Taxonomy & naming
Astyanax trierythropterus was described by M. H. Godoy in 1970 and placed in the enormous genus Astyanax, the most species-rich characid genus in the Neotropics. Type material is held at the Pirassununga Natural History Museum in São Paulo state, Brazil. Eschmeyer's Catalog of Fishes recognises the valid name as Astyanax trierythropterus Godoy, 1970; the authority is not parenthesised because the species has remained within Astyanax since its original description.
Astyanax belongs to the family Characidae — a vast, paraphyletic assemblage under ongoing revision — and the genus itself contains several hundred species ranging from the southern United States through Central America and across virtually every major South American drainage. Identifying species within the genus is notoriously difficult; members of the lambari complex in particular share similar body shapes and colouration, and many are distinguished chiefly by subtle meristic or pigment characters. A. trierythropterus is set apart within the genus by its tri-red-fin pattern, which is not universal among Astyanax and provides a useful field mark where it co-occurs with closely related congeners.
Morphology
Astyanax trierythropterus is a small, laterally compressed, deep-bodied characid with the fusiform silhouette typical of the lambari group. Published size data for this species are sparse; it is likely in the range of 2–3.5 in total length based on congeners of similar ecology, but a confirmed maximum length has not been captured in current databases. The body is silver with the faint lateral stripe common to many Astyanax, and the species is most readily identified by reddish pigmentation present in at least three fins — the basis of the Latin epithet.
The characteristic fin colouring tends to appear in the dorsal, caudal, and one or more of the paired fins, though fin pigmentation can vary with age, condition, and preservation. Like most characids the species carries an adipose fin, has a relatively large eye, and possesses the maxillary teeth characteristic of Astyanax. Sexual dimorphism, where described in similar lambari, is subtle: gravid females are noticeably deeper-bodied than males when in breeding condition, and males may show slightly brighter fin colour during the spawning period.
Habitat
The species is recorded from freshwater localities in Brazil (São Paulo state and adjacent drainages), Paraguay, and Argentina, placing it within the La Plata basin and its tributary systems — one of the largest river systems in South America. Within these drainages it occupies the benthopelagic zone of rivers, streams, and associated lagoons, typically in slower or moderate current over sandy or muddy substrate with marginal vegetation.
Water conditions across its range span the warm subtropical band; FishBase characterises the species as tropical, with stable or growing populations suggesting broad tolerance of the range of environmental conditions found across La Plata tributaries. River temperatures in south-central Brazil and the Río Paraná system fluctuate seasonally, often between roughly 64 °F and 82 °F, and pH tends toward neutral to slightly alkaline in the harder waters draining the Brazilian shield. Like most Astyanax, the species is likely resilient to a range of turbidity, oxygen, and flow conditions.
Feeding
Astyanax species in the lambari complex are generalist omnivores, and A. trierythropterus can be expected to follow the same pattern: an opportunistic diet that combines small aquatic invertebrates (insect larvae, crustaceans), terrestrial insects fallen to the surface, small fish, algae, and plant matter depending on local availability and season. Members of this complex are important mid-trophic links in Neotropical rivers, converting invertebrate and plant production into prey available to larger piscivores.
In the aquarium the species accepts a wide variety of foods readily. Flake and small pellets form an adequate base, with frozen or live foods (Daphnia, brine shrimp, bloodworm) improving condition and bringing out fin colouration. As with most tetras, a varied diet fed in small portions several times daily is preferable to a single large feeding.
Mating
No detailed courtship or mating observations specific to A. trierythropterus appear in the published literature, but the biology of congeners provides a reliable guide. Astyanax are egg-scattering, open-water spawners; males pursue females through the water column or over submerged vegetation, and spawning is initiated by brief lateral or circular contact prior to egg release. There is no pair bond and no territorial behaviour around a nest site.
In seasonally variable river systems, Astyanax spawning is often tied to the onset of the rainy season and associated rises in water level and temperature; a corresponding drop in temperature or photoperiod can suppress spawning activity. In the aquarium, a gradual increase in temperature, water changes with slightly cooler water, and the addition of live or frozen foods will often trigger spawning readiness in conditioned adults.
Breeding
Breeding follows the open-water egg-scattering pattern universal in the genus: a ripe female, chased by one or more males, scatters semi-adhesive eggs among fine-leaved plants, spawning mops, or open substrate, after which both sexes ignore the eggs entirely. Clutch sizes in small Astyanax can range from a few dozen to several hundred eggs depending on female size and condition. The eggs are small, transparent to slightly amber, and prone to fungus if water quality is poor.
At typical breeding temperatures of 72–79 °F, eggs hatch within 24–48 hours; the larvae are initially immobile and exhausting their yolk sac before becoming free-swimming after a further day or two. Fry are tiny and require infusoria or commercial liquid fry food as a first food, graduating to newly hatched brine shrimp (Artemia nauplii) within the first week. Adults will eat both eggs and fry, so separation into a dedicated breeding container or removal of adults after spawning is standard practice.
In the aquarium
Astyanax trierythropterus is rarely encountered in the hobby relative to common lambari tetras, and is primarily of interest to dedicated characin keepers and South American biotope aquarists seeking a regionally authentic assemblage. Where available it is undemanding to maintain: a well-oxygenated, modestly planted aquarium of 23.5 in or more will suit a school of six or more, with neutral to slightly alkaline, moderately hard water (pH 6.8–7.8, moderate hardness) in the range of 68–79 °F replicating the subtropical river conditions of its home range.
As a schooling fish it displays best in groups and is peaceful toward similarly sized tankmates. Appropriate companions include other South American characids, smaller Corydoras, and diminutive peaceful cichlids. The species is an active mid-water swimmer, so cover and some open swimming lane should both be provided. The red fin flashes are most visible under bright lighting with a dark substrate. Diet is as noted above: varied, with a strong invertebrate component to maintain condition and colouration.
Conservation
Astyanax trierythropterus is currently assessed as Least Concern (LC) on the IUCN Red List, reflecting a 2018 review that found the species to have a sufficiently wide distribution across Brazil, Paraguay, and Argentina and populations showing no marked decline. FishBase records a population doubling time of less than 15 months, consistent with the high reproductive output typical of small, short-lived characids — a demographic buffer against localised pressures.
The species was previously assessed as Vulnerable (VU) under an earlier IUCN review, and the change to LC represents either improved distributional data, population trend data, or a reclassification under revised criteria. Ongoing threats to freshwater fishes in the La Plata system include dam construction, agricultural run-off, riparian habitat loss, and introduced species, but none of these appear to constitute a range-wide threat to this species at present. No species-specific conservation measures are in place, nor are they currently warranted given the LC assessment.