Taxonomy & naming
Creagrutus atrisignum was described by George Sprague Myers in 1927. The valid name is Creagrutus atrisignum Myers, 1927, as listed in Eschmeyer's Catalog of Fishes (Fricke, Eschmeyer & van der Laan); the authority is not parenthetical because Myers placed the species in Creagrutus at the time of its original description and it has remained there.
Creagrutus is a large and complex genus of Neotropical characids placed within the tribe Stevardiinae (or the broader subfamily Stevardiinae, depending on the classification followed) inside the family Characidae. Phylogenetic work on the genus has confirmed the Tocantins-basin endemism of C. atrisignum and has used the secondary humeral mark as a diagnostic character in sorting species within a morphologically variable complex. The genus name derives from the Greek kreagreytos (to tear the flesh), a reference shared by many predatory characid names.
Morphology
Creagrutus atrisignum is a small, slender-to-moderately-built characid reaching approximately 2 in standard length (male/unsexed; FishBase). Like most Creagrutus it has the typical characid body plan: a compressed fusiform profile, forked caudal fin, small adipose fin, and a full complement of characid fin rays and scale rows.
The key diagnostic feature encoded in the specific epithet is the secondary humeral mark — a dark spot or blotch on the lateral flank behind the primary humeral spot that sits just posterior to the opercle. This secondary mark distinguishes C. atrisignum from many of its congeners in the Tocantins drainage. The flanks are otherwise silvery, and, as in most Creagrutus, adult males may show subtle body differences from females, though detailed published dimorphism data are sparse.
Habitat
Creagrutus atrisignum is endemic to the Maranhão River basin within the upper Tocantins River drainage in the state of Maranhão, Brazil. The Tocantins is one of the major river systems draining the Brazilian highlands (Planalto Central) toward the Atlantic via the Amazon estuary region, and its upper tributaries pass through cerrado (tropical savanna) landscapes where water chemistry is typically soft and slightly acid to neutral.
FishBase classifies the species as freshwater and benthopelagic, indicating it occupies the lower to mid water column in riverine habitats. Comprehensive faunal surveys of the Amazonian basin, including analyses of Tocantins fish distribution, confirm this endemic range. Specific substrate and microhabitat data for C. atrisignum in the literature are limited, but Creagrutus species broadly favour clear or gently turbid streams and river channels with structured habitats — woody debris, leaf litter, or sandy substrates.
Feeding
Detailed dietary studies specific to Creagrutus atrisignum have not been published. Across the genus, Creagrutus species are generally considered opportunistic omnivores or micropredators of the lower and mid water column, taking small invertebrates — insect larvae, microcrustaceans, and terrestrial invertebrates that fall to the surface — along with small amounts of plant material and organic detritus.
In the aquarium, congeners are typically offered small live and frozen foods (Daphnia, brine shrimp nauplii, microworm, finely crushed flake or micro-pellets). Given the small adult size of C. atrisignum, appropriately scaled food items are important. Creagrutus are not aggressive feeders and should not be mixed with species that will outcompete them at mealtimes.
Mating
No species-specific mating observations have been published for Creagrutus atrisignum. Within Characidae broadly, and among small Creagrutus-like characids in particular, the expected reproductive mode is open egg-scattering with no parental care: pairs or group spawners scatter small, slightly adhesive eggs among fine-leaved vegetation or over substrates, after which adults take no further interest in eggs or fry.
Male Creagrutus species can show modest secondary sexual characters — slightly more slender body or small hooks on anal-fin rays in some congeners — but courtship behaviour in the wild or aquarium has not been formally described for this species. Spawning is likely triggered by seasonal flood-cycle cues (rainfall and temperature shifts) in nature, as is typical for small characids of the cerrado-influenced Tocantins drainage.
Breeding
Captive breeding of Creagrutus atrisignum has not been documented in the hobbyist literature, and the species is not commercially bred for the trade. By analogy with small characid congeners that have been bred, the general approach would be a soft, slightly acidic to neutral, warm water setup (around 75–82 °F) with fine-leaved plants or spawning mops as egg targets, removal of adults after spawning to prevent predation of eggs, and small live foods for fry once they become free-swimming.
The adhesive or semi-adhesive eggs typical of egg-scattering characids hatch within two to three days at tropical temperatures, and fry require infusoria-grade or commercial liquid fry food before graduating to newly hatched brine shrimp nauplii. In the absence of captive breeding records, these details are inferred from the genus and family rather than confirmed observations.
In the aquarium
Creagrutus atrisignum is essentially absent from the hobby trade; it is a wild-caught, regionally endemic species that has not been established in captive breeding programmes. Its small size (2 in SL) and the general temperament of Creagrutus — peaceful, schooling, mid-to-lower water column — suggest it would suit a species or biotope aquarium replicating the cerrado-stream habitats of the upper Tocantins: fine sandy substrate, dark tannin-stained water, sparse planting or leaf litter, and gentle filtration.
Water parameters should reflect the soft, slightly acidic to neutral conditions of its native drainage. If encountered, it should be kept in a group of six or more individuals, as small characids are more settled and show more natural behaviour in conspecific company. Tankmates, if any, should be peaceful and similarly sized; robust or boisterous species would stress it. Given the complete absence of captive experience on record, any keeper would be working from first principles.
Conservation
Creagrutus atrisignum is assessed as Least Concern (LC) on the IUCN Red List, with an assessment dated 07 November 2018. The LC listing reflects that, while the species is a narrow endemic of the Maranhão River basin in the upper Tocantins, its population is not currently considered to be declining at a rate or magnitude that would qualify for a threatened category.
The upper Tocantins drainage nevertheless faces continuing pressures from agriculture, hydroelectric development, and land-use change associated with the expansion of soy and cattle production across the Brazilian cerrado. Narrow endemics in fragmented upland drainages can be sensitive to catchment modification even when population declines have not yet been formally documented. The species is not exploited by the ornamental trade to any significant degree, which removes one pressure common to more decorative characids.