Taxonomy & naming
The species was formally described by J.F. Fels and P. de Rham in 1981 as Rivulus rubrolineatus, published in the Revue française d'Aquariologie Herpétologie (volume 8, number 3, page 66). The type locality is precisely documented in Eschmeyer's Catalog of Fishes as a site 9 mi from Jenaro Herrera on the road to Colonia Angamos, in the Loreto department of Peru (4°53′S, 73°41′W), and the holotype is held at the Muséum d'Histoire naturelle de Genève (MHNG 2079.46).
In 2011, the Brazilian ichthyologist Wilson Costa published a major revision of New World annual and non-annual killifishes, reassigning the species to the genus Anablepsoides (Costa ref. 31655 in the Catalog of Fishes). The current valid name is therefore Anablepsoides rubrolineatus (Fels & de Rham, 1981), placed in the family Rivulidae, subfamily Rivulinae. The parentheses around the authority signal that the original genus — Rivulus — has been changed. The slug melanorivulus-rubrolineatus has circulated in the hobby under the pre-2011 misattribution; Melanorivulus is a distinct genus of South American rivulines established by Costa in 2006 and comprising roughly 50 species of the Brazilian Cerrado and Amazonian periphery.
The genus name Anablepsoides alludes to Anableps, the four-eyed fish, referencing the surface-dwelling, near-surface-oriented lifestyle shared by members of the genus.
This fish was long listed as Melanorivulus rubrolineatus; Eschmeyer's Catalog of Fishes places the species in Anablepsoides, and the current combination Anablepsoides rubrolineatus is followed here.
Morphology
Males reach approximately 2.6 in total length according to FishBase — a modest size typical of non-annual rivuline killifishes in the Amazonian lowlands. The body is slender and somewhat cylindrical, adapted for life at or near the water surface where the fish hunts small invertebrates. The dorsal fin is positioned well back, closer to the caudal fin than to the pectoral insertion, a classic rivuline arrangement.
Colouration is the species' most distinctive feature and the basis of its common name. Males display a pattern of red horizontal lines or streaks along the flanks against an olive to brownish background, giving an immediately recognisable appearance. Females are comparatively drab — olive or brownish overall — and carry a dark ocellus (eye-spot) on the upper caudal peduncle, a characteristic feature seen in the females of many rivuline genera and thought to function as a deflection mark against predation. The caudal spot in females contrasts with the male's more saturated red patterning, making sex determination straightforward.
The upper jaw is slightly protruding and the mouth opens upward, an adaptation for surface feeding. Overall build is more slender than an annual killifish, consistent with the longer lifespan of a non-diapausing species.
Habitat
Anablepsoides rubrolineatus is native to permanent freshwater habitats within the upper Amazon basin: the species is documented from Peru and is considered to range into adjacent Colombia and Ecuador (FishBase). The type locality in Loreto, Peru lies in lowland Amazonian forest at low elevation, where the hydrology is characterised by stable, year-round flows in small forest streams and associated pools — the permanent-water environment that non-annual killifishes depend on.
FishBase records a temperature range of 72–82 °F for the species, consistent with the warm, stable thermal regime of Amazonian lowland streams. Specific pH and hardness data for the wild populations are not captured in the available literature reviewed here; Amazonian forest streams in this region tend toward soft, slightly acid conditions, but care should be taken not to assume extreme blackwater parameters without direct measurement data from the type region.
As a surface-dwelling species — the Anablepsoides name alludes to this — the redlined rivulus occupies the uppermost water column layer, close to the film where aquatic insects and other small prey concentrate. In the wild it is likely to inhabit margins of forest streams with overhanging vegetation, root tangles, and floating leaf litter that provide both cover and hunting ground. Aquarium Glaser report that collected individuals from Peru show a migratory tendency (land-hopping in wet conditions) typical of some rivulines, suggesting the species may occasionally move between connected pools in flooded-forest conditions.
Feeding
In nature the redlined rivulus feeds as a surface and near-surface predator, taking small aquatic invertebrates, zooplankton, and terrestrial insects that fall onto the water. The upturned mouth and surface-oriented posture are well suited to ambushing prey at the film, and in Amazonian forest streams small flies, ants, and aquatic insect larvae form the bulk of the diet.
In the aquarium, Aquarium Glaser describe the species as undemanding with respect to food. Small live or frozen foods — Drosophila, mosquito larvae, Artemia nauplii, Cyclops, and Daphnia — are accepted readily and promote the best colouration in males. High-quality micro-pellets or granules formulated for small surface-feeding fish can be used as a staple, supplemented regularly with live or frozen items to keep the fish active and in breeding condition. The relatively small adult size means portions should be modest; overfeeding and the resulting water quality decline are more damaging to this species than underfeeding.
Mating
Anablepsoides rubrolineatus is a non-annual plant spawner: it reproduces continuously or seasonally in permanent water bodies and does not produce desiccation-resistant eggs that require a dry season diapause. This places it in a fundamentally different reproductive category from the annual killifishes of the same region — a distinction that governs nearly every aspect of its care in captivity.
Males display to females with spread fins and intensified red lateral striping. Courtship is relatively brief and leads to spawning embraces near the water surface or among fine-leaved plants and root tangles. Eggs are adhesive and are deposited singly or in small numbers on plant material, floating mops, or submerged roots, where they adhere and develop in water. There is no soil-burying behaviour and no requirement for the eggs to enter diapause; the pair will spawn repeatedly over days or weeks when conditions are favourable. The continuous, dribbling spawning strategy — characteristic of many non-annual rivulines — spreads reproductive effort over time rather than concentrating it in a single mass event.
Breeding
Egg incubation takes approximately two weeks at aquarium temperatures, according to Aquarium Glaser. The fry hatch fully formed as miniature adults and are free-swimming from the start, unlike the yolk-sac larvae of many egg-scattering species. Because the eggs are deposited over an extended period and incubation is brief, a breeding setup must either accommodate multiple cohorts of fry at different developmental stages or eggs must be removed regularly for separate incubation.
A classic approach is to use a heavily planted or mop-equipped spawning tank and check daily for eggs, transferring them to a rearing container with slightly reduced water flow and a small amount of methylene blue or almond-leaf extract to inhibit fungal infection. Fry are large enough at hatching to accept Artemia nauplii as a first food; Cyclops, micro-worms, and finely crushed flake can be introduced as the fry grow. Adults will consume eggs and fry if encountered, so separation is standard practice. The Aquarium Glaser source reports a captive lifespan of approximately three years — longer than any annual killifish but shorter than many community fish, a characteristic of the rivuline life history even in non-annual species.
In the aquarium
The redlined rivulus is described as a challenging species in captivity, rated as difficult to maintain by FishBase. Aquarium Glaser note that it is best kept in a species-only or near-species setup — a small, well-planted, tightly covered aquarium of perhaps 16–21 US gal is suitable for a pair or a small group. The tight cover is essential: rivulines are notorious jumpers, and the surface-dwelling habit of Anablepsoides rubrolineatus makes escape through any gap in the lid a serious risk.
Water conditions should approximate the warm, soft, slightly acid environment of Loreto, Peru: temperatures of 72–79 °F are appropriate, with gentle filtration — sponge filters are preferred — that avoids surface disruption. Floating plants (Riccia, Salvinia, Ceratopteris) and spawning mops provide both the cover the fish prefer and the substrate on which eggs are deposited. Tankmates, if any are kept, must be peaceful and similarly small; the redlined rivulus should not be combined with fish large enough to threaten it or boisterous enough to outcompete it at feeding.
Despite the challenge rating, this species is periodically available through killifish society auctions and specialist importers (Aquarium Glaser have offered wild-collected Peruvian stock). Its vivid male colouration, interesting surface behaviour, and manageable size make it an attractive target for killifish enthusiasts willing to meet its husbandry requirements.
Conservation
The IUCN Red List status for Anablepsoides rubrolineatus is assessed as Least Concern. The species' distribution across the upper Amazon basin — spanning portions of Peru, Colombia, and Ecuador — places it within one of the world's largest continuous tropical forest systems, where broad-ranging freshwater species with no known specific threats are generally assessed as LC in the absence of evidence of significant population decline.
The upper Amazon lowlands of Loreto, Peru, where the type locality is situated, face long-term pressure from deforestation, agricultural encroachment, and oil extraction activities in and around the Marañón and Ucayali catchments. Small forest-stream killifishes with limited dispersal ability can be locally vulnerable to habitat degradation even when the species as a whole is not globally threatened — the permanent, stable-flow streams on which non-annual rivulines depend are more susceptible to sedimentation and drying from forest clearance than the temporary pools that sustain annual species. Continued forest protection in the Peruvian Amazon is the primary conservation measure relevant to this species and the broader community of small endemic fish it represents.