Taxonomy & naming
Characidium nupelia was described in 2008 by Weferson da Graça, Carla Simone Pavanelli and Paulo Buckup, one of two new Characidium species erected in the same paper from the upper Rio Paraguay basin. It belongs to the family Crenuchidae — the South American darters — within the order Characiformes, a family that also holds the gentle, cave-spawning sailfin characins Crenuchus and Poecilocharax; Characidium itself is by far the largest genus in the family, a group of small, cryptically patterned, bottom-associated fishes often called darter tetras or darter characins for their stop-start, perch-and-dash locomotion.
The describing paper diagnoses C. nupelia against its close relative C. xavante, described in the same work, by a set of fine morphological differences: 12 scale rows around the caudal peduncle (versus 10 in C. xavante), a narrower interorbital distance and a deeper caudal peduncle. The species carries 12–18 dark vertical bars along the body, each roughly as wide as a scale, lacks an adipose fin, has an incomplete lateral line, and shows a conspicuous dark blotch on the caudal peduncle — a pattern typical of the genus and used across Characidium species to tell otherwise very similar-looking fish apart. As with many recently described, single-basin Characidium, no common name has taken hold in English.
Morphology
Characidium nupelia is a genuinely tiny fish: FishBase records a maximum size of only 1.5 in standard length, placing it among the smallest characiforms recorded from the Rio Paraguay drainage. The body plan is elongate and slender rather than laterally compressed like a typical tetra, an adaptation shared across the genus for holding position on and darting along the streambed rather than swimming in open water.
The original description notes 12–18 dark vertical bars crossing the flanks, each about the width of a single scale, over a pale ground colour, with a darker blotch concentrated at the base of the caudal peduncle. The species lacks an adipose fin — unusual among characiforms, but consistent with several Characidium — and its lateral line is incomplete, running only partway along the body. Breeding males in the genus typically develop small hooks on the branched rays of the pelvic and pectoral fins, a trait noted in the original description and used elsewhere in the genus to help distinguish mature males from females.
Habitat
The species is known from tributary streams of the Rio Cuiabá, part of the upper Rio Paraguay basin in Mato Grosso, Brazil — a single-drainage endemic rather than a widely distributed fish. Like other stream-dwelling Characidium, it is a benthopelagic species of running freshwater, associated with the streambed rather than open water, where it perches on fins against rocks, wood or leaf litter and makes short darting movements to intercept prey or reposition.
Detailed water-chemistry data specific to C. nupelia were not located in the sources consulted here. For the genus as a whole, darter characins are documented from well-oxygenated, flowing streams with fine-leaved vegetation and moderate, moderately soft to neutral water — conditions consistent with small upper-basin tributaries rather than the large, slow whitewater channels further downstream in the Paraguay system.
Feeding
Characidium nupelia is a micro-predator, as is typical for its genus: it takes small aquatic invertebrates picked from the substrate and drift rather than grazing algae or detritus. Genus-level accounts of Characidium husbandry identify chironomid midge larvae (bloodworm) as a primary natural prey item, alongside other small insect larvae and micro-crustaceans, and note that darter characins commonly refuse dry flake food entirely.
No diet study specific to C. nupelia was located, so this account follows the documented genus pattern rather than species-specific data. Keepers of related Characidium report best success with small live or frozen foods — bloodworm, brine shrimp, Daphnia and Cyclops — offered near the substrate where the fish naturally forages, rather than food that stays suspended in open water.
Mating
No courtship or spawning behaviour has been documented specifically for Characidium nupelia. The original description does record that breeding males of the species develop small hooks on the branched rays of the pelvic and pectoral fins, a secondary sexual character used across the genus to identify ripe males; beyond that trait, no external sexual dimorphism or courtship display has been published for this species.
Across Characidium generally, breeding is understood as substrate-associated egg-scattering with no elaborate pair-bonding display comparable to the cave-courting behaviour of its Crenuchidae relatives Crenuchus and Poecilocharax. Data on this species is sparse enough that any account of its specific mating sequence would be speculation rather than documented fact.
Breeding
As with mating behaviour, no breeding record exists for Characidium nupelia in the sources consulted — it is not a species in the aquarium trade and has not, as far as is documented, been bred in captivity. Within Crenuchidae, breeding strategy is not uniform: the cave- and leaf-spawning sailfin characins Crenuchus spilurus and Poecilocharax provide brief male care over a small clutch, genuinely breedable at nano scale, while Characidium species are understood to be substrate egg-scatterers without that cave-guarding behaviour.
Given the presence of hook-bearing breeding males noted in the original description, some form of physical contact or positioning during spawning is plausible, but nothing published describes nest-building, egg-guarding or parental care for this species. It would be inaccurate to assign it any specific parental-care mode beyond noting that documented Characidium breeding is egg-scattering with no confirmed care of eggs or fry.
In the aquarium
Characidium nupelia is not established in the aquarium hobby; it has no trade history, no common name in circulation, and this account draws on genus-level Characidium husbandry rather than species-specific keeping reports. Darter characins as a group are shy, bottom-perching nano fish best suited to a quiet, well-oxygenated aquarium furnished with fine-leaved plants, smooth stones and leaf litter that lets them perch and dart the way they do in a stream. Genus care guidance suggests keeping them in a group of five or more, since isolated individuals are typically more nervous and less likely to settle or feed confidently.
Because the genus often refuses flake and relies on small live or frozen invertebrate foods, a Characidium tank should be stocked and fed with that in mind — plenty of natural grazing surface, gentle current, and tankmates too slow-moving or too large to compete for the tiny prey items this fish depends on. At barely 1 in, C. nupelia would need correspondingly modest, carefully chosen company; nothing in the available sources suggests it is currently collected for the trade at all.
Conservation
No IUCN Red List assessment exists for Characidium nupelia; it is Not Evaluated. FishBase likewise records no conservation status for the species, and no dedicated study of its population trend or threats was located in the sources consulted here.
As a fish known only from tributary streams of the Rio Cuiabá in the upper Rio Paraguay basin, its documented range is narrow, and single-drainage headwater endemics of this kind are generally more exposed to localised pressures — deforestation, agricultural runoff and stream-habitat alteration in the upper Paraguay basin — than wide-ranging characins, but without a formal assessment or targeted survey this remains an inference rather than a documented finding. It is fair to describe the species' conservation status honestly as data-deficient in practice, even though it has not been formally evaluated under that category.