Taxonomy & naming
Myleus pachyodus was described in 2018 by Andrade, Jégu, Buckup and Netto-Ferreira in the Journal of Fish Biology, based on specimens collected from the Rio Teles Pires and Rio Jamanxim, both tributaries of the Rio Tapajós in the southern Brazilian Amazon. It belongs to the family Serrasalmidae — the piranhas, pacus and silver dollars — within the order Characiformes, and to the genus Myleus, a group of laterally compressed, herbivorous silver dollars historically confused with one another on the basis of body shape and colour pattern alone.
The species epithet pachyodus comes from the Greek pachys ("thick") and odus ("tooth"), referring to the diagnostic character the describing authors used to separate it from its congeners: teeth that are slender and incisiform in juveniles but thicken markedly into robust, bulky teeth once the fish reaches roughly 4 in standard length. As with much of Serrasalmidae, the genus-level classification of silver dollars has been revised repeatedly in recent decades, with several species once placed in Myleus moved into Myloplus, Tometes and other genera; Myleus pachyodus was described directly into Myleus and there is no indication in the literature that this placement is currently disputed.
Morphology
Myleus pachyodus has the typical silver dollar body plan: a deep, strongly laterally compressed, roughly discoid outline that lets the fish turn tightly in cover while still being an efficient open-water swimmer in a group. The holotype, a female, measured 8 in standard length (8 in), with a paratype at 7 in SL — sizes that place this species among the larger silver dollars, and consistent with an adult size in the region of 8–10 in based on comparable Myleus species.
The feature that defines the species is dental: juveniles carry slender, incisor-like (incisiform) teeth typical of young serrasalmids, but as the fish grows past roughly 4 in SL the teeth thicken into the bulky, robust form that gives the species its name. This ontogenetic shift is the primary character the describing authors used to distinguish Myleus pachyodus from its close relatives, and it likely reflects a diet that includes tougher plant material as the fish matures.
Habitat
The species is known from the Rio Teles Pires and Rio Jamanxim, tributaries that feed the Rio Tapajós in the southern Brazilian Amazon (Mato Grosso and Pará states). The Tapajós basin runs warm, clearwater-to-slightly-stained Amazon conditions, with pH ranging widely from about 4.5 to 7.8 and temperatures typically between 24 and 82 °F — soft, low-conductivity water that is often clear rather than heavily blackwater-stained.
Within this kind of river system, silver dollars generally favour open channel and margin habitat with submerged and marginal vegetation, moving along the water column in groups rather than sheltering in wood or leaf litter the way many other characins do. No population-specific habitat study has been published for Myleus pachyodus beyond the original description; the water chemistry above reflects the broader Tapajós basin rather than a site-specific measurement for this species.
Feeding
Myleus pachyodus is herbivorous, in keeping with the rest of its genus. Silver dollars in Myleus graze algae and soft aquatic and riparian plant matter, and the thickened adult dentition that gives this species its name is consistent with a diet that includes tougher plant material than juveniles can process. Related Myleus species also take some fruit and seed material opportunistically where available, in the general pattern of Amazonian floodplain characins that shift diet with the seasonal flood cycle.
In the aquarium, Myleus pachyodus should be fed a predominantly vegetable diet — blanched greens, spirulina-based foods and algae wafers — supplemented with a smaller portion of protein such as quality pellets. Keepers should not expect this species to leave live plants alone; it is a plant-shredder by nature, not by accident of hunger.
Mating
No courtship behaviour has been documented for Myleus pachyodus specifically. In the broader genus, silver dollars pair or group loosely before spawning rather than forming a defended nest site, and spawning is typically associated with the onset of softer, more acidic water conditions that mimic seasonal flooding in their native rivers. No aquarium breeding reports exist for this species, and none are known for its close congener Myleus schomburgkii either, so almost nothing can be said about sexing or pairing behaviour with confidence.
Breeding
Myleus pachyodus is presumed to be an egg-scatterer over fine-leaved vegetation, in line with the breeding mode documented for other silver dollars in the genus. There is no parental care in this reproductive strategy: eggs are broadcast among plants and left unguarded, with no nest-building, fanning or brooding behaviour of any kind.
There are no published or hobby records of Myleus pachyodus being bred in captivity. It is a recently described (2018), narrow-range species from a single Amazonian tributary system and is essentially unknown in the aquarium trade, so this account should be read as an inference from its genus rather than an observed breeding record for the species itself.
In the aquarium
Myleus pachyodus is not an established aquarium fish — it was described only in 2018 and has no meaningful trade history — but its genus gives a reliable guide to what it would need if kept. Silver dollars in Myleus are peaceful, active schoolers that require a real group (five or more individuals) and a large tank, commonly cited at 105 US gal or more, with plenty of open swimming space rather than dense cover. Given an adult size in the 8–10 in range based on the holotype and related species, that space requirement should be taken seriously rather than treated as a juvenile-tank figure.
The single most important care note is what this fish does to plants: it is a committed herbivore that will strip soft, fine-leaved vegetation to bare stems. This is a fish for a robust, hardscape-built tank with tough or artificial decor, not for a planted aquascape. Water should be kept warm (roughly 75–82 °F) and soft to neutral, and given how little is documented about this specific species, keepers should treat any account of its aquarium behaviour — including this one — as inference from its genus rather than settled fact.
Conservation
Myleus pachyodus has not been evaluated by the IUCN Red List. As a species described only in 2018 and known from a single tributary system of the Rio Tapajós, its conservation status is effectively unknown, and it carries the general vulnerability of narrow-range Amazonian tributary endemics: any large-scale change to the Teles Pires or Jamanxim watersheds — damming, mining-related sedimentation or deforestation — could affect the only known population before that population is well understood.
The Tapajós basin has been a focus of hydroelectric development and small-scale gold mining in recent decades, both of which are recognised broad-scale pressures on fish diversity in the region, though no study has assessed their specific impact on Myleus pachyodus. Given the lack of an IUCN assessment and the narrow known range, the species is best treated as data-deficient in practice even though it has not been formally classified as such.