Taxonomy & naming
Characidium crandellii was described by the Austrian ichthyologist Franz Steindachner in 1915. It sat for decades as a poorly resolved name until a 2021 redescription (published in Ichthyology & Herpetology) clarified its identity, fixed its type locality to a tributary of the Cotingo River near Boa Vista in the Rio Branco drainage of Brazil, and formally treated it as the "Guiana Shield Darter" — a species restricted to the ancient, rocky drainages of that region rather than a wide-ranging Amazonian generalist.
The species belongs to the genus Characidium within the family Crenuchidae, the South American darters and their sailfin relatives, in the order Characiformes. Crenuchidae is a small family best known in the aquarium world for the blackwater sailfin characins Crenuchus spilurus and Poecilocharax species; Characidium is the far larger, non-sailfin lineage of small, cryptically coloured, bottom-hugging darters found across rocky and sandy-bottomed streams throughout South America. As with many small-bodied Neotropical genera, species boundaries within Characidium have historically been difficult to resolve from external appearance alone, which is why the 2021 redescription of C. crandellii was needed to separate it clearly from superficially similar congeners sharing the Guiana Shield river systems.
Morphology
Characidium crandellii is a small fish even by darter standards: FishBase gives a maximum recorded length of 2.5 in standard length, and the redescription's examined series ran mostly 1–2 in SL, with a single exceptional specimen at 4 in SL described as the largest Characidium ever measured for the genus. That outlier aside, most individuals are a few centimetres long — an elongate, subcylindrical body typical of the genus, built to hug bedrock and hold position in current rather than to cruise open water.
The redescription noted that the unusually large specimen displayed golden nuptial colouration, hinting at a colour shift associated with breeding condition, though full details of standard adult colouration were not part of the cached source material here. As in other Characidium, the body shape is slender and slightly compressed, the mouth small and subterminal, and the fins unremarkable in size — nothing about the morphology suggests an open-water swimmer; everything points to a fish that perches, darts, and re-settles.
Habitat
This is a Guiana Shield endemic. FishBase and the 2021 redescription place it in the Essequibo and Takutu Rivers of Guyana, with isolated populations recorded in the Ventuari River above Tencua Falls and in the upper Paragua River, both tributaries of the Orinoco basin in Venezuela; the type locality itself is a tributary of the Cotingo River near Boa Vista in Brazil's Rio Branco drainage. Together these records describe a species confined to the rocky highland and shield-margin drainages of northern South America rather than the vast Amazonian lowlands.
The redescription characterises it as a rheophilic, benthic species — living in fast-flowing streams and rivers and collected specifically from bedrock shoals, the hard, current-scoured rock pavement typical of Guiana Shield rivers. Detailed water chemistry (temperature, pH, hardness) was not reported in the sources available for this account; given the habitat description, cooler, well-oxygenated, fast-flowing water over rock is the safest generalisation, but specific parameters should be treated as data sparse rather than assumed.
Feeding
FishBase records the diet of Characidium crandellii as small animals and insects, consistent with the genus as a whole: Characidium species are micro-predators that pick aquatic and terrestrial invertebrates, insect larvae and small crustaceans off the substrate and out of the current rather than grazing or scavenging broadly. Related, better-studied Characidium such as C. fasciatum are documented as carnivorous feeders that need live or frozen foods — insects and micro-crustaceans — and that often ignore dry flake altogether.
Given the rheophilic, bedrock-shoal habitat this species occupies, its natural feeding is almost certainly opportunistic ambush-and-dart predation on drifting or benthic invertebrates carried past its perch by the current, in keeping with the darting, perch-and-dash locomotion typical of the genus.
Mating
No courtship behaviour specific to Characidium crandellii was found in the available sources. The one relevant clue is the redescription's note that its single largest examined specimen (4 in SL) displayed golden nuptial colouration, suggesting that, as in many characins, breeding condition brings a temporary colour change. Beyond that, dedicated field or aquarium observations of pairing, courtship or spawning site selection in this species are not documented in the sources reviewed here, and this account does not extend beyond what is confirmed.
Breeding
There is no documented breeding account for Characidium crandellii specifically — it is not a species with any record of captive spawning in the sources available. Within Crenuchidae more broadly, the sailfin characins Crenuchus spilurus and Poecilocharax are cave- or leaf-spawners with brief male-only care of a small clutch, a mode that is genuinely achievable at nano scale in the aquarium; but Characidium is a different lineage within the same family, and the genus is understood to be an open substrate egg-scatterer with no parental care, not a cave-spawner. For C. crandellii itself, however, this is inference from genus-level pattern rather than a documented observation, and should be read as such — data sparse.
In the aquarium
Characidium crandellii is not an established aquarium fish; it does not appear to be part of the ornamental trade, and no hobby husbandry account for this species specifically was found in the available sources. What can be said comes from the closely related, better-documented C. fasciatum: Characidium darters are best kept in groups of six or more in a reasonably sized aquarium (30 US gal or more cited for that relative), given a hard, current-swept substrate that mirrors the bedrock shoals they occupy in the wild, and fed a varied diet of small live or frozen foods rather than flake, which micro-predatory darters often refuse outright. Given C. crandellii's tiny recorded size (typically well under 2.5 in) and its rheophilic, rocky-stream habitat, a keeper attempting it — if it were ever available — would need to replicate cool-to-moderate, well-oxygenated, flowing water over hard substrate rather than a soft, still, planted setup. This is an inference from habitat and genus, not a tested husbandry record, and should be treated accordingly.
Conservation
The IUCN Red List assessed Characidium crandellii as Least Concern in an assessment dated 23 February 2020, and FishBase reports the same status. This reflects a species with a restricted but apparently stable range across several Guiana Shield drainages (Essequibo, Takutu, Ventuari and upper Paragua) rather than any indication of population decline; iNaturalist occurrence records corroborate the same distribution across Guyana, Venezuela and Brazil.
As a narrow-range rheophilic species tied specifically to bedrock-shoal stream habitat, its long-term outlook would be sensitive to changes affecting flow regime and water quality in those specific Guiana Shield rivers — damming, mining-related sedimentation and deforestation are the general pressures on freshwater fish across that region — but no source reviewed here documents any such threat currently acting on this species, and it is not flagged as being of active conservation concern.