confined to seasonal drainage basins in the interior of South America — principally the Paraguay, Tocantins-Araguaia, Xingu, and Mamoré-Grande drainages across Bolivia, Brazil, and Paraguay. Members of the genus share several derived characters: males possess prominent contact organs on the flank scales (gland-tipped structures pressed against the female's body during spawning), and males of most species carry elongated filamentous extensions on the dorsal and anal fin rays that function as part of the breeding display. The authority is presented in parentheses because the species has been moved from its originally described genus.
Morphology
Adult males of Spectrolebias filamentosus reach approximately 2 in total length; females are somewhat smaller and more plainly marked, as is typical in annual killifishes with strong sexual selection. The body shape is deep and laterally compressed for a killifish, with a rounded head and an upward-tilted mouth suited to feeding near the water surface in shallow pools.
The most immediately striking features of the male are the elongated, thread-like rays projecting from the posterior edges of the dorsal and anal fins — the filaments that give the species its epithet. These extensions lengthen with sexual maturity and are more developed in dominant individuals. Male flanks carry contact organs on the scales, small glandular structures that produce chemical or tactile stimulation during the spawning embrace. Background coloration in males is iridescent with metallic blues and greens, overlaid with red or orange spotting and barring in patterns typical of the genus; females are drab olive-brown with faint mottling, which confers camouflage in the leaf-litter and detritus of the pool floor.
Habitat
Spectrolebias filamentosus is known from the río Mamoré drainage in Bolivia, a major tributary system of the upper Amazon whose Bolivian headwaters drain the eastern Andes foothills and the Llanos de Mojos — an immense, seasonally flooded savanna plain. During the wet season, shallow temporary pools form across this landscape; these fill with warm, slightly acidic, soft-to-moderately-hard water. Data from the closely related congener Spectrolebias brousseaui, described from the same upper Mamoré drainage in 2013, gives a window into likely conditions: surface temperatures in these pools can reach 88 °F under full sun while bottom layers stay at around 72 °F, with pH near 6.8 and hardness of approximately 80 ppm — conditions consistent with warm, gently acidic seasonal waters over a laterite or alluvial substrate.
The pools are typically shallow, densely vegetated at the margins, and floored with accumulated leaf litter, fine sediment, and detritus into which the fish spawns. As the dry season advances, pools shrink and eventually disappear entirely; the adult fish perish, but their eggs remain buried in the moist substrate in diapause, awaiting the return of rain. The distribution of S. filamentosus is almost certainly more restricted than the Mamoré basin as a whole; like many annual killies it probably occupies a handful of specific pool complexes within a larger region, making the precise locality data from collection events scientifically critical.
Feeding
Annual killifishes of Spectrolebias are opportunistic micro-predators whose diet in temporary pools consists largely of small invertebrates: aquatic insect larvae, microcrustaceans (copepods, cladocerans), ostracods, and occasionally small worms. The upward-tilted mouth is well positioned for taking prey at or just below the water surface, and the fish are likely to exploit emerging insects and surface-film invertebrates in the energy-rich but short-lived pool environment.
In aquarium conditions the species accepts a wide range of small live and frozen foods: Daphnia, Artemia nauplii, bloodworms (Chironomus larvae), small Tubifex, and finely crushed dry foods. Live food is strongly preferred and is important for conditioning breeding pairs. The metabolism is fast — appropriate for a fish racing to reproduce before its habitat disappears — and regular feeding several times daily keeps adults in peak condition during the breeding season.
Mating
Spectrolebias filamentosus is a substrate-diving annual killifish; spawning occurs not on plants or open substrate surfaces but by the pair plunging together into soft substrate — peat, mud, or detritus — and releasing eggs directly into the sediment at depth. This spawning mode, shared across the annual killifish clade, ensures that the drought-resistant eggs are buried well below the pool surface where they will not desiccate immediately as the pool shrinks.
The spawning sequence begins with male courtship: lateral displays showing off the iridescent flank coloration and the elongated dorsal and anal fin filaments, combined with contact-organ stimulation of the female's flanks. When the female is receptive the pair swim side by side and dive headfirst into the substrate, with the male arched around the female — the characteristic embrace of annual killies. One or a few eggs are deposited per dive; the pair resurface and the sequence repeats. Males are polygynous and will court multiple females across a spawning season.
Breeding
The eggs of Spectrolebias filamentosus are encased in a tough, adhesive chorion adapted to survive desiccation within the damp substrate. After spawning, the eggs enter diapause — a state of arrested embryonic development that can persist for weeks to months while the pool dries out. Development resumes when the substrate is re-wetted by seasonal rains; larvae hatch within days of contact with water and grow rapidly, reaching sexual maturity within weeks. The entire adult lifespan from pool-filling to pool-drying is compressed into a few months.
In the killifish hobby, annual species are typically bred on a spawning medium of peat moss or coconut coir laid on the bottom of a dedicated breeding tank. After a period of active spawning, the medium is removed, gently pressed to remove excess water, and stored in sealed plastic bags or containers at controlled humidity and temperature — replicating the dry-season conditions the eggs experience in nature. After a rest period of one to three months, the medium is re-wetted with aged, soft, warm water; larvae hatch within 24–48 hours and should be fed immediately with fine live foods such as microworms or freshly hatched Artemia nauplii. Multiple diapause attempts on the same medium often yield additional hatches.
In the aquarium
Spectrolebias filamentosus is a specialist hobbyist fish, kept primarily by killifish enthusiasts and annual-killi breeders rather than general aquarists. It is a small fish — males to 2 in — that can be housed in relatively modest aquaria of 10–15 US gal for a breeding pair or trio, though a longer footprint is beneficial for the courtship displays. The tank should have a soft, deep substrate layer of fine, dark material (peat, fine sand, or coconut coir) for spawning; a peat spawning bowl or container placed on the floor is a common alternative. Floating plants or surface cover reduce light intensity and make the fish bolder.
Water parameters should reflect the species' Bolivian pool origin: warm (75–82 °F), mildly acidic to neutral (pH 6.5–7.2), and soft to moderately hard. The AKA lists the species in its records, with multiple JAKA (Journal of the American Killifish Association) articles on its husbandry documenting that it has been kept and bred successfully in the US hobby. Males may be aggressive to one another; keeping one male with two or more females distributes spawning pressure and reduces harassment. Tankmates should be limited to fish too large to be harassed and not inclined to eat the small eggs; a species setup is simplest.
The species' defining husbandry challenge is the egg diapause cycle: the keeper must manage the spawning medium, storage period, and re-wetting event correctly to achieve consistent hatches. This investment of care makes Spectrolebias filamentosus a fish for dedicated annual-killi keepers rather than casual aquarists, but the iridescent male coloration and the fascinating annual life cycle reward the effort.
Conservation
The IUCN Red List assessed Spectrolebias filamentosus as Data Deficient (DD) in 2014, with the assessment credited to Sarmiento. The DD listing does not mean the species is secure — it means that the information needed to assign it to a threat category is lacking. Annual killifishes are inherently vulnerable to data scarcity: their pools are transient, their distributions can be highly localised to specific drainages or even individual valleys, and systematic field surveys require visiting sites during the narrow wet-season window when the fish are alive.
The broader genus Spectrolebias illustrates the range of threat outcomes for Bolivian and Brazilian annual killies. The congener Spectrolebias reticulatus is known to have been directly impacted by the Belo Monte hydroelectric dam complex in Brazil, which altered or destroyed pool habitats in its range — a reminder that infrastructure development in the South American interior can eliminate annual-killi localities before they have been properly surveyed. For S. filamentosus, the primary concern is the quality and extent of the seasonal pool habitat within the Mamoré drainage: drainage of the Llanos de Mojos for agriculture and cattle ranching, alteration of flood regimes, and the potential introduction of invasive mosquitofish (Gambusia) to pools for malaria-vector control all represent plausible threats. The AKA lists Bolivia as a high-priority collection locality for this species, underlining the importance of ex-situ breeding populations in the hobby as an insurance collection for annual killies whose wild situations are incompletely known.