Taxonomy & naming
Characidium samurai was described by Angela Zanata and Priscila Camelier in 2014, in Neotropical Ichthyology, from material collected at Piraí do Norte on a tributary of the rio do Peixe in coastal Bahia, Brazil. The holotype (MZUSP 108188, 2 in SL) was collected in August 2012. The species name refers directly to its most obvious field mark: a dark, sharply defined midlateral band running from the back of the head to the end of the caudal peduncle, likened by the describing authors to the stroke of a samurai's blade.
The genus Characidium sits in the family Crenuchidae, order Characiformes — the same family as the shy, cave-spawning sailfin characins (Crenuchus, Poecilocharax) that are prized nano-fish in the blackwater-aquarium hobby, though Characidium itself is ecologically and behaviourally distinct: a stream-bed darter rather than an open-swimming or cave-dwelling fish. Based on phylogeographic analysis in the original description, C. samurai is placed within the C. alipioi species group, one of several morphologically similar species complexes within the large, taxonomically active genus Characidium.
Morphology
Characidium samurai is a small, elongate, torpedo-shaped fish typical of the genus, built for perching on the substrate and making short, quick darting movements rather than sustained swimming. FishBase records a maximum size of 2 in standard length in males, making this one of the smallest characins in the Piranhas & Pacus atlas — genuinely nano scale.
The defining feature is the dark lateral stripe: a black band roughly 1.5–2 scales wide running from behind the head to the base of the caudal fin, sharply set off against the paler body. Unlike some related darter species, C. samurai lacks dark bars or blotches on the lower half of the body, a detail the original description uses to distinguish it from close congeners in the C. alipioi group. Beyond this pattern, detailed morphometric data (fin ray counts, scale counts) are documented in the original taxonomic description but are not summarized in commonly available secondary sources — readers wanting full meristics should consult Zanata & Camelier (2014) directly.
Habitat
The species is known only from small coastal river systems in the central coastal area of Bahia State, eastern Brazil — specifically the Rio das Almas and rio Vermelho basins, within the Atlantic Forest (Mata Atlântica) ecoregion. It occupies clear-water streams with moderate to fast current flowing over rock, pebble, sand and muddy substrates, at low elevations of roughly 56–581 ft.
As a Crenuchidae darter, C. samurai is a bottom-perching fish rather than an open-water swimmer, holding station against rocks and sediment in flowing water and making short dashes to feed or reposition — the same lifestyle documented across the genus. Detailed water chemistry specific to this species has not been published; a closely related congener, C. fasciatum, is documented from similar flowing habitats at roughly 64–75 °F and pH 5.6–7.5 in soft to moderately hard water, which gives a reasonable working picture for the genus but should not be read as measured data for C. samurai itself.
Feeding
Characidium as a genus are micro-predators: insectivorous fish that dig small invertebrates out of the substrate or pick them off rocks, rather than filter-feeding or grazing algae. They favour well-oxygenated water in areas of current and rapids, consistent with their darting, perch-and-dash foraging style.
No diet study specific to C. samurai has been located, but genus-wide accounts are consistent: these fish are true micro-predators that often ignore dried flake food entirely, doing best on small live or frozen foods — cyclops, baby brine shrimp, microworm, chopped bloodworm and similar. Anyone keeping this species (or any Characidium) should plan around live/frozen micro-foods rather than assume a flake-eating community fish.
Mating
No courtship or spawning behaviour has been documented for Characidium samurai specifically. Within Crenuchidae more broadly, breeding biology diverges sharply between subfamilies: the sailfin characins (Crenuchus, Poecilocharax) are cave- or leaf-spawners with brief male parental care, while Characidium species are understood to be substrate egg-scatterers, without the nest-guarding behaviour of their sailfin relatives. No sexual dimorphism has been documented for this species — data are simply sparse, and this account does not speculate beyond that.
Breeding
There is no confirmed record of Characidium samurai being bred in the aquarium, and genus-wide hobby sources note that Characidium species generally are not established captive breeders — unlike their Crenuchidae relatives Crenuchus and Poecilocharax, which are genuinely breedable cave/leaf spawners with male care. Characidium as a genus is understood to scatter eggs over the substrate with no parental care, but no clutch size, incubation period or fry-rearing account exists for this species in the sources reviewed. Given how narrowly restricted C. samurai is to a few Bahia coastal streams, and its complete absence from the aquarium trade, this is unsurprising — this is a case where honesty means saying breeding data is simply not available rather than inferring detail from a congener.
In the aquarium
Characidium samurai is not established in the aquarium hobby — there is no evidence of trade availability, and this account should not be read as a care guide from direct experience with the species. What follows is drawn from its documented biology and genus-wide keeping notes for Characidium.
At under 2 in, this would be a true nano fish, suited to a modest, well-filtered aquarium with a current-generating flow (a powerhead or strong filter outlet) to mimic its native fast-flowing stream habitat, plus rock and pebble substrate that gives it surfaces to perch on. Like other Characidium, it would need live or frozen micro-foods rather than flake, and — as a bottom-perching, somewhat territorial darter rather than an open shoaler — would likely do best kept in modest numbers with adequate perching space and sightline breaks rather than crowded together. Given its extremely restricted native range in Bahia's coastal streams, any aquarium specimens should never be assumed to be a sustainably farmed source without direct confirmation.
Conservation
The IUCN Red List assesses Characidium samurai as Least Concern, an assessment dated November 7, 2018. This may read as reassuring, but it should be weighed against how narrow the species' known range is — a handful of coastal river systems in central Bahia within the Atlantic Forest ecoregion, one of the most heavily deforested and fragmented biomes in Brazil. The Least Concern rating reflects the criteria applied at assessment rather than an absence of pressure on Atlantic Forest coastal streams generally.
There is no evidence this species is targeted by the aquarium trade, which removes one common pressure on small-range Neotropical fishes, but it also means there is essentially no hobbyist or captive population as a hedge against habitat loss. As with many small-range, recently described Crenuchidae, ongoing monitoring of Atlantic Forest stream habitat is the relevant long-term watch item for this species.