Taxonomy & naming
Laimosemion rectocaudatum was described by Sven O. Kulander and Júlio César Ferreira in 1991, with the type locality in the Amazon River basin of Peru. The original description placed it in the genus Rivulus, where most South American rivulines had long been classified. Subsequent molecular and morphological revisions led to a major reorganisation of the Rivulidae, and the species was transferred to Laimosemion — a resurrected genus now housing a clade of South American rivulines formerly grouped under Rivulus. Eschmeyer's Catalog of Fishes (Fricke, Eschmeyer & Fong) treats Laimosemion rectocaudatum (Kulander & Ferreira, 1991) as the valid combination; the parenthetical authority indicates that the species name was originally published under a different genus.
The specific epithet rectocaudatus is formed from the Latin rectus (straight, upright) and cauda (tail), describing the notably straight posterior margin of the caudal fin — the feature that gives the fish its English name, the straight-tailed rivulus. The family Rivulidae, to which Laimosemion belongs, is the largest family of annual and non-annual killifishes in the New World, with its centre of diversity in South America and extending into Central America and the Caribbean.
This fish was long listed as Laimosemion rectocaudatus; Eschmeyer's Catalog of Fishes places the species in Laimosemion, and the current combination Laimosemion rectocaudatum is followed here.
Morphology
Laimosemion rectocaudatum reaches approximately 2.4 in total length (FishBase), placing it in the mid-size range for the genus. The body form is that of a typical South American rivuline: elongate and cylindrical, with a slightly flattened dorsal profile and the upturned mouth characteristic of a near-surface predator. The caudal fin shows the straight posterior margin from which the species draws its epithet, distinguishing it from close relatives with rounded or lyre-shaped tails.
Colouration in males is the principal ornamental feature of the species, as in most rivulines: a pattern of iridescent spots or bars on the flanks and fins, typically rendered in blues, greens, and reds, though detailed published colour descriptions for wild-caught specimens of this species are sparse in the accessible literature. Females, as is general in the family, are cryptically coloured — an olive to brownish ground tone with reduced patterning — and are best identified by the combination of the straight caudal fin margin and association with displaying males.
Habitat
Laimosemion rectocaudatum is native to the Amazon River basin of Peru — the precise collection localities and drainage systems are not well documented in the accessible literature, reflecting the Data Deficient assessment. As a non-annual rivuline, it inhabits permanent or near-permanent water bodies rather than the temporary pools exploited by annual killifishes. This typically means small forest streams, pools along stream margins, flooded forest floor, and associated shallow, shaded habitats in lowland Amazonian rainforest.
FishBase records a temperature range of 72–77 °F for this species, consistent with the warm, relatively stable thermal regime of lowland Peruvian Amazonia. Water chemistry in these forest habitats tends toward the soft and acidic — dark, tannin-stained water with low conductivity is typical of the region — though specific pH and hardness data for the collection sites of L. rectocaudatum are not recorded in the reviewed sources. The species is described as difficult to maintain, suggesting some sensitivity to captive water conditions that may reflect particular demands associated with its natural water chemistry.
Feeding
Like most members of the Rivulidae, Laimosemion rectocaudatum is a carnivore, feeding on small invertebrates in the wild — aquatic insect larvae, small crustaceans, worms, and other invertebrates accessible near the water surface and in the upper water column. The upturned mouth is adapted for taking prey at or just below the surface film, a foraging mode typical of surface-oriented rivulines in still or slow-moving water.
In captivity, the species accepts live and frozen small invertebrates: Drosophila fruit flies, micro-worms, small Daphnia, and baby brine shrimp nauplii are suitable offerings. A varied diet of appropriately sized live or frozen prey helps maintain condition and supports the natural surface-feeding behaviour. As with many killifishes, a healthy feeding response is a reliable indicator of fish in good condition — reluctance to feed often precedes visible signs of stress or illness.
Mating
Laimosemion rectocaudatum reproduces as a non-annual, plant-spawning killifish. Rather than burying eggs in substrate to undergo seasonal diapause, the pair spawn adhesive eggs into floating spawning mops, fine-leaved plants, or roots near the water surface — a strategy suited to permanent water bodies that do not dry seasonally. Spawning is a protracted process: the pair produce a small number of eggs at each courtship encounter, the male displaying alongside the female before a brief embrace in the plant or mop material.
The hobbyist literature records approximately three eggs per spawning session, an unusually low per-encounter output that distinguishes this species from high-fecundity plant-spawners in the family. This low egg production, combined with a comparatively slow development rate relative to many rivulines, contributes to the species' reputation for being slow to establish a population in captivity. Hobbyists note that lower pH shifts the sex ratio toward females — an observation that may have relevance both in captive management and in natural population dynamics, where local water chemistry could influence population sex ratios.
Breeding
Aquarium breeding follows the standard non-annual killifish approach: floating spawning mops of synthetic wool or natural Java moss suspended near the surface are offered, and the pair deposit their small clutch of adhesive eggs over multiple sessions. Eggs can be collected by hand and incubated individually in small containers of aquarium water — a technique reported in hobbyist practice for this species. Incubation at ambient Amazonian temperatures takes approximately 16 days before larvae hatch.
Fry are small at hatching and require appropriately sized first foods: infusoria, vinegar eelworms, or commercial fry powder in the first days, graduating to baby brine shrimp nauplii and micro-worms as they grow. Growth to adulthood is slower than in many rival genera, and hobbyists report that the fish are longer to reach sexual maturity than many popular killifish. Captive lifespan reaches up to 3.5 years, which is relatively long among killifishes and contrasts sharply with the single-year lifespan of annual species. The extended lifespan partially compensates for the low clutch size by giving each breeding pair more reproductive seasons.
In the aquarium
Laimosemion rectocaudatum is considered a challenging species, recommended for experienced killifish keepers rather than beginners. FishBase flags the species as very difficult to maintain, and hobbyist accounts echo this — the low egg output, slow development, and apparent sensitivity in captivity demand attentive husbandry. The fish should be housed in a well-covered aquarium: like nearly all killifishes, it is an accomplished jumper and will exploit any gap in the lid.
A species or near-species setup in a small, heavily planted or moss-furnished tank suits the fish's temperament and feeding habits. Soft, slightly acidic water reflecting its Amazonian origin — temperate to warm (72–77 °F), pH toward the acid end of neutral, low conductivity — is likely to give the best results, though precise water parameters for optimal captive performance are not extensively documented in the literature. Males may spar or show aggression to each other, and keeping one male with one or two females is the standard killifish approach. Because the species is Data Deficient with a restricted known range, responsible captive husbandry that sustains a healthy, diverse aquarium population has genuine conservation value for this understudied rivuline.
The species appears in killifish society databases and specimen exchange networks (including American Killifish Association records) under both its current combination and the former name Rivulus rectocaudatus. Hobbyists sourcing the fish should confirm provenance where possible, as collection locality data is particularly valuable for an understudied species.
Conservation
The IUCN Red List assessed Laimosemion rectocaudatum as Data Deficient (DD) on 24 April 2014 — a category that reflects genuine uncertainty rather than confirmed security. Assessors noted a restricted distribution within the Amazon basin of Peru, but insufficient data on the species' current population status, the extent and condition of its habitat, and the threats it faces to support placement in any threatened or non-threatened category with confidence.
The Amazon basin faces mounting pressure from deforestation, agricultural expansion, and infrastructure development, including road-building that opens previously inaccessible forest to settlement and land clearance. Small headwater streams and marginal forest pools — the typical habitats of non-annual rivulines — are disproportionately vulnerable to local deforestation because their hydrology depends on intact riparian and upland forest cover. Siltation, water temperature increases, and chemical run-off from agriculture can degrade or destroy these microhabitats. Given the species' apparently low reproductive rate and restricted range, localised habitat loss could have outsized population effects. A formal field survey to document current range, population structure, and habitat condition would be the necessary foundation for a more informative IUCN reassessment.