Taxonomy & naming
Astyanax depressirostris was described by the Brazilian ichthyologist Alípio de Miranda Ribeiro in 1908, from material collected in the Rio Ribeira drainage of São Paulo state. The valid name, as recorded in Eschmeyer's Catalog of Fishes (Fricke, Eschmeyer & Fong), is Astyanax depressirostris Miranda Ribeiro, 1908.
The species belongs to Astyanax, one of the largest and most taxonomically complex genera in the Neotropical family Characidae. Astyanax is distributed from Mexico to Argentina and encompasses several hundred described species, many of which are difficult to separate on external characters alone. The genus has long been regarded as paraphyletic, and ongoing molecular and morphological revisions continue to reorganise its boundaries; A. depressirostris falls within the Brazilian Atlantic Forest clade, a group of closely related species centred on coastal drainages of southeastern and southern Brazil. Its specific epithet, depressirostris, refers to the flattened or depressed snout — a morphological character noted in the original description.
Morphology
Like the majority of Astyanax, A. depressirostris is a small to medium-sized, laterally compressed characin with the typical tetra body plan: a roughly oblong, moderately deep body, a forked caudal fin, the single dorsal fin of the Characiformes, and the characteristic adipose fin placed between the dorsal and caudal. The specific epithet draws attention to the depressed or somewhat flattened snout profile that distinguishes it from allied species with more rounded or pointed rostra.
Detailed published morphometric data for this species are sparse in the readily accessible literature. As with most Astyanax, the body is silvery, often with a lateral stripe of varying intensity running from behind the operculum toward the caudal peduncle, and a dark spot or smudge at the caudal fin base — characters typical of the genus but variable across populations and individuals. Precise maximum length data are not well documented in the sources available; the species is presumed to be in the small Astyanax size range typical of Atlantic Forest drainages.
Habitat
Astyanax depressirostris is recorded from the Rio Ribeira de Iguape basin, the major coastal river system of the southern coast of São Paulo state, Brazil, draining roughly southward through the Serra do Mar and the remnant Atlantic Forest before emptying into the Lagamar estuarine complex near Iguape. The Ribeira basin sits within the southern Atlantic Forest ecoregion, a region recognized for its exceptional freshwater fish endemism.
The waters of the Ribeira system vary from clear, fast-flowing hill streams in the upper reaches to broader, slower river channels in the lowlands. Atlantic Forest streams generally run cool to moderate in temperature, with soft, slightly acid to neutral water reflecting the granite and sandstone geology of much of the catchment. The species is recorded as a freshwater inhabitant; detailed microhabitat data — substrate preference, association with vegetation or particular flow regime — are not documented in the accessible literature for this species specifically.
Feeding
No detailed dietary studies specific to Astyanax depressirostris have been published in the sources available. Within the genus Astyanax, most species are opportunistic omnivores taking a wide range of small invertebrates, plant matter, algae, and allochthonous material (terrestrial insects and plant fragments falling from the riparian forest canopy), with the exact diet varying by season, habitat, and food availability.
By analogy with well-studied Atlantic Forest Astyanax, A. depressirostris likely feeds on small aquatic invertebrates, insect larvae, microcrustaceans, and plant or algal material. In the aquarium, species of this type accept standard small-particle prepared foods alongside frozen and live foods such as Daphnia, brine shrimp, and bloodworm.
Mating
No published mating observations specific to Astyanax depressirostris are available. The genus Astyanax is characterised by egg-scattering open spawning without parental care — the modal reproductive pattern across the Characidae. Males in breeding condition typically pursue females actively, with spawning occurring in the water column or among fine-leaved vegetation and submerged roots.
In Atlantic Forest stream communities, Astyanax breeding activity is often correlated with the rainy season and associated rises in water level and temperature, though the specifics vary by species and drainage. No sexual dimorphism has been documented for this species in the available literature, though gravid females of most small Astyanax become noticeably plumper around the abdomen before spawning.
Breeding
Breeding has not been documented in the aquarium for Astyanax depressirostris in the sources consulted. General Astyanax breeding biology is well understood from numerous congeners: the eggs are small, non-adhesive or weakly adhesive, scattered over fine-leaved plants or substrate, and receive no parental attention. Both adults and eggs should be separated promptly after spawning to prevent egg predation.
Fry are typically small and free-swimming within a few days of hatching at tropical temperatures. First foods for newly hatched Astyanax fry are usually infusoria-grade particles or commercial liquid fry preparations, transitioning to newly hatched brine shrimp nauplii and finely powdered dry food as the fry grow. Water quality must be maintained carefully during the early fry stage, as small characins are sensitive to ammonia spikes in rearing containers.
In the aquarium
Astyanax depressirostris is not established in the aquarium hobby and is not commercially available through normal trade channels. It is an Atlantic Forest endemic from a restricted river basin, and no captive-bred population or regular import stream exists for this species at the time of writing.
Should it ever become available, husbandry would follow the pattern established for similar small Atlantic Forest Astyanax: a well-filtered aquarium with some water movement, soft to moderately hard, neutral to slightly acid water in the low to mid 20s Celsius, kept as a small school of six or more, with a varied diet of small prepared and live or frozen foods. Atlantic Forest tetras of this type are generally undemanding once established, and their natural habitat in the Ribeira basin suggests they can tolerate cooler temperatures than the typical tropical aquarium.
Conservation
Astyanax depressirostris has not been assessed by the IUCN Red List at the time of writing and carries a status of Not Evaluated (NE). Its known range is restricted to the Rio Ribeira de Iguape drainage of São Paulo state, Brazil — a basin within the southern Atlantic Forest, one of the world's most threatened biodiversity hotspots. The Atlantic Forest as a whole has been reduced to less than 12 percent of its original extent, and freshwater fish diversity in its coastal drainages is under pressure from deforestation, agricultural runoff, urban expansion, and modification of river flow.
The Ribeira valley retains proportionally more intact Atlantic Forest than most coastal basins of southeastern Brazil, partly due to the topography and the lower density of intensive agriculture in the upper drainage. This relative land-cover integrity offers some protection to the freshwater communities of the basin, though ongoing development pressure means the status of endemic fishes like A. depressirostris warrants monitoring. A formal IUCN assessment would help clarify whether the species' restricted range represents a genuine conservation concern.