Taxonomy & naming
Astyanax dissimilis was described by Júlio César Garavello and Fernando Antônio Abrantes Sampaio in 2010, in a paper published in the Brazilian Journal of Biology presenting five new Astyanax species from the Rio Iguaçu, Paraná, Brazil. The type locality is the Rio Iguaçu drainage in the upper Paraná basin of southern Brazil. Eschmeyer's Catalog of Fishes and the Plazi TreatmentBank both record the valid name as Astyanax dissimilis Garavello & Sampaio, 2010, placed in Order Characiformes, Family Characidae.
Some recent molecular and taxonomic revisions of the vast genus Astyanax have transferred a number of South American species into the segregate genus Psalidodon. FishBase references Psalidodon dissimilis as a current valid combination, reflecting this reclassification; however, the Catalogue of Life cross-reference confirms Astyanax dissimilis as the original combination. This atlas follows the original description pending authoritative stabilisation in Eschmeyer's Catalog. Because Astyanax contains several hundred described species and the Iguaçu system is unusually species-rich owing to the biogeographic isolation imposed by Iguazu Falls, A. dissimilis is one of many closely related small tetras whose genus-level placement continues to be debated.
Morphology
Astyanax dissimilis is a small characin in the typical mould of its genus: a laterally compressed, silvery-bodied fish with the large eyes, terminal mouth, and rounded fins characteristic of Iguaçu Astyanax. Published size data are limited; the species is estimated at roughly 1–1.5 in standard length based on comparisons within the genus as described from Iguaçu material, placing it among the smaller members of Astyanax.
The diagnostic characters described by Garavello and Sampaio centre on the osteology of the orbital region and the dentition. Astyanax dissimilis is distinguished by the third infraorbital bone not contacting the preopercle, leaving a naked area between the two bones — a character that separates it from several sympatric congeners. The tooth formula is also distinctive: the premaxillary external series carries 2–5 tricuspid teeth; non-symphysial teeth are pentacuspid and form a ventral arch; and the maxillary bears 0–3 teeth. A faint, narrow, elongate vertical bar is present at the humeral region, the only obvious external colour mark beyond the general silvery flanks typical of small Astyanax.
Habitat
Astyanax dissimilis is endemic to the Rio Iguaçu (Iguazu River), a major left-bank tributary of the lower Paraná River that drains much of the state of Paraná and parts of Santa Catarina in southern Brazil, with its lower reaches forming the Argentina–Brazil border at Iguazu Falls. The falls constitute a near-absolute upstream barrier to fish dispersal from the Paraná, and the fauna above and below the cataracts is largely distinct; it is this isolation that has driven the remarkable species radiation of small characins, including several Astyanax, within the upper Iguaçu.
Within the Iguaçu system the species likely inhabits freshwater rivers and streams with moderate current and vegetated margins, consistent with the general habitat preferences of small Astyanax in subtropical South America. Water parameters for the upper Iguaçu basin fall within a range of approximately pH 6.0–8.0 and temperatures of 72–82 °F (72–82 °F), reflecting the subtropical climate of Paraná state, which experiences cool winters relative to tropical Brazilian drainages.
Feeding
No dedicated dietary study has been published for Astyanax dissimilis. Based on its membership in Astyanax and its small size and tooth morphology — tricuspid and pentacuspid teeth suited to handling small invertebrates and plant matter — it is consistent with the opportunistic omnivory characteristic of the genus. Iguaçu Astyanax generally exploit a wide range of small invertebrates, aquatic insect larvae, algae, and allochthonous organic matter available in lotic habitats.
In the aquarium, small Astyanax species of similar size and ecology typically accept good-quality flake food, small pellets, and frozen or live foods such as Daphnia, baby brine shrimp, and micro-worms. A varied diet supports condition and coloration, though the species carries no striking colour requiring enhancement.
Mating
No specific observations of courtship behaviour in Astyanax dissimilis have been published. Within Astyanax broadly, mating involves egg-scattering open spawning without pair bonding; males pursue ripe females and spawning is typically non-selective, with eggs broadcast among fine-leaved aquatic vegetation or over the substrate. The genus shows no parental investment of any kind, and spawning adults will readily consume their own eggs.
For aquarium-kept fish of this genus, conditioning with live or frozen foods promotes spawning readiness. Displays, where noted in small Astyanax, are brief chasing behaviours rather than elaborate rituals, and multiple males may participate in a single spawning event.
Breeding
Astyanax dissimilis has not been bred in captivity to any documented degree, and no aquarium breeding accounts are known. The reproductive biology is inferred from close congeners: like most small Astyanax, the species is presumed to be an egg-scatterer that deposits small, adhesive eggs among fine-leaved plants or over open substrate, with no parental care. Eggs would be abandoned immediately after spawning.
Should captive specimens become available, standard small-tetra breeding protocols would apply: a separate, well-planted breeding tank or one furnished with fine synthetic mops, slightly warmer water to trigger spawning, and prompt removal of adults or eggs to a separate rearing vessel to prevent predation. Incubation periods for comparably sized subtropical Astyanax are typically several days at temperatures in the mid-20s Celsius, with free-swimming fry requiring very fine first foods before progressing to newly hatched brine shrimp.
In the aquarium
Astyanax dissimilis is not established in the aquarium trade and is not commercially available. Because the entire known range lies within a system where export of ornamental fish from Brazil faces regulatory constraints, and because the species has no common name and remains little known outside specialist ichthyology, it is unlikely to appear in the hobby in the near term. Any captive specimens would arrive as incidental by-catches in scientific collections rather than through the ornamental trade.
For reference, a small Astyanax of approximately 1–1.5 in standard length would be suited to a modest aquarium of 23.5 in or more in length, kept in a school of six or more individuals with soft, slightly acidic to neutral water in the low-to-mid 20s Celsius. Being subtropical in origin it tolerates temperatures lower than many traded tropicals. As an active, peaceful schooling fish it would suit a community aquarium with other small peaceful species. Until the species enters trade and captive experience accumulates, care guidance must remain tentative and based on congener parallels.
Conservation
Astyanax dissimilis has not been formally assessed by the IUCN Red List and carries a status of Not Evaluated (NE). The species was only described in 2010, and many recently described small endemic fishes from South American river systems have yet to receive formal conservation evaluation.
The Rio Iguaçu drainage faces pressures typical of the heavily populated and agriculturally intensive state of Paraná: water abstraction, sedimentation from land clearance, agricultural run-off, and impoundment from hydroelectric development. The upper Iguaçu in particular has been affected by dam construction, which can fragment stream habitats and alter the flow regimes and sediment dynamics that small riverine characins depend on. As a species with an apparently restricted range within a single drainage system — one already experiencing significant anthropogenic modification — A. dissimilis may warrant conservation attention, but no population data exist to support an assessment at this time.