Taxonomy & naming
Characidium borellii was described by the Belgian-British ichthyologist George Albert Boulenger in 1895. It belongs to the family Crenuchidae, commonly called the South American darters or sailfin characins, a group that sits apart from most other characins in its benthic, perching lifestyle. Within Crenuchidae it falls in the subfamily Characidiinae — the true darters — as distinct from the subfamily Crenuchinae, which contains the shier, cave-spawning sailfin characins such as Crenuchus and Poecilocharax.
Characidium is a large genus of small, superficially similar stream-bed darters distributed across the Amazon, Orinoco, Paraná-Paraguay and Guianas drainages, and species-level identification within the genus is difficult without close examination of fin-ray counts and colour pattern; C. borellii is recognised as a valid species distinct from its many congeners by its combination of fin-ray counts (dorsal ii,8; anal ii,7) and vertebral count (37) alongside its characteristic zebra-like banded flank pattern.
Morphology
Characidium borellii is a slender, cylindrical, torpedo-shaped fish reaching a modest maximum of about 2.5 in standard length, placing it firmly among the nano-sized characins. The mouth carries a single row of small conical teeth in both jaws, suited to picking small invertebrates from the substrate and water column rather than to any predatory slashing bite. The dorsal fin holds two spines and eight soft rays and the anal fin two spines and seven soft rays, with 37 vertebrae recorded for the species.
Colouration follows the darter-characin template: a pale to olive-brown body overlaid with a series of dark vertical bars or blotches running down the flank, giving a zebra-like banded appearance that provides effective camouflage against a mottled gravel or sand stream bed. The paired fins, particularly the pectorals, are proportionally large and used actively to prop the fish against the substrate, a feature shared across the genus and central to its distinctive perching behaviour.
Habitat
Characidium borellii is known from Andean tributaries of the Paraná River basin in northwestern Argentina, part of a genus otherwise distributed widely across fast-flowing streams throughout South America, including the Amazon, Orinoco, Paraná and Guianas systems. It is a benthopelagic freshwater species, living on and just above the stream bed rather than in open water.
Habitat data specific to this species is sparse, but the genus as a whole is strongly associated with clear, well-oxygenated, moderate-to-fast-flowing streams over sand, gravel or rock, generally in soft, acidic-to-neutral water. Reported ranges for Characidium species more broadly span roughly 64–81 °F and pH 5.5–7.5, and as an Andean-tributary population, C. borellii likely experiences cooler, more subtropical conditions than its lowland Amazonian relatives, consistent with its northwestern Argentine range.
Feeding
Characidium borellii is a micro-predator, picking tiny aquatic insect larvae and micro-crustaceans from the stream bed and drift rather than grazing or scavenging broadly. Its perching, dart-and-settle foraging style — holding a position against the current on its pectoral and pelvic fins before darting a short distance to seize prey — is typical of the genus and reflects its adaptation to flowing water over a firm substrate.
In the aquarium, darter characins are reported to feed frequently on live or frozen micro-foods such as bloodworm, mosquito larvae, cyclops and daphnia, and they do not readily accept dry flake. A keeper should plan on a consistent supply of small live or frozen foods rather than relying on flake or pellet as a staple.
Mating
Specific courtship behaviour for Characidium borellii has not been documented in the sources available. Within Crenuchidae more broadly, the darter subfamily (Characidiinae, which includes Characidium) is understood to scatter eggs over the substrate with minimal courtship display and essentially no parental care, in contrast to the cave- and leaf-spawning sailfin characins of the subfamily Crenuchinae, where males briefly guard a small clutch.
No reliable account of external sexual dimorphism in this species could be confirmed from the cache; data on sexing C. borellii is sparse.
Breeding
Based on genus-level accounts, Characidium species are understood to be substrate egg-scatterers: eggs are released over sand, gravel or fine-leaved vegetation with no parental guarding, unlike the brief male care documented in the related sailfin characins Crenuchus and Poecilocharax. Hobby breeding reports for the genus describe soft, acidic water, fine-leaved plants such as Java moss, and good water movement — ideally from a sponge-covered filter inlet to protect eggs and fry from being drawn in — as the conditions most likely to trigger a spawning attempt.
Documented breeding of Characidium borellii specifically in the aquarium is sparse to absent in available sources; this species should be considered rarely if ever home-bred, and any spawning attempt should follow the general genus-level conditions above rather than a confirmed species-specific protocol.
In the aquarium
Characidium borellii is a genuine nano species, topping out around 2.5 in, and a single specimen or small group can be kept in a tank as modest as 23.5 in in length, provided the aquarium favours current and oxygenation over standing water. Strong filtration with visible flow, a sand or fine-gravel substrate, and driftwood or smooth stones to break up sightlines all suit its stream-bed lifestyle far better than a heavily planted, still-water setup. As an accomplished jumper, it needs a tight-fitting lid.
This is a shy, bottom-dwelling fish, best kept alone or with a handful of its own kind rather than in a boisterous community; fast, nippy or aggressive tankmates will keep it hidden and off its food. Because it seldom takes flake, feeding must be active and varied — small live or frozen foods offered regularly — and a keeper unwilling to maintain that feeding routine should look elsewhere. It is not a piranha-style predator and poses no threat to tankmates beyond its own tiny prey.
Conservation
Characidium borellii has not been formally assessed by the IUCN Red List and carries a status of Not Evaluated. Its known range in Andean tributaries of the upper Paraná basin in northwestern Argentina is comparatively restricted next to many of its wider-ranging Amazonian congeners, but no population trend or specific threat data is available in the sources reviewed here.
As with many small, non-commercial stream fishes in this genus, the lack of a formal assessment most likely reflects limited survey and research attention rather than any confirmation of a secure or declining status; general pressures on Andean and Paraná headwater streams, such as water diversion, sedimentation and habitat degradation, would be relevant to a population like this one if documented, but no such specific data was found.