Taxonomy & naming
Parodon hilarii was described by the Danish zoologist Johannes Theodor Reinhardt in 1867. It belongs to the genus Parodon within the family Parodontidae, a small Neotropical family of rheophilic (current-loving) characiforms commonly called scrapetooths for their specialised algae-scraping dentition. The genus Parodon is distinguished from its closest relative, Apareiodon, by the presence of dentary (lower-jaw) teeth — Apareiodon largely lacks them.
Parodontidae sits apart from the large, showier characin lineages covered elsewhere in this atlas — the true piranhas of Serrasalmidae, the migratory pacus and flannelmouths, the predatory dorado — as a family of small, drab, current-adapted grazers. The Catalog of Fishes (Eschmeyer, California Academy of Sciences) is the governing authority on the valid name and combination; nothing in the available sources suggests a synonymy or revision affecting Parodon hilarii, so it is treated here under its original combination.
Morphology
Parodon hilarii is a small, slender-bodied characin, reaching a maximum recorded length of about 5.5 in standard length. Like other members of its family, it has a subterminal, ventrally positioned mouth adapted for scraping rather than biting, set behind a somewhat pointed, downturned snout that lets the fish work its teeth against rock and gravel surfaces while holding position in current.
The genus's diagnostic feature is dental: Parodon species carry functional teeth on the dentary (lower jaw) as well as the premaxilla, an arrangement used to compare it against the closely related genus Apareiodon. Body colouration in Parodontidae generally runs to muted silvers, olives and browns with faint banding — cryptic patterning suited to a life spent pressed against streambed substrate rather than schooling in open water. Specific colour-pattern detail for Parodon hilarii is not covered in the available sources.
Habitat
Parodon hilarii is native to the São Francisco River basin of Brazil, one of the country's major river systems, running from the interior highlands to the Atlantic. As a rheophilic species, it favours flowing water — riffles, rapids and rocky-bottomed stretches of river rather than still lagoons or floodplain lakes — where current keeps stone surfaces scoured and productive for algal growth.
Detailed water-chemistry data specific to Parodon hilarii was not located in the available sources. A related congener, Parodon pongoensis, has been reported at roughly pH 6.0–7.5 and 72–79 °F, which is broadly consistent with the moderate, well-oxygenated conditions typical of Parodontidae as a family, though this should be read as a genus-level approximation rather than a confirmed figure for this species.
Feeding
Parodontidae are peraphytophagous specialists — a technical way of saying they graze the perilithon layer, the algae, biofilm and associated microfauna that coats submerged rock in flowing water. Parodon hilarii fits this pattern as a grazer rather than a predator or opportunistic omnivore; its specialised jaw and scraping teeth are built for rasping surfaces, not for taking fish, fruit or large invertebrate prey.
In the wild this diet is continuous grazing on riffle and rapid substrate. No aquarium feeding data for this species was found in the available sources, but a herbivore/grazer of this kind would be expected to need algae-based and vegetable-forward foods with steady access to grazing surfaces rather than a heavy protein diet — an inference from its documented biology, not a confirmed aquarium record.
Mating
No courtship or spawning-behaviour data for Parodon hilarii was located in the available sources. FishBase notes that the species carries a 2n=54 chromosome complement with a ZZ/ZW sex-determination system, which is a genetic rather than a behavioural or morphological detail — it does not by itself indicate any visible external dimorphism, and none is documented for this species. Data sparse; nothing further can be stated without guessing.
Breeding
There is no documented account of Parodon hilarii being bred in the aquarium, and the broader literature on Parodontidae describes the family as a captive-breeding rarity with data sparse on reproduction generally. Rheophilic scrapetooths of this kind are presumed, by analogy with related riffle-dwelling characiforms, to spawn in flowing water rather than to guard a nest or brood young, but this is not confirmed for Parodon hilarii specifically and should not be taken as an established breeding mode.
Given the complete absence of aquarium-breeding records, this species should be treated as essentially unbred in captivity. Anyone encountering claims of successful home breeding of Parodon hilarii should treat them with real skepticism until backed by a documented source.
In the aquarium
Parodon hilarii is not an established aquarium fish, and no meaningful hobbyist husbandry record exists in the available sources. What can be inferred responsibly from its biology: this is a small (to roughly 5.5 in), current-loving grazer, not a predator and not a fish that grows to outgrow a home tank the way the family's pacus and wolf fish do. A setup built around its wild habitat would need strong water flow, oxygenation, and hard surfaces (rock, gravel) for it to graze, plus tankmates that are not aggressive competitors for that same grazing niche.
Because this species is essentially unknown in the trade and undocumented in captivity, prospective keepers should treat any care claims beyond these basics as unverified. This is a case where the honest answer is that data is sparse, not that the fish is easy or thoroughly worked out.
Conservation
Parodon hilarii has not been evaluated by the IUCN Red List; its status is Not Evaluated. This is unsurprising for a small, non-commercial river grazer restricted to part of a single South American basin, and it places the species among the many obscure, single-region characins that simply have not been assessed.
Its native range, the São Francisco basin, is one of Brazil's most heavily dammed and altered river systems, which is a relevant long-term watch item for any rheophilic species dependent on flowing, well-oxygenated riffle habitat — but no population-trend or threat data specific to Parodon hilarii was found in the available sources, so no conservation status beyond Not Evaluated can be responsibly stated here.