Taxonomy & naming
Myleus setiger was described by Johannes Müller and Franz Hermann Troschel in 1844, in the same wave of nineteenth-century German ichthyological work that established much of the Serrasalmidae — the family uniting the piranhas with the pacus and silver dollars within the order Characiformes. The genus Myleus remains one of the family's silver-dollar lineages, though the Catalog of Fishes (Eschmeyer, California Academy of Sciences) reflects a long history of taxonomic revision across Serrasalmidae, with related silver-dollar species having been redistributed at various points among Myleus, Myloplus, Tometes and Ossubtus as the family's internal relationships have been reworked; this article follows Myleus setiger as currently recognised and does not attempt to relitigate that history.
In the aquarium trade the species circulates almost exclusively under its common name, Sailfin Red Hook Silver Dollar — a reference to the sail-like, elongated dorsal and anal fins adult males and females both develop, and to the reddish, hook-shaped fin margins that darken with maturity. It is one of relatively few Myleus species regularly seen in the hobby, and confusion with other red-finned silver dollars in the trade is possible without close attention to fin shape and adult colour pattern.
Morphology
Like other silver dollars, Myleus setiger has the classic Serrasalmidae body plan turned toward grazing rather than predation: a strongly laterally compressed, near-circular disc of a body, a small terminal mouth suited to picking at plant material and fruit, and a bright, mirror-silver flank that gives the group its common name. FishBase records a maximum size of 10.5 in standard length in males and 11.5 in standard length in females, with a maximum recorded weight of 1.1 kg — a substantially larger adult than many silver dollars kept in the trade, and the source of the aquarium-size figure of roughly 9 in commonly cited for tank-raised fish.
The species' signature feature, and the source of its common name, is the fin development of maturing adults: the dorsal and anal fins elongate and sweep upward and rearward into a sail-like profile, with margins that deepen into a reticulated, blotchy dark red. This ornamentation is most pronounced in well-conditioned, mature specimens and is a large part of the fish's appeal in the trade.
Habitat
Myleus setiger is native to South America, recorded across the lower and middle Amazon basin, the Orinoco basin, and rivers and streams of the Guiana Shield, including drainages in Guyana and Suriname. Across this range it occupies tropical, generally soft-to-neutral river and tributary habitat typical of Amazonian and Guiana Shield lowlands, where seasonal flooding brings it into contact with fallen fruit, seeds and flooded marginal vegetation — the resources that shape its feeding ecology.
The cached sources do not report a documented specific pH or temperature range for wild populations beyond the general "soft-to-neutral, tropical" characterisation FishBase gives for the species; aquarists should treat exact water-chemistry targets as inferred from the family and genus rather than pinned to a study of this species specifically.
Feeding
Myleus setiger is fundamentally a plant- and fruit-eater. In the wild it is described as a frugivore, taking fruit and nuts that fall into the water from the flooded forest canopy along with other plant material, and trade sources describe it more broadly as an omnivore that will also accept prepared aquarium foods readily. This plant-forward diet is the single most important fact for a prospective keeper: a hungry group of silver dollars this size will graze soft-leaved aquarium plants down to the ribs in short order.
In the aquarium, feed a vegetable-heavy diet — blanched courgette, cucumber, spinach or other soft greens, spirulina-based flake or pellet, and algae wafers — supplemented with some protein such as quality omnivore pellets. Aquarium-coop-style guidance for Serrasalmidae more broadly notes that conditioning ahead of any breeding attempt benefits from a higher-protein diet, but day-to-day feeding should stay vegetable-forward to match the species' natural grazing habit and to reduce (though not eliminate) plant damage in a furnished tank.
Mating
Specific courtship behaviour for Myleus setiger has not been documented in the cached sources, and the species is not reliably reported as sexually dimorphic — no consistent difference in colour, fin shape or body form between males and females is described despite this being one of the more commonly traded Myleus species. Breeders working with Serrasalmidae egg-scatterers generally condition groups on a richer diet ahead of a spawning attempt and rely on softer, more acidic water to help trigger spawning behaviour, but data specific to this species is sparse and this account does not extend beyond what the sources support.
Breeding
As a silver dollar within Serrasalmidae, Myleus setiger is understood to be an egg-scatterer, releasing eggs over fine-leaved plants rather than building a nest or mouthbrooding. Aquarium-trade sources note that tank-bred specimens of this species are available, indicating it has been spawned in captivity, though detailed accounts of the process for this particular species were not found in the cached sources.
Consistent with egg-scattering Serrasalmidae generally, the eggs and resulting fry receive no parental care once released — adults will consume their own eggs and fry given the chance, so a dedicated spawning setup with fine-leaved plants or a mesh layer to let eggs fall out of reach of the adults, followed by separating fry as soon as they are free-swimming, is the standard approach for this style of breeder. Set facts.breeding.mode as an egg-scatterer among fine-leaved plants with no parental care.
In the aquarium
Myleus setiger is a peaceful schooler that should be kept in a group, and it needs to be sized for what it actually becomes: close to 10.5–11.5 in at full wild size, with tank-raised aquarium specimens commonly reaching around 9 in. That is a large-bodied fish that needs a correspondingly large tank with plenty of open swimming space, not a species to buy small and outgrow quickly. Trade sources describe it as peaceful but capable of intimidating smaller or slower tankmates simply through its size and activity level, so stock companions accordingly.
The defining care consideration is that this is a committed plant-shredder: a group of silver dollars this size will strip soft-leaved aquarium plants efficiently, so the species suits a robust, hard-scaped tank with tough, unpalatable or artificial plantings rather than a delicate aquascape. Provide strong filtration for the bioload of a large schooling group, stable warm tropical temperatures, and a vegetable-forward diet as described above. Because it is a schooling fish, keeping too few individuals or a single specimen tends to produce a more skittish, stressed fish rather than a calmer one.
Conservation
Myleus setiger has not been evaluated by the IUCN Red List — no species-specific assessment was located, so its status is Not Evaluated (NE) rather than a documented category such as Least Concern or threatened. This is common among Serrasalmidae silver dollars that are not major food-fish targets and are not flagged as being of particular conservation concern, but the absence of an assessment means there is no formal, data-backed statement on population trend or threats for this species.
The aquarium trade sources cited here note that tank-bred specimens are available, which — if that supply is meaningful at scale — would reduce reliance on wild collection from its Amazon, Orinoco and Guiana Shield range; the cached sources do not otherwise document specific pressures on wild populations, so this account does not speculate beyond what is supported.