Taxonomy & naming
Characidium stigmosum was described by Marcelo Rodrigues de Souza Melo and Paulo Andreas Buckup in 2002, in a paper reviewing Characidium species of the Characidium fasciatum species complex from the rio Paranã system of the upper Tocantins basin. It belongs to the family Crenuchidae — the South American darters — within the order Characiformes, and specifically to the subfamily Characidiinae, the true darters, which the Catalog of Fishes (Eschmeyer, California Academy of Sciences) separates from the blackwater cave-and-leaf spawners of the subfamily Crenuchinae (Crenuchus, Poecilocharax).
The genus Characidium is large and taxonomically difficult, with many species distinguished by fine details of squamation, fin-ray counts and pigmentation pattern rather than obvious external differences, and stigmosum is no exception: the species epithet is Latin for "marked" or "spotted," from the Greek stigma (a mark, prick, or brand), referring to the dark elliptical blotches along the body that anchor its identification against close relatives in the same species complex. No subsequent taxonomic revision or synonymy affecting this species is documented; it remains a valid, narrowly distributed member of the genus.
Morphology
Characidium stigmosum is a small fish even by darter standards: the type description gives a holotype standard length of 1.5 in, roughly 1.5 in, which is consistent with the compact, elongate body shape typical of the genus — a slim, faintly cylindrical profile built for holding station against current rather than for open-water cruising. As in other Characidium, the pectoral and pelvic fins are proportionally large and set low and forward on the body, letting the fish brace against rock and gravel in flowing water in a way that visually recalls a small darter or goby, even though the resemblance is purely functional convergence rather than any relationship.
The diagnostic feature for this species is its namesake pigmentation: a series of dark, elliptical marks along the sides of the body, associated with a pattern of vertical bars, that together distinguish it from other members of the C. fasciatum complex described from the same general region. Beyond the type description, detailed data on maximum size, longevity or any sexual difference in body proportions have not been located; adult size is presumed to stay close to the small end of the genus, which generally tops out under 4 in.
Habitat
The species is known from tributary streams of the rio Paranã, part of the Tocantins river drainage in Goiás State, central Brazil — a tropical Neotropical system with the seasonal high- and low-water cycle typical of the Cerrado and central Brazilian plateau. Like other Characidium darters, it is presumed to occupy flowing, well-oxygenated stream reaches over sand, gravel or rock substrate, often along margins with leaf litter, submerged wood and marginal vegetation, rather than the still floodplain lakes and backwaters favoured by many of its larger characin relatives.
Specific water-chemistry data for C. stigmosum itself have not been located; genus-level accounts for related, better-studied Characidium species point to soft, mildly acidic water in the general range of pH 6.0–7.5 and warm tropical stream temperatures, and this account follows that pattern rather than asserting numbers specific to this species. Given its restricted range in a single Brazilian drainage, its wild habitat should be treated as narrowly defined and, without dedicated surveys, incompletely documented.
Feeding
Characidium species are micro-predators of the stream bed, and there is no reason from the available record to expect C. stigmosum to differ: they pick chironomid and other small insect larvae, micro-crustaceans and similar invertebrates from the substrate and drift, darting forward from a resting perch rather than actively cruising to hunt. This lifestyle means the species is likely to largely ignore standard flake food in the aquarium, a common frustration reported for the genus generally.
In captivity, related Characidium are best maintained on small live and frozen foods — microworm, baby brine shrimp (Artemia nauplii), chopped bloodworm and cyclops — offered in a way that lets the fish dart out from cover to intercept them, mimicking natural foraging. No species-specific feeding data for C. stigmosum have been located, so this guidance is extrapolated from the genus rather than documented directly for this species.
Mating
No courtship or spawning behaviour has been documented specifically for Characidium stigmosum; the original description is a taxonomic account based on preserved specimens and does not address reproductive biology. Within the darter subfamily Characidiinae generally, Characidium species are understood to be substrate egg-scatterers rather than nest builders or mouthbrooders, with no elaborate pairing display reported, in contrast to the cave- and leaf-spawning behaviour with brief male care documented in the related crenuchine genera Crenuchus and Poecilocharax.
Sexual dimorphism has not been described for this species; data are sparse enough that this account makes no claim about external differences between males and females.
Breeding
As a Characidiinae darter rather than a crenuchine cave-spawner, Characidium stigmosum is presumed — following the pattern documented for better-studied congeners — to be an egg-scatterer over stream-bed substrate and fine vegetation, without the nest-guarding and brief male parental care known in Crenuchus and Poecilocharax. No parental care of eggs or fry has been reported for the genus's darter lineage generally, and none has been documented for this species specifically.
There is no record located of C. stigmosum being bred in the aquarium hobby. General aquarium accounts of Characidium darters note that some species can be induced to spawn in soft, acidic water with fine-leaved plants such as Java moss providing egg-scatter cover, but this remains unconfirmed for stigmosum itself, and given how rarely the species appears to be kept, captive breeding should be treated as undocumented rather than assumed.
In the aquarium
Characidium stigmosum is not established in the aquarium trade under that name, and this account is written from genus-level husbandry for Characidium darters rather than species-specific experience. Where kept, darter characins do well in a modest nano-to-small aquarium (accounts for the genus suggest 20 US gal upward) with a fine sand or gravel substrate, driftwood, and moderate water flow that lets the fish perch and dart the way it would in a stream; still, stagnant setups do not suit this lifestyle.
Diet needs active management, since these fish are micro-predators that often refuse dry food — small live or frozen foods should be offered regularly rather than relying on flake. Characidium are generally peaceable, bottom-and-mid-water fish best kept in a small group of their own species with calm tankmates that will not outcompete them at feeding time or bully them off their favoured perches. Given the very limited information available for this exact species, a cautious keeper should treat any specific numbers as genus-level defaults rather than confirmed facts for C. stigmosum.
Conservation
Characidium stigmosum has not been formally assessed by the IUCN Red List and is currently listed as Data Deficient, reflecting how little has been published on the species since its 2002 description — there is no population trend data, no dedicated threat assessment and no subsequent survey work located for its rio Paranã tributary habitat. This is a common status for narrowly distributed, taxonomically obscure Characidium species that receive attention primarily from systematic revisions rather than ecological or conservation studies.
In the absence of species-specific data, the general pressures on small-range Cerrado and central Brazilian stream fish — agricultural land-use change, damming and associated sedimentation, and water-quality degradation in the Tocantins drainage — are relevant long-term considerations for the type locality, though none of these has been documented as an active, quantified threat to this particular species.