Taxonomy & naming
Steindachnerina gracilis was described by Richard P. Vari and Carol J. Williams Vari in 1989. It belongs to the genus Steindachnerina, one of several genera of small detritivorous characins placed in the family Curimatidae — the "curimatas" — within the order Characiformes. Curimatidae is only loosely related, at the ordinal level, to the pacus, silver dollars and true piranhas that make up most of the "Piranhas & Pacus" group; curimatids lack the bladed, interlocking teeth that define Serrasalmidae and instead have reduced or absent jaw dentition adapted to sifting fine organic sediment rather than biting flesh or plant material.
The genus Steindachnerina honours the Austrian ichthyologist Franz Steindachner, a prolific describer of Neotropical fishes in the nineteenth century. The Catalog of Fishes (Eschmeyer, California Academy of Sciences) is the governing authority for the valid combination Steindachnerina gracilis Vari & Williams Vari, 1989, and no revision affecting this species' placement is reflected in the available sources; the species should not be confused with the unrelated Hemiodus gracilis, a similarly named but taxonomically distinct hemiodontid that turns up in some aquarium-trade references and appears to be a frequent source of mix-ups online.
Morphology
Steindachnerina gracilis is a small, slender, laterally compressed characin reaching a documented maximum of about 3 in standard length — modest even by the standards of the smaller minor characins, and far removed from the large-bodied pacus, dorado and wolf fish that share this group's page. As with other curimatids, the body plan favours an unremarkable, streamlined silvery fish built for steady grazing rather than pursuit or ambush; the family's defining feature is a reduced or toothless mouth suited to sifting sediment and biofilm rather than biting.
Detailed morphological description beyond size and family-level body plan is not available from the cached sources for this species specifically; readers should treat any finer-grained claims about fin shape, colour pattern or scale counts as unconfirmed until a dedicated description or FishBase morphometric account can be checked directly.
Habitat
The species is documented from the Tocantins River basin in Brazil, a major Amazon-adjacent drainage in north-central Brazil. FishBase characterises it as a benthopelagic, tropical freshwater fish, consistent with a life spent near the riverbed and just above it, foraging on fine detritus and organic sediment rather than schooling in open water.
Specific water chemistry data for S. gracilis itself was not located in the cache. Related Steindachnerina species from the same Tocantins drainage and family (S. insculpta, S. elegans) are documented as non-migratory, detritivorous fish typical of warm tropical Amazonian-basin rivers, broadly in the range of roughly 75–82 °F with neutral-to-slightly-acidic water — figures offered here as a reasonable family-level expectation rather than a confirmed reading for this species, and they should be treated as provisional.
Feeding
Steindachnerina gracilis is a detritivore, the typical feeding mode for its family: curimatids graze fine organic debris, biofilm and sediment from the substrate rather than hunting live prey or browsing on plants and fruit the way many of the larger characins in this group do. This puts it in the same broad dietary camp as the hemiodus and curimata grazers of the group, and at the opposite end of the spectrum from the predatory Brycon and Salminus or the fin-nipping piranhas.
No aquarium-specific feeding notes were found in the cache. A detritivorous curimatid of this size would be expected to do well on sinking, algae- and detritus-based foods in a planted or driftwood-furnished setup, but this is an inference from its documented biology rather than a reported aquarium account, and it is not currently known to be established in the hobby.
Mating
No courtship or pairing behaviour specific to Steindachnerina gracilis was found in the cache. Congeneric Steindachnerina species from the same Tocantins drainage (S. insculpta, S. elegans) are documented as having a prolonged reproductive season running from roughly September to March, with fractionated (batch) spawning rather than a single annual event, and no evidence of long-distance migration. Whether S. gracilis shares this exact pattern is not confirmed and should be treated as a reasonable family-level expectation rather than a documented fact for this species.
Breeding
Steindachnerina gracilis is not known to have been bred in the aquarium hobby, and no aquaculture or captive-breeding account was located in the cache. Based on the reproductive biology documented for its closest congeners in the same river system, it most likely follows the broader curimatid pattern of open-water or substrate egg-scattering with no parental care, spread across a prolonged spawning season rather than concentrated into a single migratory event — the pattern seen in far larger relatives like Prochilodus. This should be read as an informed inference from related species, not a confirmed account, and the honest summary is that its breeding biology in captivity is essentially undocumented.
In the aquarium
Steindachnerina gracilis does not appear to be an established aquarium fish. The cache turned up no reliable aquarium-trade, care or tankmate information for this species — searches for its common names and care requirements largely surfaced results for the unrelated, similarly named Hemiodus gracilis instead, which underscores how easily this quiet little curimatid gets lost in the shadow of showier, better-documented characins.
What can be said with confidence is scale-appropriate: at a maximum size of roughly 3 in, this is a genuinely small fish that would need to be kept as a group of its own kind rather than singly, in a modestly sized well-planted or driftwood-furnished tank with sediment for it to graze, and would have no place housed with anything predatory. Given the absence of documented aquarium history, prospective keepers should treat any care regimen as provisional and modelled on other small Curimatidae rather than on confirmed experience with this species.
Conservation
Steindachnerina gracilis is assessed by the IUCN Red List as Least Concern, an assessment dated 07 November 2018. This reflects a species that, while restricted to the Tocantins River basin, does not appear to face documented population decline or major identified threats in the available sources.
As with many single-basin Neotropical characins, the longer-term watch items are the general pressures facing the Tocantins system — damming, deforestation-linked sedimentation and water-quality change — rather than any species-specific threat currently on record. It is not part of the aquarium or food-fish trade in any documented capacity, so trade pressure is not a relevant conservation factor for this species.