Taxonomy & naming
Andromakhe stenohalina was described by Messner in 1962, published in mimeographed form in the Boletin de la Asociación Latino-Americana de Ictiólogos y Herpetólogos. The original type material comprised two syntypes (MHNM 758, 759) deposited at the Museo Nacional de Historia Natural de Montevideo, with the type locality given as Arroyo Tomás Cuadra, a tributary of the Río Negro (itself a tributary of the Río Uruguay), in the department of Durazno, Uruguay.
The species was originally placed in Astyanax and later assigned to Hyphessobrycon. A 2020 reclassification erected the genus Andromakhe and transferred this species into it, where it is currently treated as Andromakhe stenohalina. The genus name alludes to the figure of Andromakhe in Greek mythology — notable as the mother of Astyanax — reflecting a conceptual link to the earlier generic placement. The specific epithet stenohalinus derives from Greek roots meaning narrow tolerance of salt, referencing the species' freshwater restriction.
This fish was long listed as Hyphessobrycon stenohalinus; Eschmeyer's Catalog of Fishes places the species in Andromakhe, and the current combination Andromakhe stenohalina is followed here.
Morphology
Andromakhe stenohalina is a slender characid reaching up to 3.1 in standard length. Among its most distinctive features is the presence of hooks on all fins in adult males — a characteristic seen in some other members of this lineage and useful in distinguishing the species from superficially similar sympatric forms. The anal fin is notably long, with 25 to 30 branched rays, and the lateral line carries 37 to 39 perforated scales.
The dentition is relatively modest: 2 to 3 maxillary teeth are present, and the teeth themselves are not expanded. Like many characids of the Río de la Plata basin, the body is fusiform and laterally compressed, suited to life in flowing and still inland waters. Detailed meristic data from museum specimens and redescription work have confirmed the consistency of these characters across the species' range in Argentina, Uruguay, and southern Brazil.
Habitat
The species is distributed across the Río de la Plata basin in Argentina (Buenos Aires province), the Atlantic slope drainages of Uruguay including the Laguna Merín system, and the Vacacaí drainage in Rio Grande do Sul, Brazil. Its geographic range spans subtropical latitudes roughly between 32°S and 33°S, at low elevations.
Within these drainages it occupies freshwater habitats including lowland streams, tributaries, and associated still and slow-flowing waters. The type locality — Arroyo Tomás Cuadra in the Río Negro–Río Uruguay tributary network — typifies the inland, warm-temperate to subtropical stream systems the species uses. Like other lowland South American characids, it likely frequents marginal vegetation and areas with structural complexity.
Feeding
No detailed dietary studies specific to Andromakhe stenohalina are available in the accessible literature. Based on its body form and placement within the broader Hyphessobrycon-allied lineages, it is presumed to be an opportunistic omnivore in the manner of most small characids, feeding on small invertebrates, microcrustaceans, insects, and organic particles in the water column and at the surface.
In an aquarium setting, fish of this size and build typically accept commercial flake and micro-pellet diets supplemented with small live or frozen foods such as Daphnia, brine shrimp nauplii, or bloodworm. Dietary data remain sparse, and detailed gut-content studies have not been published in the sources available.
Mating
Specific information on the mating behaviour of Andromakhe stenohalina has not been published in accessible sources. As a member of a lineage of open-water egg-scatterers, it is expected to follow the general pattern seen in allied small characids: males displaying toward females, with spawning occurring without pair bonding or territory defence and no lasting social structure around reproduction.
The presence of fin hooks in adult males — a widespread feature among certain characid groups — is associated with mating behaviour in several related species, where hooks are thought to play a role during spawning contact, potentially assisting in the alignment or entanglement of spawning partners. In aquarium conditions, closely related egg-scattering characids typically spawn among fine-leaved vegetation or over substrate, releasing adhesive eggs that receive no further parental attention.
Breeding
No captive breeding accounts for Andromakhe stenohalina have been documented in the accessible hobby or scientific literature. Given the species' membership in the broadly defined Hyphessobrycon alliance and its apparently similar morphology to other open-water spawning characids, the general pattern of egg-scattering with adhesive eggs deposited among plants or over fine substrate, followed by no parental care, is the most likely breeding mode.
Fish of this type are typically induced to spawn by a drop in water temperature, light simulation of rain, and the provision of fine-leaved plants or synthetic spawning mops. Eggs hatch in one to several days depending on temperature, and the small fry require infusoria or very fine commercial fry foods initially before accepting larger items. These details are inferred from the biology of congeners; breeding data specific to this species are sparse.
In the aquarium
Andromakhe stenohalina is rarely encountered in the ornamental trade and is primarily of interest to scientists and collectors with a focus on Río de la Plata basin fauna. For hobbyists who do keep it, the general care requirements of a subtropical small characid apply: cool to warm freshwater conditions in a well-filtered, planted aquarium with open swimming space and some vegetated cover.
Given its subtropical distribution, it likely tolerates a wider temperature range than many popular aquarium tetras, and cooler conditions typical of temperate-to-subtropical Argentina and Uruguay may be appropriate for long-term maintenance and conditioning for breeding. It should be kept in groups, as shoaling behaviour is common to virtually all small characids. Tankmates should be peaceful and of similar size. The long anal fin and fin hooks of males make this species identifiable to experienced observers, though it lacks the vivid colours of ornamental tetras bred for the trade.
Conservation
Andromakhe stenohalina (as Andromakhe stenohalina) is assessed as Near Threatened (NT) on the IUCN Red List, with the assessment completed on 18 February 2021 by José Bessonart and Marcelo Loureiro. The assessment applied criterion B2b(iii), indicating a restricted extent of occurrence with inferred continuing decline in the quality of the occupied habitat.
The Río de la Plata basin is one of South America's most heavily impacted freshwater systems, facing ongoing pressures from agricultural runoff, wetland conversion, water extraction, and urbanisation across its Argentine, Uruguayan, and Brazilian extent. Species with restricted or patchy distributions within this basin — particularly those tied to specific drainage systems or habitat types — are considered at elevated risk from these cumulative pressures. The Near Threatened listing signals that while the species is not currently classified as threatened, the trajectory of habitat quality gives cause for monitoring and precautionary management.