Taxonomy & naming
Characidium heirmostigmata was described by da Graça & Pavanelli in 2008 as part of a broader revision of characidiin darters from the upper rio Paraná basin. It belongs to the family Crenuchidae — the South American darters and sailfin characins — within the order Characiformes, and to the genus Characidium, a large assemblage of roughly 60-plus small, elongate, bottom-associated species distributed across South American coastal and inland drainages from the Guianas to the Paraná-Paraguay system. The Catalog of Fishes (Eschmeyer, California Academy of Sciences) is the governing authority for the valid name and placement of this species within Characidium.
The genus Characidium is taxonomically active, with new species continuing to be split out of species complexes on the basis of fine details of colour pattern, squamation and osteology, and C. heirmostigmata itself was diagnosed from co-occurring congeners largely on the pattern of dark bars along its flanks — a diagnostic feature reflected in its species epithet (see Etymology). No dedicated common name has entered general use for this species; it is referred to by its scientific binomial in the sources available.
Morphology
Characidium heirmostigmata is a small, slender-bodied darter with the characteristic Characidium silhouette: an elongate, sub-cylindrical body, a pointed head, and fins built for short, controlled bursts rather than sustained cruising. The type series ranged from 1–2 in standard length, with the holotype measuring 2 in SL — giving the species a practical adult maximum of roughly 2 in, well within the "nano" size range for a characin.
The most distinctive feature of the species, and the one that gives it its name, is a series of 8 to 11 oblique dark bars running down the sides of the body, each incomplete and extending obliquely from around the lateral line both upward and downward rather than forming complete vertical bands. This barred pattern, set against a pale to olive body ground colour typical of the genus, is the primary field mark separating C. heirmostigmata from other Characidium species sharing the upper Paraná basin.
Habitat
Characidium heirmostigmata is known only from the rio Ivaí drainage, a tributary system of the upper rio Paraná basin in Paraná State, southern Brazil — a comparatively narrow, single-drainage distribution typical of many recently described characidiin darters from that region. Like other members of the genus, it is a fish of flowing freshwater streams and river channels rather than still floodplain lakes.
Within those streams, Characidium species characteristically occupy the bottom directly over sand, gravel or fine substrate in areas with noticeable current, perching motionless on their paired fins for long stretches before darting a short distance to feed or reposition — a habit that gives the genus its "darter" common name and distinguishes it ecologically from open-water shoaling characins. Specific water chemistry data for this species were not located in available sources; genus-level accounts describe Characidium generally in slightly acidic to neutral, soft-to-moderate streams, and the subtropical latitude of the upper Paraná basin means this species likely experiences cooler seasonal water temperatures than typical Amazonian lowland characins.
Feeding
Characidium heirmostigmata is a micro-predator, consistent with the diet documented across the genus: small aquatic invertebrates and insect larvae taken from the streambed and drift, rather than plant matter, detritus or fruit. Darters of this genus typically hunt by sight from a stationary perch, darting forward to intercept prey rather than actively foraging over wide areas.
In the aquarium, Characidium species are reported to largely ignore dry flake food and do best on small live or frozen foods — items such as bloodworm, mosquito larvae, brine shrimp, daphnia and cyclops — offered in a way that lets the fish dart and snatch rather than compete in open water. No species-specific feeding data for C. heirmostigmata were located, so this account follows genus-level husbandry knowledge.
Mating
No species-specific courtship or spawning behaviour has been documented for Characidium heirmostigmata in the sources available. At the genus level, Crenuchidae darters are not known for elaborate courtship displays comparable to the cave- and leaf-spawning rituals of their sailfin relatives (Crenuchus, Poecilocharax); breeding in Characidium is generally reported as a simpler pairing-off behaviour associated with soft, slightly acidic water and good current, conditions that are thought to trigger spawning readiness in aquarium settings.
External sexual dimorphism has not been documented for this species specifically. Data are sparse enough that this account does not speculate further.
Breeding
Characidium, as a genus, are egg-scatterers: a conditioned pair or group disperses adhesive eggs over the substrate or into fine-leaved vegetation, typically without nest construction and without the sustained parental care seen in cave-spawning crenuchids such as Crenuchus and Poecilocharax. This is a meaningfully different breeding mode from those sailfin relatives, and no evidence in the available sources suggests C. heirmostigmata departs from the scattering pattern typical of its genus.
No species-specific breeding accounts, clutch sizes or hatching times were located for Characidium heirmostigmata, and the species does not appear to have an established presence in aquarium breeding programmes. This account does not assign parental care behaviour beyond noting that, unlike the sailfin crenuchids, Characidium generally show no sustained brood care.
In the aquarium
Characidium heirmostigmata is essentially absent from the aquarium trade, and this account should be read as an extrapolation from genus-level Characidium husbandry rather than species-specific experience. Darter characins of this genus are well suited to nano and small stream-style aquaria: a shallow footprint with sand or fine gravel substrate, moderate-to-strong current from a powerhead or filter outflow, and open bottom space for the fish to perch and dart is more important than tank volume. Given the species' small adult size of around 2 in, a modest aquarium is appropriate, but the current and substrate matter more than raw litres.
The genus's well-documented reluctance to take flake food is the single most important care note: keepers should be prepared to offer small live or frozen invertebrate foods rather than relying on dry diets. Characidium are not aggressive or territorial toward other fish in the way some tetras are, and their darting, resting behaviour makes them an interesting but easily overlooked bottom-dwelling addition to a quiet, well-oxygenated stream biotope. Because this species has no established trade presence, any specimens encountered would likely be scientific collections or very occasional imports rather than regular stock.
Conservation
Characidium heirmostigmata has not been evaluated by the IUCN Red List; no dedicated assessment was located in the sources available, so its conservation status here is recorded as Not Evaluated. This is unsurprising for a narrow-range characidiin darter described relatively recently (2008) and known only from a single drainage.
As a single-drainage endemic confined to the rio Ivaí system within the upper rio Paraná basin, the species' conservation profile would plausibly track the general pressures facing that basin — damming, water-quality change and habitat alteration in the heavily developed Paraná watershed — but no source-backed assessment of population trend or specific threats was available, and this account does not speculate beyond that general context.