Taxonomy & naming
Melanocharacidium depressum was described by the Brazilian ichthyologist Paulo Buckup in 1993, as part of a revision that split a group of small, dorso-ventrally flattened, bottom-dwelling characins away from the more slender-bodied genus Characidium. It belongs to the family Crenuchidae — the darter characins — within the order Characiformes, a family whose members are unusual among characins for perching on the substrate rather than swimming freely in the water column.
The genus Melanocharacidium sits alongside Crenuchus and Poecilocharax in the subfamily Crenuchinae, the group generally referred to as the South American darters. Its closest and much better documented relative in the aquarium hobby is Crenuchus spilurus, the sailfin characin, but the two genera are readily told apart: Crenuchus has an elongated, sail-like dorsal fin in mature males, while Melanocharacidium species are flatter-bodied, plainer-finned, cryptic darters. The specific epithet depressum is Latin for "pressed down" or "flattened," referring to the species' relatively depressed body profile, a name chosen in contrast to a congener described in the same revision, M. compressum (laterally compressed rather than flattened).
Morphology
Melanocharacidium depressum is a small fish, reaching a maximum recorded length of around 4 in, though most individuals seen in the wild and in the rare aquarium import are considerably smaller. Its body is noticeably flattened dorso-ventrally — depressed rather than the laterally compressed profile typical of most characins — an adaptation consistent with its habit of resting directly on submerged wood, leaf litter and other structure rather than swimming in open water.
Colouration in the genus runs from an almost uniform blackish-brown to patterns showing a dark horizontal stripe paired with a brighter band along the flank, and individuals can shift between darker and paler states depending on mood, background and lighting — a common trait among cryptic, substrate-associated fish that rely on camouflage. Fins are unremarkable compared to the showy dorsal sail of its relative Crenuchus spilurus; overall the fish reads as a small, plain, well-camouflaged bottom-percher rather than a display species.
Habitat
The species is native to South America, recorded from the Amazon and Orinoco basins, the Araguaia basin and river systems draining the Guiana Shield. Within these drainages it is a fish of the substrate and structure zone — resting on submerged wood, root tangles and accumulated leaf litter in slower stretches of forest streams and river margins, in keeping with the darting, perch-and-dash behaviour that gives the group its common name.
Detailed water-chemistry data specific to M. depressum was not found in the sources consulted for this article, so specific pH and hardness figures are not given here rather than guessed. Based on the broader habitat of Crenuchidae in these drainages, the fish is expected to favour soft, acidic, warm Amazonian and Guiana Shield stream conditions, but that should be treated as a reasonable inference rather than a documented fact until better sourcing is available.
Feeding
Melanocharacidium depressum is a micro-predator, not a grazer or general omnivore. In the wild it is expected to take small aquatic invertebrates, insect larvae and micro-crustaceans picked directly off the substrate and leaf litter it perches on, consistent with the documented feeding ecology of related darter characins.
In the aquarium, the closely related sailfin characin Crenuchus spilurus is documented as a fish that often ignores dry flake and does best on small live and frozen foods — microworm, baby brine shrimp and cyclops — and the same approach is the sound default for Melanocharacidium depressum given its similar size and bottom-perching, invertebrate-hunting habit. Feeding should be aimed at the bottom of the tank, where this genus spends essentially all of its time.
Mating
No courtship account specific to Melanocharacidium depressum was found in the sources consulted for this article. Within the darter-characin group generally, breeding activity has been linked to environmental cues associated with the rainy season, which is consistent with the seasonal flooding patterns that shape reproduction across many small Amazonian and Guiana Shield stream fish.
The best-documented relatives, Crenuchus spilurus and Poecilocharax species, are cave- and leaf-spawners in which the male takes up a defended site and briefly tends a small clutch of eggs. Whether M. depressum follows the same pattern specifically has not been confirmed in the sources available here, so this should be read as a plausible pattern for the genus rather than a documented fact for this species.
Breeding
There is no confirmed aquarium breeding account for Melanocharacidium depressum in the sources consulted for this article; it should be treated as essentially unbred or undocumented in the hobby. Within the darter-characin subfamily, however, the pattern is distinctive and worth noting: Crenuchus spilurus and Poecilocharax species are cave or leaf spawners, with the male guarding a small clutch for a brief period after spawning — a genuine, if nano-scale, form of parental care that is unusual among characins as a whole, most of which are simple egg-scatterers with no care at all.
Given the taxonomic proximity of Melanocharacidium to Crenuchus and Poecilocharax within Crenuchinae, a similar cave/leaf-spawning strategy with brief male care is a reasonable expectation for this genus, but it has not been confirmed specifically for M. depressum and should not be presented as established fact. Anyone attempting to breed this species in the aquarium would be working from very little precedent.
In the aquarium
Melanocharacidium depressum is essentially absent from the mainstream aquarium trade; almost everything written online about "Melanocharacidium" as an aquarium fish in fact describes the genus in general or its far more commonly kept relative, Crenuchus spilurus. Anyone who does encounter this species should treat it as a true nano fish: a maximum size of around 4 in makes it manageable even in a modestly sized tank, but its bottom-perching, camouflage-dependent habit calls for a quiet aquarium with soft lighting, plenty of driftwood, leaf litter and structure to perch on, rather than an open, brightly lit community setup.
As with its relatives, it is best kept peacefully — the genus is documented as gentle towards tankmates and completely unproblematic around plants — but shy, cryptic fish of this kind do poorly with boisterous or fast-moving companions that outcompete them at feeding time. A species tank or a very calm community, with small live or frozen foods offered near the substrate, is the safest approach given how little specific husbandry information exists for this exact species.
Conservation
Melanocharacidium depressum has not been assessed by the IUCN Red List and carries a status of Not Evaluated. This is unsurprising for a small, cryptic, rarely traded characin restricted to specific South American drainages — many similarly obscure single-region or multi-basin characins simply have not been formally assessed.
No specific population trend, threat data or trade-pressure information for this species was found in the sources consulted for this article. Given its wide recorded range across the Amazon, Orinoco, Araguaia and Guiana Shield drainages and its near-absence from the aquarium trade, wild collection pressure is likely minimal, but broader threats facing Amazonian and Guiana Shield freshwater habitats — deforestation, damming and water-quality change — remain relevant long-term watch items for the region as a whole.