Taxonomy & naming
Aphyocharax erythrurus was described by the American ichthyologist Carl H. Eigenmann in 1912, from material collected at the Rockstone sand-bank locality on the Essequibo River in Guyana. Eschmeyer's Catalog of Fishes recognises the valid name as Aphyocharax erythrurus Eigenmann, 1912; as the species was described in its current genus, no parenthetical authority is needed.
The genus Aphyocharax belongs to the large neotropical family Characidae. The genus name is compounded from the Greek aphye (a small fish, sometimes glossed as sardine or anchovy) and charax (a marine fish with sharp teeth), a construction shared across several tetra genera. The specific epithet erythrurus derives from the Greek erythros (red) and oura (tail), a direct description of the species' most conspicuous feature. Several members of Aphyocharax carry vivid red on the fins — a common colour theme in the genus — and A. erythrurus sits within a radiation of small, open-water characids distributed across the river systems of the Guiana Shield and adjacent lowlands.
Morphology
A slender, laterally compressed tetra reaching a maximum recorded standard length of 2.5 in, though many individuals in both wild populations and aquarium stocks are smaller. The body is the typical torpedo shape of the genus: moderately elongate, with a small head, a somewhat upturned mouth suited to surface feeding, and a pronounced lateral line running along the mid-flank.
The base colouration is silver with a faint bluish or olive sheen along the dorsal surface, fading to pale silver or cream on the belly. The caudal fin carries the defining red pigmentation, which can extend to varying degrees onto the caudal peduncle and the bases of the dorsal and anal fins. In well-conditioned, actively displaying fish the red is intense; in stressed or subdued individuals it may fade significantly. The species shows mild sexual dimorphism: females develop a rounder, deeper body profile when gravid, while males tend to be slightly slimmer.
Habitat
Aphyocharax erythrurus is endemic to the Essequibo River basin of Guyana, which drains a large section of the Guiana Highlands and coastal plain before reaching the Atlantic. Its range within that basin is associated with the Coastal Rivers of the Guianas ecoregion, and the type locality — the Rockstone sandbank on the Essequibo — is a broad, slow-flowing reach of the main stem characterised by sandy and silty substrates.
The species is benthopelagic in classification but schools primarily in the water column rather than close to the bottom. It favours fresh, clear to lightly tannin-stained water in the acidic to neutral range, consistent with the soft, low-mineral conditions typical of Guiana Shield river systems. FishBase records water parameters of 72–79 °F, pH 6.0–7.5, and hardness in the range of 10–20 dH, reflecting both field conditions and aquarium keeping data. Floodplain margins with overhanging or floating vegetation appear to be favoured microhabitat.
Feeding
Like other Aphyocharax species, A. erythrurus is an opportunistic microinvertebrate feeder in the wild, taking small aquatic insects, insect larvae, tiny crustaceans such as cladocerans and copepods, and surface-trapped prey. The upturned mouth is adapted to surface and mid-water foraging rather than bottom feeding.
In the aquarium the species is undemanding and accepts dry foods readily — good-quality small-granule or flake foods form a sound staple. Colour and condition improve markedly with regular supplements of live or frozen foods: brine shrimp, Daphnia, and micro-worms are all taken eagerly. The species benefits from several small daily feedings rather than a single large meal, and it competes actively at feeding, so care is needed with shy or slow-eating tankmates.
Mating
Aphyocharax erythrurus is an egg-scattering, open-water spawner with no parental care, following the typical pattern for the genus. In wild populations spawning is likely tied to the seasonal rise in water temperature and photoperiod associated with the wet season, when flooding brings additional food and fine-leaved vegetation into range.
Males court females with brief lateral displays and chasing, and spawning takes place in open water or over fine-leaved plants. The adhesive eggs are deposited among vegetation or substrate clutter and are immediately abandoned by both parents. No pair bond forms. Males in peak condition tend to show intensified red on the caudal fin, which may function as a visual signal to competing males as well as to females.
Breeding
Breeding records in captivity are limited and FishBase notes no specific breeding data for the species, but the general reproductive biology can be inferred from the genus and from closely related species such as Aphyocharax anisitsi and A. rathbuni. The species is an egg-scatterer that deposits small, slightly adhesive eggs among fine-leaved plants or spawning mops; the adults show no parental instinct and will consume the eggs if given the opportunity.
Breeders working with this and similar Aphyocharax species condition a small group of adults on live and frozen foods before introducing them to a separate, dimly lit breeding tank furnished with Java moss or synthetic mops. Spawning typically occurs in the early morning. Eggs should be removed promptly; at temperatures in the mid-20s Celsius they hatch within 24–36 hours. The tiny, adhesive fry require very fine first foods — infusoria or commercial liquid fry food — before transitioning to newly hatched brine shrimp within a week or so.
In the aquarium
The flametail tetra is a peaceful, active schooling fish that does best in groups of eight or more individuals, where the collective movement and mutual security of a proper school brings out natural behaviour and the boldest colouration. In smaller groups it may be timid or spend much of the time hidden. A tank of 15 US gal or more in length provides adequate swimming room; the species is active but not a strong current-seeker, and gentle to moderate filtration suits it.
Water conditions should reflect its Guiana Shield origins: soft to moderately hard (10–20 dH), slightly acidic to neutral (pH 6.0–7.5), and maintained at 72–79 °F. Planted aquaria with open swimming lanes, subdued or dappled lighting, and a dark substrate show the red tail to good advantage and reduce stress. Tankmates should be similarly small and peaceful — other small tetras, small rasboras, pencilfish, and non-predatory catfishes such as Corydoras are all compatible. The species is compatible with community setups and is not known to nip fins on long-finned species, though direct observation with specific tankmates is always advisable.
Conservation
Aphyocharax erythrurus is assessed as Least Concern (LC) on the IUCN Red List, with the assessment dated 4 March 2021. The Essequibo River basin supports extensive areas of relatively intact freshwater habitat, and the species has no severe range restriction of the kind that typically elevates conservation concern in small neotropical fish.
It enters the international ornamental-fish trade, though it is not among the most frequently encountered Aphyocharax in the hobby. Wild collection has not been flagged as a population-level threat, and the extensive Guianese river systems that form its range remain substantially forested and hydrologically intact relative to river systems further south. No specific conservation measures are in place, and the current LC status reflects a genuinely low-risk profile given what is known of its range and ecology.