Taxonomy & naming
Myleus knerii was described by the Austrian ichthyologist Franz Steindachner in 1881, from a type specimen collected in the Maroni River, French Guiana. It sits in the family Serrasalmidae — the piranhas, pacus and silver dollars — within the order Characiformes, and in the genus Myleus, a group of laterally compressed, herbivorous characins generally associated with rapids and fast-flowing stretches of South American rivers rather than the sluggish floodplain waters favoured by many of their pacu relatives.
A 2015 taxonomic revision re-examined the status of M. knerii alongside the closely related Myleus setiger, clarifying the two as distinct species and providing a complementary description of both. The species epithet knerii is a Latinized eponym honouring Rudolf Kner (1810–1869), an Austrian ichthyologist and colleague of Steindachner, in keeping with the era's convention of naming new species after fellow naturalists. As with much of Serrasalmidae, the family has undergone repeated genus-level revision — several former Myleus species have since been moved into Myloplus, Tometes and other genera — but M. knerii itself remains placed in Myleus per the Catalog of Fishes.
Morphology
Myleus knerii has the classic silver dollar shape: a strongly laterally compressed, roughly disc-like body that presents a large, mirror-bright flank to a predator or a passing aquarist. FishBase records a maximum size of 5 in standard length, making it a small to mid-sized member of its genus and considerably more modest than the larger pacus in the same family. The body carries the typical Myleus silhouette — a small, blunt head, a steep dorsal profile and a deeply forked tail suited to holding station in the faster water it favours in the wild.
Colouration follows the silver dollar pattern common to the genus: a bright, silvery ground colour that flashes as the fish turns, without the bold barring or patterning seen in some related species. Precise details of fin colouration and any subtle marbling are not well documented for this species specifically; much of what is known about its external appearance comes from the 2015 taxonomic redescription rather than extensive aquarium observation, reflecting how rarely this particular Myleus is kept or studied compared to its more common relatives.
Habitat
Myleus knerii is known from the Maroni River basin of French Guiana, which forms the border between French Guiana and Suriname. Like other rapids-associated Myleus species, it is understood to favour faster-flowing stretches of river rather than still floodplain lakes, though detailed habitat descriptions specific to this species are sparse in the available literature.
As a tropical South American stream fish, it lives in warm water typical of the region; specific published pH, hardness and temperature ranges for wild M. knerii populations are not well documented, but general guidance for the genus and for silver dollars broadly points to soft to neutral, mildly acidic water in the roughly pH 5.5–7.0 range and warm tropical temperatures around 75–82 °F, consistent with the Maroni basin's lowland tropical river conditions.
Feeding
Myleus knerii is herbivorous, with FishBase describing a diet dominated by aquatic macrophytes — the plant material lining and rooted in its rapids and stream habitat. This plant-heavy diet is typical of the genus and of silver dollars generally, distinguishing them sharply from the flesh-eating reputation of their piranha relatives in the same family.
In the aquarium, silver dollars in general take a vegetable-forward diet readily: blanched courgette, cucumber and spring greens, spirulina flake or wafer, and algae wafers, along with an occasional protein supplement such as bloodworm or brine shrimp. Given its wild diet of macrophytes, M. knerii should be fed primarily on vegetable matter, and keepers should expect the fish to treat any live plants in its tank as a food source rather than decoration.
Mating
Specific courtship behaviour has not been documented for Myleus knerii in the available literature. Based on general silver dollar biology, spawning in the genus is typically associated with a rise in temperature and softer, more acidic water, conditions that in the wild would correspond to seasonal rains. Data on this species is otherwise sparse, and no reliable external sexual dimorphism has been documented — sexes are not readily distinguished by colour or fin shape in silver dollars generally.
Breeding
Myleus knerii has not been documented as bred in the aquarium hobby, and specific breeding accounts for this species are not available. Based on the well-documented breeding mode of silver dollars generally, M. knerii would be expected to be an egg-scatterer, releasing eggs over fine-leaved plants or vegetation in soft, acidic water, with no parental care of the eggs or fry afterward — the pattern seen in related, better-studied silver dollar species. This should be treated as an inference from genus-level biology rather than a documented account of this species specifically; a keeper attempting to breed M. knerii would be working largely without a precedent in the literature.
In the aquarium
Myleus knerii is rarely, if ever, seen in the aquarium trade, and dedicated care information for this specific species is sparse. What can be said with confidence follows from its size and its genus: at a modest maximum of around 4.5 in, it is a manageable tank resident by silver dollar standards, but it remains a schooling fish that needs to be kept in a real group and given open swimming space rather than housed alone or crowded into a small tank.
As with silver dollars generally, any aquarist considering this species should plan a robust, hardscaped setup rather than a delicate planted aquascape — the wild diet of aquatic macrophytes translates directly into a fish that will graze and shred live plants in a home tank. Water should be kept warm and soft to neutral, mildly acidic, echoing its tropical South American stream habitat. Given how little is documented about this species in captivity, prospective keepers should treat it as an advanced or speculative acquisition rather than a well-established aquarium fish.
Conservation
Myleus knerii is currently assessed by the IUCN Red List as Data Deficient, an assessment dated January 2021, meaning there is not enough information on its population size, trend or the specific pressures it faces to assign a threat category. This is a common status for narrow-range, single-basin characins that receive little targeted research attention.
The species' known range is confined to the Maroni River basin of French Guiana and neighbouring Suriname, a region under less intensive agricultural and urban pressure than much of the Amazon basin, but subject to the general threats facing South American freshwater systems, including mining-related sedimentation and habitat disturbance. Without dedicated population surveys, the true conservation status of M. knerii remains unknown, and it is not a species that appears in the aquarium trade at any meaningful scale, so trade pressure is not considered a relevant factor.