Taxonomy & naming
Myloplus planquettei was described in 2003 by Michel Jégu, Philippe Keith and Régis Le Bail from specimens collected in the Maroni River at Twenke, French Guiana, which serves as the type locality. It belongs to the family Serrasalmidae — the piranhas, pacus and silver dollars — within the order Characiformes, in the genus Myloplus, one of several silver-dollar genera split out of the old, broadly defined Myleus during the family's ongoing taxonomic revision.
The specific epithet planquettei honours Paul Planquette (1940–1996) of the Institut National de la Recherche Agronomique in Kourou, French Guiana, in recognition of his contributions to the study of Guianese freshwater fishes. Serrasalmidae taxonomy has shifted considerably since the species was described, with silver dollars repeatedly reshuffled between Myleus, Myloplus, Tometes and other genera as the family's phylogeny is worked out; this article follows Myloplus planquettei, the combination under which the species continues to be recorded by FishBase and under which it is known in the aquarium trade.
Morphology
Like other silver dollars, Myloplus planquettei has the classic laterally compressed, disc-shaped body of a Serrasalmidae herbivore — a deep, rounded profile built for manoeuvring in current rather than for the streamlined shape of an open-water pursuit fish. FishBase records a maximum published length of 23 in standard length, making this one of the larger species in its genus, well beyond the size most aquarium-trade silver dollars reach.
The species carries 20–23 dorsal soft rays and 32–34 anal soft rays, and is distinguished in part by a black distal margin on the caudal fin, a useful field mark against similar Guiana Shield silver dollars. As in related species, the mouth and dentition are adapted for grazing rather than piercing, with a battery of teeth suited to rasping algae and tough aquatic plant tissue from rock and current-swept substrate.
Habitat
Myloplus planquettei is restricted to the Guiana Shield, recorded from the Mana, Maroni and Essequibo basins spanning French Guiana, Suriname and possibly Guyana. Within this range it is a specialist of upper-river reaches with fast current and clear water rather than the sluggish, sediment-laden floodplain habitats used by many other Serrasalmidae — a narrower habitat niche than most silver dollars occupy.
Its primary food source, the rheophilic (current-adapted) plant family Podostemaceae, only grows attached to rock in fast-flowing, well-oxygenated, clear-water rapids and cataracts, and the species' distribution tracks that habitat closely. This tight link between fish and a specific current-swept plant community likely explains its narrow known range and makes it more habitat-sensitive than many of its wider-ranging relatives.
Feeding
Myloplus planquettei is phytophagous — a dedicated plant-eater. Its documented primary food is Podostemaceae, the family of rheophilic aquatic plants that carpet submerged rock in the fast, clear rapids it inhabits, though like other silver dollars it likely also takes algae and other plant material opportunistically where available.
In the aquarium this translates to a diet built around vegetable matter: blanched greens (courgette, cucumber, spinach), spirulina-based flake or pellet, and algae wafers, supplemented with only modest amounts of protein. Keepers should not expect this species to accept a carnivore-leaning diet, and should plan feeding — and tank decor — around the certainty that it will graze any soft-leaved plant offered to it.
Mating
No species-specific courtship behaviour for Myloplus planquettei has been documented in the available literature; data is sparse for this recently described, narrow-range species. Silver dollars in general are reported to pair or group-spawn over fine-leaved vegetation, with spawning often associated with a shift toward softer, more acidic water, but there is no confirmed observation of this species' own courtship sequence to draw on.
Breeding
As in other silver dollars of the genus Myloplus, the expected breeding mode for this species is egg-scattering among fine-leaved plants, with no parental care of eggs or fry once spawning is complete. This mode has occasionally been achieved in the hobby for related silver dollar species, generally triggered by soft, acidic water.
However, there is no confirmed record of Myloplus planquettei being bred in captivity, and given its narrow native range and specialised rheophilic habitat, it should be considered essentially unbred in the aquarium at present — the aquarium population, where it exists, relies on wild-collected fish rather than any established captive-breeding pipeline.
In the aquarium
Myloplus planquettei is a large silver dollar — up to 23 in — and prospective keepers should size a tank to that adult length rather than to the smaller juvenile offered in the trade; this is a fish for a large, robust aquarium, not a nano or standard community tank. Strong water movement and good oxygenation, echoing its fast-flowing native rapids, are worth replicating, along with a hardscape of smooth rock and driftwood rather than a delicate planted layout.
The single most important care note is that this species will eat live plants: it is a committed grazer, and any soft-leaved aquatic vegetation in the tank should be expected to be shredded rather than left to grow. Keepers should feed a vegetable-forward diet and plan the tank as a hard-scaped grazing environment rather than an aquascape. Like most silver dollars it is a shoaling, generally peaceable fish best kept in a group of its own kind alongside similarly robust tankmates rather than delicate or slow-moving fish.
Conservation
Myloplus planquettei is assessed by the IUCN Red List as Vulnerable, an assessment dated 2 January 2021. Its narrow range across the Mana, Maroni and Essequibo basins of the Guiana Shield, combined with a specialised dependence on fast, clear-water rapids and the Podostemaceae plants that grow only in that habitat, leaves it more exposed to localised threats than wide-ranging Amazonian relatives.
The principal pressures identified are habitat degradation and water-quality decline from gold mining across the Guiana Shield — a major regional driver of sedimentation and mercury contamination in exactly the clear-water rapid habitat this species depends on — together with overfishing across its restricted range. Because the species is not established in captive breeding, any aquarium-trade specimens draw on wild populations already under this pressure.