Taxonomy & naming
Astyanax argyrimarginatus was described by Garutti in 1999 from material collected in the upper Paraná River basin in Brazil. Eschmeyer's Catalog of Fishes records the valid name as Astyanax argyrimarginatus Garutti, 1999, with no parenthetical authority, indicating the species was placed in Astyanax from the outset.
Astyanax is among the largest and most taxonomically turbulent genera in the family Characidae — itself the largest family of freshwater fishes in the world. The genus spans North, Central, and South America and encompasses well over 100 valid species, with the boundaries of individual taxa frequently revised as morphological and molecular studies accumulate. A. argyrimarginatus belongs to the South American radiation of the genus centred on the Paraná–Paraguay and adjacent basins, a region with particularly high Astyanax diversity. Precise diagnosis relative to congeners relies on counts and measurements set out in the original description; these have not been superseded by a subsequent revision specific to this species.
Morphology
Astyanax argyrimarginatus conforms to the general body plan of small South American Astyanax: a moderately deep-bodied, laterally compressed characin with large cycloid scales, a short adipose fin, and a terminal mouth armed with small conical and tricuspid teeth. The species epithet — from the Latin argyri (silver) and marginatus (margined or edged) — refers to a silver edging on the fins that distinguishes it from closely similar congeners in the original description.
Detailed meristic data (lateral-line scale counts, fin ray counts, scale rows above and below the lateral line) are given in Garutti (1999) and are not replicated here. In overall appearance, the fish is silvery with the dark lateral stripe reaching the caudal peduncle that is typical of many small Astyanax, though the intensity and extent of this stripe varies among individuals and populations. Specific size data for this species are sparse in the literature; it is a small characin in the range typical for the genus in the upper Paraná.
Habitat
Astyanax argyrimarginatus is known from the upper Paraná River basin in Brazil. The upper Paraná system drains a large portion of southern Brazil, including the states of Goiás, Minas Gerais, Mato Grosso do Sul, and São Paulo, encompassing a diverse range of stream and river habitats from fast-flowing upland tributaries over rocky substrates to slower, silted lowland channels.
Small Astyanax of the upper Paraná typically occupy shallow, warm, well-oxygenated streams with moderate current, often associated with marginal vegetation and submerged root masses that provide cover. Water parameters across the basin vary but are generally soft to moderately hard and circumneutral to slightly acidic in headwater tributaries, with temperatures ranging from the low to upper 20s Celsius through seasonal variation. Specific microhabitat records for A. argyrimarginatus beyond the type locality have not been widely documented in available literature.
Feeding
No dietary study specific to Astyanax argyrimarginatus appears in the available literature. Small Astyanax of the upper Paraná basin have been extensively studied as a group, however, and consistently prove to be opportunistic omnivores. Their natural diet shifts with season and availability and typically includes aquatic insect larvae, small terrestrial insects fallen at the surface, microcrustaceans, plant matter, periphyton, and algae.
In the aquarium, small Astyanax are straightforward feeders that accept quality flake foods, micro-pellets, and frozen or live invertebrates such as Daphnia, brine shrimp, and bloodworm. Variety improves condition and coloration. As active midwater foragers they compete readily at feeding time and are not prone to dietary problems in a community setting.
Mating
No mating behaviour specific to Astyanax argyrimarginatus has been documented in the literature. The genus Astyanax is uniformly composed of egg-scattering open spawners, and it is reasonable to infer that A. argyrimarginatus follows the same pattern. Males in condition typically pursue females through open water and among fine-leaved vegetation, with no pair bond established and no territory defended prior to or after spawning.
As is typical in the genus, spawning activity in wild populations is likely tied to seasonal cues — the onset of the rainy season in the upper Paraná basin, accompanied by rising temperatures and water levels, is the most common trigger for breeding in small characins of this region. In captivity, conditioning with live or frozen foods and a modest water change with slightly cooler water often stimulates spawning attempts.
Breeding
No captive breeding records specific to Astyanax argyrimarginatus are known. Based on the uniform reproductive biology of small Astyanax, breeding would be expected to follow the standard characin pattern: eggs scattered among fine-leaved aquatic vegetation or over spawning substrate, adhesive or semi-adhesive, receiving no parental care whatsoever. Adults should be separated from the eggs promptly, as they will consume them.
At typical tropical temperatures of 75–81 °F, eggs of small Astyanax generally hatch within 24–48 hours, with fry becoming free-swimming a few days later. First foods need to be very small — infusoria, rotifers, or commercial liquid fry food — before the young can take newly hatched brine shrimp nauplii. These general principles should guide any first captive breeding attempt with this species, though published data confirming specific incubation times or clutch sizes for A. argyrimarginatus are not available.
In the aquarium
Astyanax argyrimarginatus does not appear in the hobby trade and is unknown as an aquarium fish outside research collections. It has not received coverage in specialist aquarium publications. Should it become available, the husbandry appropriate for small upper Paraná Astyanax generally would apply: a group of at least six fish in a well-filtered aquarium of 23.5–31.5 in or more, with soft to moderately hard water at a neutral to mildly acidic pH and temperatures in the 72–81 °F range.
Small Astyanax are active, shoaling fish that are peaceful toward tankmates of similar size. Dense planting along the margins with open swimming space in the centre suits their natural microhabitat. As with most small characins, a varied diet including live or frozen foods alongside quality dried foods will keep them in good health. Their appeal as aquarium fish would rest mainly on the silver-margined fin detail that gives the species its name — a subtle character more likely to interest the specialist than the general hobbyist.
Conservation
Astyanax argyrimarginatus has not been assessed by the IUCN Red List; it is listed as Not Evaluated. The species is known from the upper Paraná River basin, a heavily modified freshwater system subject to extensive impoundment, agricultural runoff, and urban development across southern and central Brazil. Many small characins endemic or near-endemic to this basin have experienced range contractions linked to reservoir construction and water-quality degradation, though the majority have not been formally assessed.
Without population data, range estimates, or threat assessments specific to this species, its conservation status cannot be characterised with confidence. Its position within the taxonomically complex Astyanax — where synonymies and new descriptions continue to shift species limits — also means that any future assessment would require careful taxonomic verification. The upper Paraná ichthyofauna overall is well studied, and A. argyrimarginatus's small-stream habitat is among the most impacted by Brazilian agricultural expansion.