Taxonomy & naming
Myloplus nigrolineatus was described in 2020 by Ota, Machado, Andrade, Collins, Farias & Hrbek as part of a broader revision of the silver-dollar genus Myloplus, using an integrative approach that combined external morphology, COI molecular barcoding and habitat data to separate it from morphologically similar congeners collected across the same interfluvial region. The type locality lies in the Purus-Madeira interfluvium of Brazil, an area of blackwater lakes and slow-flowing channels between two of the Amazon's major southern tributaries.
The species belongs to the family Serrasalmidae — the piranhas, pacus and silver dollars — within the order Characiformes. Serrasalmidae taxonomy has been reworked substantially in recent decades: many silver dollars once lumped into the old genus Myleus have since been redistributed among Myloplus, Tometes and Ossubtus on the basis of dentition, fin-ray counts and molecular data, and Myloplus itself has grown considerably as new species like this one have been split out of what were once treated as single wide-ranging forms. As a species named in 2020, M. nigrolineatus has not yet accumulated a long literature trail, and much of what is documented about it comes directly from the original description.
Morphology
Myloplus nigrolineatus shows the classic silver-dollar body plan: a deep, strongly laterally compressed, disc-like body with a small head and a mouth set for grazing rather than seizing prey. FishBase records a maximum size of 9.5 in standard length, a modest figure relative to some Myloplus and Metynnis relatives, keeping this species within reach of a large but not enormous home aquarium.
The original description separates M. nigrolineatus from its close congeners using a combination of external morphology and molecular (COI) evidence rather than a single obvious field mark, which is typical of recent Myloplus splits — several of these silver dollars look extremely similar to the naked eye and are best told apart by subtle differences in fin-ray counts, dentition and coloration pattern documented in the type series. Data on colour pattern beyond the descriptive literature is sparse; as with other Myloplus, expect a bright, silvery, laterally flattened fish typical of the group.
Habitat
The type series for Myloplus nigrolineatus was collected in the Purus-Madeira interfluvium of Brazil, and FishBase describes the species as inhabiting slow-flowing backwaters and lakes rather than fast, oxygenated river channels. These are blackwater systems — acidic, oligotrophic, tannin-stained waters typical of much of the central Amazon basin — a habitat type that shapes both the low mineral content and the low pH commonly associated with Myloplus species generally.
Within this floodplain-lake and backwater habitat, the species would be expected to follow the same broad seasonal pattern as other Amazonian characins, using flooded marginal vegetation during high water and concentrating in more open lake and channel habitat as water levels recede, though detailed seasonal-movement data specific to M. nigrolineatus is not yet documented in the available literature.
Feeding
FishBase records that Myloplus nigrolineatus feeds mainly on aquatic and terrestrial plants, consistent with the herbivorous diet typical of the Myloplus and wider silver-dollar group. In the wild this likely includes grazing on submerged and marginal vegetation plus whatever plant material — leaves, fruit, seeds — falls into the blackwater lakes and backwaters it inhabits from the surrounding flooded forest, a food source shared with many Amazonian characins.
In the aquarium, keepers should plan around this herbivorous bias: a varied vegetable-forward diet of blanched greens (spinach, cucumber, courgette), spirulina-based flake or pellet, and algae wafers, supplemented with a modest amount of protein such as quality pellets, will suit the species far better than a meat-heavy diet designed for predatory characins. Given the plant-grazing habit, any live plants in the tank should be treated as food rather than décor — see "In the aquarium" below.
Mating
Behavioural notes associated with the type material and figured specimens describe Myloplus nigrolineatus as a diurnal, migratory schooler that forms schools for external fertilisation, with spawning occurring in areas of marginal vegetation. Breeding males show orange-red patches developing on the gill plates and fins, while females display orange colouring along with dark blotches — a seasonal breeding coloration change documented from field specimens rather than aquarium observation.
Beyond this colour-based signalling, detailed courtship behaviour — pairing, chasing, site selection — has not been documented for this recently described species; data is sparse and this account does not extend beyond what the source material supports.
Breeding
Myloplus nigrolineatus is an egg-scatterer: schools spawn together in areas of marginal vegetation, releasing eggs and sperm into the water column and among fine plant growth rather than building or defending a nest. Consistent with this strategy, the eggs and resulting fry are not guarded — there is no parental care once spawning is complete, and the school disperses rather than remaining near the eggs.
The species has occasionally been bred in the wider hobby, as have several Myloplus and Metynnis silver dollars, typically triggered by soft, acidic water conditions that echo the blackwater habitat of its native range; however, given how recently M. nigrolineatus was described, documented aquarium breeding reports specific to this species are scarce, and this account does not claim more than that general pattern for the genus.
In the aquarium
At a maximum recorded size of 9.5 in SL, Myloplus nigrolineatus is a manageable but still substantial fish that needs a large aquarium and, as a schooling species, real numbers — a lone silver dollar is a stressed, skittish fish. Keepers should plan for a group in a spacious tank with plenty of open swimming room, since this is an active, disc-bodied schooler rather than a fish suited to a cramped or heavily obstructed layout.
The single most important care note is the diet: this is a committed plant-grazer, and a genuinely planted, aquascaped tank will be reduced to stems and stumps in short order. The tank should be robustly hard-scaped instead — driftwood, rock and hardy, unpalatable or artificial plants — with feeding built around vegetable matter (blanched greens, spirulina, algae wafers) so the fish is not driven to strip whatever greenery is available. Soft, acidic to neutral water reflecting its blackwater origin, warm tropical temperatures and strong filtration for a large, actively swimming school round out the basic setup; specific numeric water parameters are not yet well documented for this species beyond its blackwater habitat description, and this account does not invent them.
Conservation
Myloplus nigrolineatus has not been formally assessed by the IUCN Red List and is classified Not Evaluated. This is unsurprising for a species described as recently as 2020, and it reflects a broader gap in the conservation literature for many of the newer, range-restricted Myloplus splits rather than any evidence of population decline.
As a species known from the Purus-Madeira interfluvium blackwater systems of central Brazil, it shares the general pressures facing that region's freshwater fish — deforestation-linked changes to floodplain hydrology, water quality shifts and, more broadly, the cumulative effect of damming across Amazon tributaries — though no species-specific assessment of these pressures exists yet in the available record. Given the very recent description and limited surveyed range, this account does not speculate about population trend or threatened status beyond the Not Evaluated designation.