Taxonomy & naming
Characidium pterostictum was described by the Brazilian ichthyologist Aloysio de Mello Filho Gomes in 1947. It sits in the genus Characidium within the family Crenuchidae, the South American darters — a family that also holds the prized, gentler blackwater nano-fish Crenuchus spilurus and Poecilocharax, though Characidium itself is a bottom-perching, stream-bed group rather than a mid-water shoaler.
Characidium is a large and taxonomically active genus, and species-level identification within it has historically relied on fine details of squamation, fin-ray counts and colour pattern that are easy to confuse between sympatric species sharing the same stream. Research on this species has specifically compared it against its close relative C. lanei, with which it co-occurs in some Atlantic Forest coastal drainages, underlining that field identification of Characidium is not always straightforward and museum-level comparison is often needed to confirm a given population's identity.
Morphology
Characidium pterostictum is a small, elongate, cylindrical fish, reaching a maximum recorded standard length of about 3.5 in — modest even by the standards of a nano-appropriate genus. Like other Characidium, it has a reduced swim bladder relative to open-water characins, which contributes to its habit of resting directly on the substrate rather than holding position in the water column.
The body carries the muted, cryptic patterning typical of darter characins — pale flanks broken by darker blotches or speckling that help it disappear against rock and gravel — well suited to a fish that relies on stillness and camouflage between short bursts of movement rather than on open-water speed.
Habitat
This species is native to coastal river basins of the Atlantic Forest region of South America, with a documented range spanning from the Ribeira de Iguape basin in southeastern Brazil south to the Uruguay River basin, crossing through Brazilian and Uruguayan coastal drainages. It is a benthopelagic freshwater fish, found specifically in creeks and small rivers with rocky bottoms and clear, fast-flowing water.
That combination of clear water, hard substrate and strong current is central to its ecology: Characidium pterostictum is a rheophilic (current-loving) species that perches on rock and gravel to hold position against flow, rather than a still-water or floodplain fish. Any aquarium approximation of its habitat needs to reproduce that moving, well-oxygenated water rather than the slack blackwater conditions often assumed for South American characins generally.
Feeding
Characidium pterostictum is a specialized insectivore. A study of its feeding ecology alongside the sympatric C. lanei in an Atlantic Forest coastal stream found its wild diet dominated by immature aquatic insects and other small invertebrates, consistent with a fish that darts from a stationary perch to intercept prey drifting or crawling nearby rather than grazing or filtering.
In the aquarium this translates into a genuine micro-predator that frequently refuses dry flake on its own. Keepers report the best results offering a varied diet of small live or frozen foods — bloodworm, brine shrimp and daphnia — alongside high-quality flake or pellet as a supplement rather than a mainstay, echoing the insect-larvae-heavy diet documented in the wild.
Mating
Specific courtship behaviour for Characidium pterostictum is not well documented in the available literature. Within the darter-characin family more broadly, its close relatives Crenuchus spilurus and Poecilocharax are known cave- or leaf-spawners with brief male care of the clutch, but Characidium itself is understood to be an egg-scatterer over the substrate rather than a cave spawner, and hobby accounts note that gregarious, well-conditioned groups kept with fine-leaved plants or spawning mops may display natural breeding behaviour without a documented courtship ritual specific to this species.
Breeding
Data on captive breeding of Characidium pterostictum specifically is sparse. As an egg-scattering Characidium rather than a cave-spawning Crenuchus or Poecilocharax, it is not expected to show the brief male parental care documented in those genera; available hobby notes describe only that well-maintained aquarium groups given fine-leaved plants or mops as potential spawning sites may breed on their own, with no substantiated account of dedicated fry-rearing protocols for this species. Prospective breeders should treat this as an open area — data sparse — rather than assume a specific spawning trigger or parental-care pattern beyond scattering adhesive eggs with no ongoing care.
In the aquarium
Characidium pterostictum is a nano-to-small-aquarium species by virtue of its modest 3.5 in maximum size, but its needs are specific: strong water flow or current, a hard substrate of rock and fine gravel, and tight-fitting cover, since darter characins are noted as excellent jumpers. It is a social species that should be kept in a proper group — commonly cited as a dozen or more individuals — where it will spend most of its time perched on or just above the substrate, darting briefly to intercept food or reposition, rather than swimming freely in open water.
Water should be kept cool-to-moderate for a Neotropical fish, roughly 72–79 °F, with a soft, mildly acidic to neutral pH around 6.0–7.5, reflecting its clear, flowing coastal-stream habitat rather than warm, static blackwater. Feed a varied diet weighted toward small live or frozen invertebrates rather than relying on flake, and expect a shy, watchful fish that is easily outcompeted for food by faster mid-water tankmates — a species or quiet-tank setting with similarly current-loving bottom dwellers suits it best.
Conservation
Characidium pterostictum has not been formally assessed by the IUCN Red List and is classified as Not Evaluated. Its range along Atlantic Forest coastal drainages from the Ribeira de Iguape to the Uruguay River basin places it within one of South America's most heavily deforested and urbanized freshwater ecoregions, where habitat degradation, sedimentation and water-quality change are ongoing pressures on small-range coastal stream fishes generally, though no species-specific population data are available to quantify the effect on this fish. Data sparse — no dedicated conservation assessment or trend data was located for this species.