Taxonomy & naming
Argonectes robertsi was described by the Brazilian ichthyologist Francisco Langeani in 1999. It belongs to the genus Argonectes, a small group within the family Hemiodontidae — the hemiodontids, a Neotropical characiform lineage of slender, often silvery, mid-water to benthopelagic shoaling fish distinct from the deep-bodied Serrasalmidae (piranhas, pacus and silver dollars) that dominate the popular idea of "characins" in this atlas group. The Catalog of Fishes (Eschmeyer, California Academy of Sciences) is the authority for the currently valid combination Argonectes robertsi within Hemiodontidae.
The genus Argonectes itself contains only a couple of recognised species, the better-known being Argonectes longiceps, which occasionally reaches the aquarium trade and gives the best available proxy for how this group behaves and feeds. Argonectes robertsi is the more range-restricted and far less commonly encountered of the two, known primarily from Brazilian river systems rather than from broad international collection.
Morphology
Argonectes robertsi has the elongated, laterally compressed, streamlined body typical of hemiodontids — a shape built for holding position and moving efficiently in flowing water rather than the deep, disc-like profile of a pacu or silver dollar. FishBase records a maximum length of 14 in total length and a maximum recorded weight of 483 g, making it a solidly medium-sized characin rather than a small nano fish or a giant food fish; this is the real adult size and should be sized for accordingly, not assumed to stay small.
Colouration in the genus runs to bright, uniform silver flanks with a fine dark lateral stripe or band toward the caudal peduncle in related species, a countershaded pattern consistent with an open-water, schooling clearwater fish. Detailed morphological description specific to A. robertsi beyond the original 1999 description is sparse in the sources available here; readers should treat fine-grained colour and fin-shape detail as data sparse pending access to the primary literature.
Habitat
Argonectes robertsi is documented from the Tapajós, Xingu, Tocantins and Capim river basins in Brazil — southern and eastern tributaries of the Amazon system known for extensive stretches of clearwater habitat. FishBase classifies it as a tropical freshwater, benthopelagic species, meaning it forages both near the bottom and up into the water column rather than staying tied to either zone exclusively.
Specific water chemistry data (temperature range, pH, hardness) for A. robertsi is not captured in the sources available for this article; data sparse. Broadly, Tapajós, Xingu and Tocantins clearwater habitats run warm and are typically soft to moderately mineralised with near-neutral to mildly acidic pH, but that regional generalisation should not be treated as a confirmed figure for this species specifically.
Feeding
FishBase gives Argonectes robertsi a trophic level of 2.8 ± 0.4, placing it as an omnivore with a micropredatory component — consistent with a fish that forages over the riverbed and through the water column for a mixed diet rather than one that is either a pure grazer or a dedicated hunter. The closely related Argonectes longiceps, better documented in hobbyist and importer sources, is likewise described as an omnivore/micropredator that readily accepts small frozen and prepared foods in captivity, which is the best available proxy for how A. robertsi likely feeds given the shared genus and body plan.
No aquarium-specific feeding account for A. robertsi itself is available in the sources used here; keepers of the related A. longiceps report success with a varied diet of small meaty frozen foods (such as bloodworm and brine shrimp) alongside quality flake or small pellet, and that general approach is a reasonable starting point pending species-specific confirmation.
Mating
FishBase describes Argonectes robertsi as oviparous with "distinct pairing behavior" — language that indicates the species forms discrete male-female pairs around spawning rather than spawning as an undifferentiated mixed shoal, but the source does not elaborate on courtship displays, triggers, or how pairs are formed. No further detail on courtship behaviour, seasonal timing, or external sexual dimorphism is available in the sources consulted for this article; data sparse.
Breeding
Beyond FishBase's note of oviparous reproduction with distinct pairing behaviour, no documented account of spawning site, egg type, parental care, or captive breeding exists in the sources available for this article. There is no record of Argonectes robertsi being bred in the aquarium hobby or in aquaculture. Given the pairing behaviour noted by FishBase, an egg-scattering or loosely guarded pair-spawning strategy typical of hemiodontids is plausible by analogy with other characins in this family, but that inference is not confirmed for this species and should not be stated as fact. Prospective breeders should treat captive reproduction of this species as essentially unattempted and undocumented.
In the aquarium
Argonectes robertsi is not an established aquarium fish; it is rarely if ever offered in the trade, and virtually all practical care guidance available comes by extension from its better-known relative Argonectes longiceps, for which importer sources recommend a minimum tank size around 55 US gallons (roughly 55 US gal) — appropriate for an active, shoaling, mid-sized characin of this body plan rather than a small community species. Given a documented adult size of 14 in TL, a keeper who did source this fish should plan for a considerably larger, long, well-filtered tank with strong water flow to mimic clearwater river conditions, not a standard community setup.
As a benthopelagic, presumably shoaling hemiodontid, A. robertsi is likely best kept in a group rather than alone, alongside other robust, similarly fast-moving fish that can hold their own in current — again reasoning from genus-level behaviour rather than species-specific husbandry reports, since none exist in the sources used here. This is a fish for the specialist importer and the researcher rather than the general hobbyist.
Conservation
The IUCN Red List category for Argonectes robertsi as reported via FishBase is Least Concern, assessed 14 August 2020; no dedicated IUCN Red List species page was located in the sources used for this article, so the status here is taken from FishBase's citation of that assessment rather than from a directly verified iucnredlist.org record. As a species restricted to specific Brazilian Amazon tributaries — the Tapajós, Xingu, Tocantins and Capim basins — that are all subject to ongoing hydroelectric damming, deforestation and mining pressure, its Least Concern status should be read as reflecting current abundance and range rather than an assurance against future habitat change in those river systems.
There is no evidence in the sources consulted of a significant aquarium trade or fishery pressure on this species specifically; it does not appear to be a targeted food or ornamental fish at any meaningful scale.