Taxonomy & naming
Mylossoma acanthogaster was described by the French zoologist Achille Valenciennes in 1850, originally placed in the now-obsolete genus Myletes before being moved to Mylossoma, where the Catalog of Fishes (Eschmeyer, California Academy of Sciences) confirms it as the currently valid combination. It belongs to the family Serrasalmidae — the piranhas, pacus and silver dollars — within the order Characiformes, sitting in the pacu lineage of the family rather than among the predatory, blade-toothed true piranhas of Serrasalmus and Pygocentrus.
The genus Mylossoma is a small group of deep-bodied, silver-flanked pacus distributed across northern and central South America; most of its species range across the Amazon and Orinoco basins, which makes M. acanthogaster's confinement to the single, comparatively small Lago Maracaibo drainage of Venezuela an unusual pattern within the genus and a large part of why its conservation status differs from its more widespread relatives.
Morphology
Mylossoma acanthogaster shows the classic pacu-relative body plan: a strongly laterally compressed, deep, almost disc-shaped silver body, a small blunt head, and a row of flattened, molar-like teeth built for crushing rather than shearing flesh — the same basic dental architecture that lets its much larger relatives in Colossoma and Piaractus crack open nuts and hard seed coats. FishBase records a maximum published length of 11 in standard length, a solidly mid-sized fish for the family — larger than the true silver dollars of Metynnis, but well short of the near-metre-long true pacus.
Colouration is a simple, bright silver over most of the body, often with a faint golden-orange wash on the lower flanks and fins typical of the genus, and no strongly distinctive markings have been documented for this species beyond the general Mylossoma pattern. No reliable external differences between the sexes have been recorded.
Habitat
Mylossoma acanthogaster is endemic to the Lago Maracaibo basin in Venezuela — a single, large brackish-influenced lake system fed by numerous rivers draining the Venezuelan Andes and the Sierra de Perijá, rather than the sprawling multi-country Amazon or Orinoco ranges typical of its genus. FishBase describes it as a benthopelagic species of tropical freshwater, meaning it moves through the water column and near the bottom rather than staying tied to one zone.
Temperatures in the 75–86 °F range are typical for the genus and consistent with the warm lowland tropical waters of the Maracaibo basin. As with most Serrasalmidae, seasonal flooding likely shapes its access to the fruiting and seeding vegetation that dominates its diet, though detailed hydrological or water-chemistry data specific to this species is sparse in the available literature.
Feeding
Mylossoma acanthogaster is an omnivore with a strong frugivorous lean, reflected in a documented trophic level of 2.6 — consistent with a diet weighted toward plant material rather than animal prey. Across the genus, wild Mylossoma feed heavily on fallen fruit, seeds and other plant matter from flooded forest and marginal vegetation, supplemented by insects, and in doing so the group functions as a seed disperser for its native flooded forests, much like its larger pacu relatives.
In the aquarium, sources describing the genus report that Mylossoma readily take green foods — salad greens, dandelion and similar vegetable matter — and will graze on robust live plants if kept in a planted tank, so keepers should plan on vegetable-forward feeding with sturdy or artificial plantings rather than a delicate aquascape.
Mating
No species-specific courtship behaviour has been documented for Mylossoma acanthogaster in the available literature. Data is sparse for this Maracaibo-basin endemic compared with the much better-studied Amazonian pacus, and nothing should be assumed about its courtship beyond the general pattern of migratory Serrasalmidae, which typically pair or aggregate ahead of a flood-triggered spawning migration rather than displaying fixed courtship rituals in place.
Breeding
As with its pacu relatives in the family Serrasalmidae, Mylossoma acanthogaster is presumed to be a long-distance, flood-pulse spawning migrator rather than a fish that pairs off and guards a nest. Commercial pacu aquaculture in South America relies on hormone-induced spawning of migratory species precisely because these fish do not reliably reproduce on their own in static, confined water — a pattern documented across Colossoma, Piaractus and the wider pacu lineage that Mylossoma sits within.
No aquarium or hobby breeding reports exist for this species in the sources reviewed, and given its modest but still substantial adult size and its migratory relatives' reproductive biology, it should be considered essentially unbred in captivity. There is no parental care documented or expected for this reproductive mode.
In the aquarium
Sources describing the genus report that Mylossoma is peaceful within its own species and toward other fish, with no special water-chemistry demands, and that it does best kept in a swarm within a genuinely large aquarium — a shoaling temperament shared with silver dollars rather than the solitary or paired behaviour of true piranhas. At a documented maximum of 11 in SL, this is nonetheless a mid-to-large characin that outgrows small community tanks and needs real swimming room and robust filtration for a group.
Because the species leans frugivorous and will graze on plants, keepers should offer vegetable matter as a diet staple and expect that soft-leaved live plants will be eaten; tough, well-anchored or artificial plantings hold up better. As with other Serrasalmidae, this fish should never be released into open water if a keeper can no longer accommodate it — feral pacu relatives are an invasive-species problem on multiple continents, and specimens should be rehomed responsibly instead.
Conservation
The IUCN Red List assesses Mylossoma acanthogaster as Vulnerable (assessment dated 15 January 2021), a notably more concerning status than most widespread Amazonian and Orinoco Mylossoma species carry. The distinction traces directly to range: this species is endemic to the single Lago Maracaibo basin in Venezuela rather than distributed across a multi-country river system, which concentrates any pressure on habitat quality, water use or fishing within one lake drainage instead of spreading the risk across a continental range.
Detailed threat data specific to this species is sparse in the sources reviewed, but Lago Maracaibo and its tributaries face well-known regional pressures from urban and industrial development, agricultural runoff and oil-related infrastructure around the lake, any of which plausibly bears on a single-basin endemic like this one. Given the limited published detail, this article does not go further than the IUCN's own Vulnerable listing and the endemism that underlies it.