Taxonomy & naming
Myleus micans was described by the Danish ichthyologist Christian Frederik Lütken in 1875. It sits in the family Serrasalmidae — the piranhas, pacus and silver dollars — within the order Characiformes. The genus Myleus has historically been used as a catch-all for many disc-shaped serrasalmids, and the family has since been split hard by later revisions, with a number of former Myleus species moved into Myloplus, Tometes and Ossubtus; Myleus micans, however, remains in Myleus as a valid combination.
The species is endemic to the São Francisco River basin of eastern Brazil, a large, largely isolated drainage that does not connect directly to the Amazon or Paraná systems, which helps explain why it is a geographically restricted species rather than one spread across the wider Neotropics like many of its relatives. In the aquarium trade it circulates under the common name "pacu azul" (blue pacu) alongside the generic label "silver dollar," though the latter is shared with several other genera and is not specific to this species.
Morphology
Myleus micans has the classic silver dollar shape: a strongly laterally compressed, nearly circular body that reflects light like a coin, giving the group its common name. The body is deep relative to its length, with a small terminal mouth adapted for grazing rather than for seizing prey, and dorsal and anal fins that sweep back toward a forked tail. FishBase records a maximum size of 15 in standard length and up to roughly 2 kg in weight, making this one of the larger silver dollars kept in the hobby — considerably bigger than the small Metynnis species often sold under the same generic name.
Colouration is predominantly silver with a bluish sheen that gives rise to the "pacu azul" name, and depending on individual and mood, some fin edges may show darker or reddish tones. As with many serrasalmids, colour intensity and fin coloration can vary with water conditions, age and social status within the group.
Habitat
Myleus micans is confined to the São Francisco River basin in eastern Brazil, a large, biogeographically distinct system draining much of the country's interior. Within this range it is described as benthopelagic, occupying the water column over and near the bottom, and it favours moderately fast-flowing or rapid water rather than still backwaters, consistent with a disc-bodied fish adapted to hold position and forage in current.
Water conditions across its native range run warm and soft to moderately hard, broadly in the range of 75–82 °F and pH 6.0–7.5, with general hardness around 2–10 dGH. These are typical tropical South American river conditions rather than extreme blackwater, and a keeper should aim to replicate warm, soft-to-neutral water with good flow and oxygenation rather than the very dark, acidic tannin water associated with some other Amazonian characins.
Feeding
Myleus micans is primarily herbivorous in the wild, grazing on fruit, seeds and vegetable matter that falls into or grows along its home river, supplementing this with some incidental animal material. This diet is central to understanding the species in the aquarium: it is a determined, efficient plant-shredder, and soft-leaved aquatic plants placed in a tank housing this fish should be expected to disappear rather than thrive.
In captivity, keepers should offer a vegetable-forward diet — blanched vegetables such as courgette, cucumber and spinach, algae wafers, spirulina-based foods — along with occasional protein such as quality pellets, to round out nutrition without encouraging the fish to view every soft plant in the tank as a meal. Feeding a diet too heavy in animal protein is not appropriate for a fish built around a herbivorous digestive system.
Mating
Specific courtship behaviour for Myleus micans is not well documented in the available literature, and this article does not speculate beyond what is known for silver dollars generally. Hobby accounts of silver dollar breeding describe conditioning a group toward sexual maturity at one to two years of age, with a rise in temperature and a shift toward shallower, softer, more acidic water often cited as a spawning trigger in related species.
External sexual dimorphism has not been reliably documented for this species; keepers of silver dollars generally report no consistent, easily read difference in colour or fin shape between males and females outside of behaviour and relative body fullness in ripe females.
Breeding
Myleus micans is best understood, in line with silver dollars generally, as an egg-scatterer over fine-leaved plants rather than a nest-builder or mouthbrooder. Spawning in related silver dollars has been described in shallow water (roughly 4–6 in), warm temperatures around 79–82 °F and softer, slightly acidic water near pH 6.0–6.5, with a female capable of scattering large numbers of adhesive eggs among vegetation.
There is no parental care: eggs and fry are not guarded once released, and breeders of related silver dollar species typically remove the adults after spawning to prevent the eggs being eaten. Myleus micans is only occasionally bred in the hobby, and its large adult size and specific range make it a less commonly spawned aquarium fish than the smaller, more widely traded Metynnis species.
In the aquarium
The defining facts for a prospective keeper are size and diet. Myleus micans can approach 15 in and several kilograms as an adult, well beyond many silver dollars sold small in shops, so it needs a genuinely large aquarium — several hundred litres at minimum for a mature group — with plenty of open swimming space and strong filtration to handle its bioload. It is a schooling fish and should be kept in a proper group of five or more; a lone or thinly-kept specimen is a stressed fish prone to skittish, erratic behaviour.
Because it is a committed herbivore, this is not a fish for a planted aquascape: soft-leaved plants will be grazed down quickly, so tanks are usually hardscaped with driftwood and tough or artificial decor instead. Water should be kept warm (75–82 °F) with moderate to strong flow and soft-to-moderately-hard, mildly acidic to neutral chemistry (pH 6.0–7.5). Tankmates should be robust, similarly sized fish that will not be outcompeted for food or intimidated by an active, fast-moving school.
Conservation
Myleus micans is assessed by the IUCN Red List as Least Concern. As a species endemic to a single large river basin — the São Francisco — rather than one spread across multiple major drainages, its conservation status is nonetheless worth watching over time given the general pressures facing South American freshwater systems, including damming, water abstraction and habitat modification.
The aquarium trade for this species draws on a range that is more limited than that of widely distributed serrasalmids from the Amazon or Orinoco, so keepers should be mindful that its availability may depend more directly on the health of a single river system than is the case for more broadly ranging relatives.