Taxonomy & naming
Acnodon senai was described by Michel Jégu and Geraldo M. dos Santos in 1990, part of a wave of Serrasalmidae taxonomy from Brazilian and French ichthyologists working through the Amazon's small-bodied pacu genera. It sits in the genus Acnodon, a group of South American serrasalmids distinguished from the well-known dollar genera Metynnis and Myloplus by dentition and other anatomical details, within the family Serrasalmidae — the piranhas, pacus and silver dollars — in the order Characiformes.
The species epithet honours Anazildo Mateus de Sena, a Brazilian fisheries worker with Brazil's National Institute for Amazonian Research (INPA) who collected the holotype from the Jari River in 1987. As with several small-range Serrasalmidae described from single river systems, A. senai has not been the subject of a dedicated post-description revision, and its taxonomic status rests on the original 1990 paper; the Catalog of Fishes (Eschmeyer, California Academy of Sciences) is the reference point for its current valid combination.
Morphology
Acnodon senai is a small serrasalmid, reaching a maximum recorded standard length of 5 in — modest even by silver-dollar standards, and far short of the pacu giants elsewhere in the family. Like other Acnodon species it shows the compressed, disc-shaped body plan typical of the silver-dollar lineage: a deep, laterally flattened silhouette built for holding position in current and grazing rather than for burst predation.
Detailed colouration and fin-ray counts for A. senai specifically are not well documented outside the original species description. The closest relative for which husbandry-relevant morphology has been published in the hobby literature, Acnodon normani, reaches a similar 4.5–5 in SL and shows the silvery, laterally compressed body typical of the genus; A. senai can reasonably be expected to share this general body plan, though specific colour pattern should be treated as data sparse pending direct confirmation.
Habitat
The species is known from the Jari River basin, a Guiana Shield tributary that forms part of the Amapá–Pará border in northern Brazil and drains into the Amazon near its mouth. The type locality lies downstream on the Rio Jari, at Carapatinho Island (approximately 0°35'45" S, 52°38'20" W), and the species is currently treated as endemic to this basin — it has not been reported from the wider Amazon mainstem or other tributaries.
Specific water chemistry data for the Jari population have not been published, but the genus Acnodon as a whole is associated with the soft, acidic blackwater and clearwater conditions typical of Guiana Shield drainages. FishBase classifies the species as a freshwater, benthopelagic fish of tropical distribution.
Feeding
Acnodon senai is herbivorous, with FishBase assigning it a low trophic level (approximately 2.0) consistent with a diet built around plant material rather than animal prey. This places it firmly with the grazing, algae- and plant-eating end of the Serrasalmidae rather than with the predatory true piranhas in the same family.
In the related species A. normani, feeding studies note a primarily herbivorous diet with some observed scale-eating, a pattern documented elsewhere in silver-dollar-type serrasalmids. Whether A. senai shares the occasional scale-eating habit is not documented; in the aquarium, a diet weighted toward vegetable matter — blanched greens, spirulina-based foods and algae wafers — with some supplementary protein is the safe, biologically grounded approach for this genus.
Mating
No courtship or spawning behaviour has been documented for Acnodon senai in the available literature. Breeding in the closely related A. normani is likewise unrecorded, which is typical for small-range, poorly studied Serrasalmidae outside the handful of commercially important pacus and popularly kept piranhas. Data here should be treated as sparse rather than absent by inference; nothing specific can be stated about pairing, courtship displays or spawning triggers for this species.
Breeding
As with its close relative Acnodon normani, no breeding has been recorded for Acnodon senai in aquaria or reported from the wild. Where silver dollars in general have been bred — in genera such as Metynnis and Myloplus — the pattern is egg-scattering over fine-leaved plants with no parental care, and it is reasonable to expect Acnodon would follow the same broad reproductive strategy typical of the silver-dollar lineage within Serrasalmidae. That expectation is not, however, a documented fact for this species, and it has essentially no track record in the hobby.
Given the complete absence of published breeding accounts, the honest statement is that Acnodon senai is not a species with any established aquarium breeding history.
In the aquarium
Acnodon senai is essentially unknown in the aquarium trade — it is a narrow-range Jari River endemic with no significant commercial collection history, and hobbyist experience with the species specifically does not exist in any documented form. Anyone encountering it in the trade would be working from congener data rather than species-specific care sheets.
What can be said comes from the genus: Acnodon species are compact by pacu standards (4.5–5 in SL) but are still committed herbivores that will graze and shred live aquarium plants, so they belong in a robustly planted or hardscaped setup rather than a delicate aquascape, with soft, acidic water in the AquaSoc-typical range of roughly 72–82 °F and pH 5.5–7.5 reported for the related A. normani. Some intraspecific aggression has been noted in that species, suggesting a group kept with adequate space rather than a lone specimen or an overcrowded tank is the safer approach. None of this is confirmed for A. senai itself, and a prospective keeper should treat it as provisional, congener-derived guidance rather than established fact.
Conservation
The IUCN Red List has not published a species-specific assessment for Acnodon senai that could be independently verified; FishBase records the species' conservation status as Data Deficient, with an assessment dated 2018. This reflects the species' narrow known range — a single Amazonian tributary system — and the general lack of population, threat and trend data rather than any evidence of stability or decline.
As a single-river endemic in a basin subject to the broader pressures facing Amazonian freshwater systems — damming, deforestation-linked sedimentation and mining activity in the Jari basin — Acnodon senai is a reasonable candidate for future conservation attention once more field data become available, but no specific threats have been documented for it to date.