Taxonomy & naming
Aphyosemion ecucuense was described by Rainer Sonnenberg in 2008, with the original publication appearing in Ichthyological Exploration of Freshwaters volume 18 (number 4, for 2007), pages 361 and following, with figures illustrating the type material. The holotype is a male measuring 1 in standard length (MRAC 73039.P.1170), collected at a locality catalogued as GEMLBJ 03/36: a stream crossing the Bata–Niefang road approximately 22 mi east of Bata in the Río Ecucu basin, at an elevation of 633 ft.
The species was placed in the subgenus Chromaphyosemion, a grouping of West and Central African plant-spawning Aphyosemion with characteristic red dot patterning in the dorsal, caudal, and anal fins. Eschmeyer's Catalog of Fishes records the current valid combination as Aphyosemion ecucuense (Sonnenberg, 2008), with Chromaphyosemion ecucuense listed as a synonym — the subgenus is not treated as a separate genus in the current authoritative classification. Prior to its description, material assignable to this species was misidentified as Aphyosemion bivittatum, A. bitaeniatum, and A. splendopleure, all of which overlap its range or share superficial colour features.
The specific epithet ecucuense is a Latinised toponym derived from the Río Ecucu, the river that defines the type locality and the centre of the species' known distribution.
Morphology
Aphyosemion ecucuense is a small fish; the holotype male measures 1 in SL, and FishBase records a maximum of 1 in SL for the species — placing it among the more diminutive members of the genus. The body form is typical of Chromaphyosemion: moderately elongate and laterally compressed, with a pointed snout, upturned mouth suited for surface feeding, and rounded unpaired fins.
Male colouration is the defining field character: an orange-copper base to the body with red dots distributed through the dorsal, caudal, and anal fins — a pattern shared at the subgeneric level with close relatives but differentiated from each by the precise arrangement and density of markings. As with all Aphyosemion, females are considerably plainer, with a brownish body and little or no red ornamentation in the fins. Sexual dimorphism is pronounced in colour but less so in body form; mature females become slightly deeper-bodied when gravid.
Habitat
The species is known from tributaries of the middle and upper Río Ecucu system on the coastal plain of northwestern Equatorial Guinea — a relatively small drainage basin flowing westward through forested and forest-margin terrain to the Bight of Biafra. The type-locality stream crosses a road 22 mi east of Bata at 633 ft elevation, suggesting a foothills-to-lowland gradient rather than true highland habitat.
Like other Chromaphyosemion, it inhabits permanent or near-permanent streams rather than the seasonally drying pools that support annual killifish. These are typically clear to lightly tannin-stained, slow-flowing or still forest watercourses with leaf litter substrates, overhanging riparian vegetation, and accumulations of aquatic plant matter and submerged roots. Water chemistry across this region is characteristically soft and slightly acidic: the AKA reference data for aquarium maintenance (pH 6.5, hardness approximately 2°dGH) is consistent with conditions in comparable Equatorial Guinean lowland streams. The coastal plain setting means the surrounding landscape is subject to rapid deforestation and agricultural conversion.
Feeding
Field data on the diet of Aphyosemion ecucuense specifically is limited, but the CLOFFA fauna database records the diet of this species as ants and other insects — consistent with the surface-oriented, insectivorous feeding habit shared broadly across Aphyosemion. The upturned mouth and position near the water's surface are adaptations for taking terrestrial invertebrates that fall onto the water, as well as aquatic invertebrates in the surface film and upper water column.
In the aquarium the species accepts a wide range of small live and frozen invertebrates: Drosophila and other small flies, mosquito larvae, bloodworm, Daphnia, and Artemia. Prepared foods are taken once the fish is settled, but live and frozen items should form the core of the diet to sustain colouration and reproductive condition. Feeding frequency should be moderate — small quantities several times daily rather than single large meals.
Mating
Aphyosemion ecucuense is a non-annual killifish, reproducing by the plant-spawning strategy characteristic of the subgenus Chromaphyosemion. Unlike annual killifish that bury eggs in substrate and undergo obligate diapause through a dry season, this species lives in permanent water and deposits adhesive eggs on fine-leaved plants, moss, or spawning mops over an extended period rather than in a single mass-spawning event.
Spawning occurs as a series of brief, repeated embraces or side-by-side contacts between the male and female among plant cover near the substrate or mid-water. Each contact deposits one or a small number of eggs, which cling to the plant material by a short adhesive filament. The AKA reference data records approximately 20 eggs per week from a conditioned pair — a low but sustained production that continues over many weeks when conditions are right. Males actively court females and can harass them; in smaller tanks a single pair or a larger ratio of females to males is advisable.
Breeding
Eggs are adhesive and deposited on Java moss, fine-leaved plants, or synthetic spawning mops, where they incubate in the water column without any substrate burial or diapause. The AKA reference data records incubation at 72–73 °F as 14 to 17 days under normal conditions, with some eggs taking up to 21–28 days. Egg diameter is recorded by CLOFFA as approximately 0.5 in. Water temperature is critical: spawning ceases above 77 °F and below 70 °F, meaning the species requires cooler conditions than many tropical aquarium fish.
Conditioned water at pH 6.5 and very low hardness (around 2°dGH) is recommended for spawning and egg development. Newly hatched fry are very small and require appropriately sized first foods — infusoria or commercial fry powder — for the first week or two before advancing to microworms and, eventually, freshly hatched brine shrimp nauplii. Sexual maturity is reached slowly relative to annual killifish: the AKA notes record 9–10 months as the time to reproductive readiness. Because adults will consume eggs, mop or moss containing eggs should be removed and incubated separately, or the adults moved out of the breeding tank after each collection.
In the aquarium
Aphyosemion ecucuense is a specialist aquarium fish suited to dedicated killifish keepers rather than general community aquaria. Its small adult size (under 1 in SL), requirement for cool water, and sensitivity to elevated temperature make it incompatible with the warmer tanks kept for most tropical fish. A small, well-planted aquarium of 10–15 US gal with a tight-fitting lid, gentle filtration, shaded lighting, and a good cover of Java moss or fine-leaved plants is ideal.
Water quality requirements: soft (around 2°dGH), slightly acidic (pH 6.5), and cool — kept strictly between 70 °F and 77 °F, with 72–73 °F the recommended optimum for breeding. Temperatures above 77 °F suppress spawning and can cause health problems; this limits the species' suitability in warm climates without air conditioning. Regular partial water changes with soft, slightly acidic water are important. Tankmates, if used, should be small, non-aggressive fish of similar water requirements; in practice most keepers maintain the species in species-only or genus-only tanks to simplify husbandry and avoid competition for spawning sites.
The species is rarely available in the mainstream trade and is primarily kept and exchanged within killifish society networks. It has no established common name in the hobby.
Conservation
Aphyosemion ecucuense has not been evaluated by the IUCN Red List and carries no formal conservation status. However, the basis for concern is clear: it is known only from tributaries of the Río Ecucu system on the coastal plain of Equatorial Guinea, a small and geographically bounded watershed. The broader coastal lowlands of Equatorial Guinea and Cameroon — the most killifish-rich region on the continent — are subject to accelerating deforestation, agricultural expansion, and oil-industry infrastructure, all of which degrade the shaded, leaf-litter forest streams that Chromaphyosemion species depend on.
The combination of a narrow endemic range, dependence on intact riparian forest, and an undescribed trajectory of habitat change means that a precautionary view of its status is warranted. The species appears in no monitored captive-breeding programme as of its description, and aquarium records are sparse. Formally assessing it under IUCN criteria — particularly gathering distributional data to determine whether it is restricted to a single sub-basin or present more broadly in the Río Ecucu system — would be a useful contribution from regional herpetofauna and ichthyology surveys.