Taxonomy & naming
Oligosarcus amome was formally described by Adriana Almirón, Jorge Casciotta, Lubomír Piálek, Vendula Doubnerová, and Oldřich Říčan in 2015, published in Zootaxa 3915(4):581–590. The type locality is arroyo Oveja Negra, a tributary of Arroyo Yabotí-Guazú in Misiones Province, Argentina (27°08'15.5"S, 53°55'28.8"W), within the Río Uruguay drainage. The species is placed in the genus Oligosarcus Günther, 1864, a group of small to moderately large predatory characids distributed across southeastern South America.
Phylogenetic analysis by the authors recovered O. amome as the sister taxon to all remaining Oligosarcus species with the exception of O. itau, placing it near the base of the genus and underscoring its significance for the group's biogeographic and systematic history. The valid name is governed by Eschmeyer's Catalog of Fishes (CAS), which lists Oligosarcus amome Almirón, Casciotta, Piálek, Doubnerová & Říčan, 2015.
Morphology
Oligosarcus amome reaches a maximum length of 5 in standard length (SL), a modest size for the genus. Like other oligosarcines, it is a slender, fusiform fish with a relatively large mouth armed with well-developed teeth — the jaws of a pursuit predator built for seizing prey smaller than itself.
The most diagnostic feature distinguishing O. amome from congeners is its dentition: the outer series of the premaxilla bears pentacuspidate (five-cusped) teeth, and the species is further characterized by two conspicuous series of teeth on the premaxilla. These dental characters, along with meristic data presented in the original description, separate it from other Oligosarcus species in the upper Río Uruguay and Paraná systems. Body colouration in life has not been extensively documented in the published literature; like most Oligosarcus it is presumed to be silvery with a dark lateral stripe or midlateral pigmentation typical of the genus.
Habitat
Oligosarcus amome inhabits the small forested streams and tributaries of the upper Río Uruguay basin in Misiones Province, northeastern Argentina — a subtropical region of the Atlantic Forest biome characterized by warm temperatures, high humidity, and streams flowing over rocky and sandy substrates under dense riparian canopy.
Based on broader Oligosarcus ecology, the species likely occupies shallow areas with dense marginal vegetation in small tributaries and river channels. The arroyo Oveja Negra and Arroyo Yabotí-Guazú systems where it was collected are tributaries of the middle Uruguay drainage in one of Argentina's most biodiverse freshwater regions. The rarity implied by its Guaraní name — 'once in a while' — suggests it is not an abundant constituent of these assemblages, at least in the localities surveyed during collection.
Feeding
The genus Oligosarcus is predominantly piscivorous. Species in this group are active predators in the water column, pursuing smaller fishes in open and vegetated areas of streams. Aquatic insects, terrestrial arthropods, and other invertebrates supplement the diet in some species, particularly in smaller-bodied individuals and in streams where small fishes are less available.
Given O. amome's modest maximum size of 5 in SL and its dentition — multi-cusped teeth suited for gripping — it likely targets small fishes and large invertebrates in proportion to its own body size. No specific dietary study of O. amome has been published; inferences draw on patterns documented across the genus in southeastern South America.
Mating
No published mating observations specific to Oligosarcus amome are available. Based on the reproductive biology documented for other Oligosarcus species and the broader characid family pattern, the species is expected to be an egg-scattering open spawner with no parental care. Males likely court females with brief chasing behaviour, and spawning probably occurs over submerged vegetation or rocky substrate in stream shallows.
Egg adhesion to fine-leaved plants or submerged surfaces is probable given the eggs' need for oxygenated water in swift-flowing stream environments. No secondary sexual characters distinguishing males from females have been described in the original publication; sexual dimorphism, if any, is likely subtle at standard length.
Breeding
Oligosarcus amome has not been bred in captivity to any documented extent, and no published accounts of aquarium reproduction exist. Extrapolating from the general Oligosarcus reproductive template: spawning would be non-adhesive to weakly adhesive egg-scattering over vegetation or gravel, with no parental care and adults indifferent or predatory toward their own eggs after spawning.
Incubation and fry development data are unavailable for this species. Any captive breeding attempt would need to provide good water quality, live or frozen prey to condition adults, and a separate rearing container given the predatory nature of the adults. The rarity of the species in the hobby means captive-bred stock essentially does not exist at present.
In the aquarium
Oligosarcus amome is essentially unknown in the aquarium hobby and is not commercially available. It has not appeared in the ornamental-fish trade to any meaningful extent, and there are no husbandry accounts in the hobbyist literature. Any care guidance must be extrapolated from the wider Oligosarcus genus: these are active, predatory characids that require a spacious aquarium with a long swimming lane, good oxygenation, and tankmates large enough not to be eaten.
A subtropical temperature range appropriate for Misiones stream fish — roughly 64–79 °F — would be most suitable. Feeding on live or frozen meaty foods (small fish, large invertebrates) would be expected. Because the species is so rarely encountered even in its native range, dedicated collectors or research institutions would be the only realistic keepers; the species has no established hobby presence.
Conservation
Oligosarcus amome has not been formally assessed by the IUCN Red List; as of the date of this article it carries no Red List category. The species was described in 2015 from a restricted set of localities in the Arroyo Yabotí-Guazú system in Misiones Province, Argentina — a region that falls partly within the Yabotí Biosphere Reserve, which affords some habitat protection.
The rarity implied by its Guaraní name and its apparent low encounter rate in collecting surveys suggest that the species may have a naturally limited distribution and low abundance. The Atlantic Forest streams of Misiones face ongoing pressures from deforestation, agriculture, and water abstraction. Given its very recently described status and the paucity of occurrence data, a precautionary approach to any wild collection would be appropriate until population surveys can establish whether the species merits formal listing.