Taxonomy & naming
Laimosemion agilae was first described by the Dutch ichthyologist J. J. Hoedeman in 1954, with the type locality given as Agila, Surinam (now Suriname). For most of the following half-century the species was placed in the broadly conceived genus Rivulus, and it is still referenced in much of the hobby literature and in some databases under the synonym Rivulus agilae. The transfer to Laimosemion follows the revision of New World rivuline killifishes that recognised Rivulus as paraphyletic; Catalog of Fishes treats Laimosemion agilae (Hoedeman, 1954) as the valid combination.
Laimosemion is a South American rivuline genus comprising non-annual, plant-spawning species distributed through the Guianas and the wider Amazonian and Atlantic coastal drainages. Within the genus, L. agilae is part of the lowland Guianan assemblage and is sympatric in parts of its range with members of the Anablepsoides urophthalmus complex. The authority is presented in parentheses to indicate that Hoedeman's original genus placement has since been revised.
Morphology
Males reach approximately 2 in total length (FishBase; the British Killifish Association gives up to about 2.5 in for particularly large individuals), while females are smaller and more plainly coloured. The male body is elongate and laterally compressed, with an earthy green-brown ground colour overlaid by reddish-brown spots on the flanks. The caudal fin is conspicuously two-toned: the upper lobe is pale green and the lower lobe dark brown, a combination that varies in intensity across the range. Coastal French Guiana specimens tend toward a deep red body with a strong black caudal margin and an often-absent yellow sub-marginal band; Surinamese fish carry more blue on the body, an intense yellow sub-marginal caudal band, and a weaker black margin with a less elongated body; Guyanese individuals show more blue overall with the black and yellow margins subdued or missing entirely.
Females are pale grey-blue with a marbled pattern and a bright orange anal fin — a useful field and aquarium character for sex determination. Sexual dimorphism is therefore pronounced, and the species follows the typical rivuline pattern in which males are the more vividly coloured sex.
Habitat
Laimosemion agilae occupies the coastal lowland zone of the Guianan shield — a belt of shallow, often sandy-bottomed waterbodies that includes fast-moving rocky streams, sluggish savannah creeks, and black-water swamps in open to semi-open areas. The substrate is frequently white sand, vegetation is sparse or floating, and sunlight penetrates to the bottom. The species can travel overland between isolated pools — a trait observed across many rivuline killifishes of seasonally variable habitats — allowing it to colonise or recolonise pools after dry-season draw-down.
Water conditions across the range are soft and mildly acidic to near-neutral. FishBase records a temperature range of 72–77 °F and pH 6.0–7.0 with hardness of 5–12 dH, consistent with soft Guianan lowland water; the British Killifish Association gives a slightly wider temperature tolerance of approximately 70–82 °F in the aquarium. The species is described as non-migratory and benthopelagic — occupying the lower to mid-water column of shallow waterbodies rather than open deep water.
Feeding
In the wild, Laimosemion agilae feeds predominantly on insects and their aquatic larvae, with mosquito larvae forming a significant part of the diet — a feeding ecology consistent with surface-oriented, slow-water habitats where mosquitoes breed. The trophic level is recorded as 3.3 (FishBase), placing it in the lower range of secondary consumers, consistent with a diet of small invertebrates rather than fish predation.
In the aquarium the species takes live and frozen foods readily: daphnia, white worm, micro worm, brine shrimp nauplii, and bloodworm are all accepted. Dried and prepared foods are also taken. A varied diet with a strong live-food component maintains colour and condition in males and stimulates regular spawning behaviour.
Mating
Laimosemion agilae is a non-annual, plant-spawning killifish. Rather than burying eggs in substrate and deferring development through a dry season via diapause — the strategy of annual killifishes — pairs and trios spawn continuously over extended periods by depositing individual adhesive eggs on floating vegetation, plant roots, or suspended spawning mops near the water surface. This mop-spawning reproductive mode is characteristic of Laimosemion and other Guianan rivuline genera, and reflects life in permanent or near-permanent water where there is no selective pressure for diapausing eggs.
Courtship is relatively low-key: a male displays to a receptive female and the pair align side by side to deposit a single egg at each spawning act. Females produce up to approximately 5–15 eggs per day throughout adult life. The eggs are amber-coloured and approximately 0.5 in in diameter. An unusual observation from It Rains Fishes is that sex ratio among the fry is influenced by incubation pH: hatching at lower pH produces a higher proportion of females, a rare form of environmental sex determination documented in rivulines.
Breeding
Breeding Laimosemion agilae in the aquarium is straightforward. A small tank of 2.5–4 US gal with gentle filtration, a dark substrate or peat moss bottom, and one or more floating synthetic mops is sufficient. Water should be soft and slightly acidic to neutral (pH 6.8–7.2, hardness 75–150 ppm; temperature 72–79 °F). A pair or a male with two females is the standard approach, with varied live food offered daily to stimulate spawning.
Eggs are collected from the mop daily or every few days and transferred to a shallow container with a little aquarium water and an antifungal agent. At 72 °F, FishBase records incubation of approximately four weeks, though the British Killifish Association gives a shorter period of 10–14 days at somewhat higher temperatures — the difference reflects temperature sensitivity. Freshly hatched fry are about 0.5 in long and accept micro worm and artemia nauplii as first foods. Fry are best reared in very shallow water (approximately 1 in) initially, with the depth increased gradually as they grow. Juveniles sexually differentiate during their first week of life. Sexual maturity is reached at approximately 4–10 months (sources vary), and the species is comparatively long-lived for a killifish, with a lifespan approaching three years.
In the aquarium
Laimosemion agilae is a rewarding, manageable killifish for hobbyists with some experience of the group. Its relatively small size and non-aggressive temperament make it suitable for a species tank or a carefully chosen community with similarly sized, non-boisterous fish. Males are territorial toward one another and a single male is usually kept per tank unless space is generous; a male with two or three females is the typical setup and produces near-continuous spawning.
The aquarium should be densely planted or furnished with floating mops, and a tight-fitting lid is advisable — rivulines are capable jumpers. Lighting can be moderate to bright, consistent with the open, sunny habitats the species occupies in nature. Water changes of around 20–30% weekly maintain the clean, soft conditions that promote health and colour. The species is tolerant of a temperature range spanning roughly 70–82 °F, making it adaptable to most temperate and tropical home aquaria without specialist heating. Its willingness to spawn continuously and the ease of collecting and raising eggs make it a popular introduction to killifish breeding in the British and continental European killifish hobby communities.
Conservation
The IUCN Red List assessed Laimosemion agilae as Least Concern in January 2021 (version 2025-2), citing a broad distribution across the coastal Guianas and the absence of evidence for rapid population decline. FishBase records the species as having high resilience — a population capable of doubling in under 15 months — which buffers it against localised disturbance.
Nevertheless, the coastal savannah habitat of the Guianas is under increasing pressure from agricultural expansion, drainage projects, and infrastructure development in the coastal zone. The species' association with shallow, often isolated pools and slow creeks means that individual water bodies can be lost to land-use change without affecting the species-level IUCN status. The presence of geographically distinct colour morphs across French Guiana, Suriname, and Guyana also raises the question of whether populations showing marked phenotypic divergence may represent evolutionarily significant units deserving separate conservation attention — a question that would benefit from molecular population-level study.