Taxonomy & naming
Acestrocephalus stigmatus was described by Brazilian ichthyologist Naércio Aquino Menezes in 2006, in a paper titled "Description of five new species of Acestrocephalus Eigenmann and redescription of A. sardina and A. boehlkei," published in Neotropical Ichthyology. The type material was collected from river systems draining Brazil's central plateau. Eschmeyer's Catalog of Fishes records the valid name as Acestrocephalus stigmatus Menezes, 2006; because it was described in Acestrocephalus from the outset, no parenthetical authority is applied.
The genus Acestrocephalus Eigenmann, 1910 belongs to the subfamily Characinae within the family Characidae, the largest family of freshwater fishes. Characinae as broadly conceived encompasses a variety of small-to-medium predatory and insectivorous characids distributed across tropical South America. Within Acestrocephalus, A. stigmatus is distinguished from the closely related A. maculosus by a higher anal-fin ray count (29–31 versus 25–27) and a lower gill-raker count (5–6 versus 7–8), as documented by Menezes (2006). The specific epithet derives from the Greek stigma (mark, spot), chosen in reference to the scattered dark spots — stigmata — that mottle the flanks and dorsal fin and form the species' most visible diagnostic feature.
Morphology
Acestrocephalus stigmatus reaches approximately 5.5 in total length, placing it among the larger members of its genus. The body is elongated and laterally compressed in the manner typical of open-water characids built for pursuit predation. Like other Acestrocephalus, it carries the slender profile, terminal mouth with well-developed dentition, and the forked caudal fin characteristic of active characid hunters.
The species is identified by a combination of pigmentation characters: a conspicuous dark humeral blotch sits immediately behind the operculum, and scattered dark spots (the stigmata of the epithet) are distributed across the flanks and onto the dorsal fin. This spotted patterning separates A. stigmatus visually from several congeners that lack such discrete marks. Anal-fin ray count (29–31) and gill-raker count (5–6) provide the meristic separation from the sympatric or geographically close A. maculosus. Detailed colour descriptions in life are not extensively documented in the accessible literature beyond these core pigmentation features.
Habitat
Acestrocephalus stigmatus is known from three river systems: the Rio Tocantins, the Rio das Mortes (a significant southern tributary of the Rio Araguaia), and the Rio Xingu — all flowing across or from Brazil's central plateau, the Planalto Central, before descending toward the Amazon and its margins. These are large, predominantly clear-to-moderately turbid rivers cutting through cerrado and transitional forest landscapes.
FishBase classifies the species as freshwater and benthopelagic, occupying the open water column and lower reaches rather than a strictly bottom-associated niche. The Tocantins–Araguaia and Xingu systems are renowned for their high fish diversity and endemic fauna; they drain ancient crystalline and sedimentary substrates under a strongly seasonal rainfall regime, with pronounced dry and wet seasons driving water-level fluctuations that influence fish movement, feeding, and reproduction. Specific microhabitat associations within these rivers — whether the species favours main-channel runs, flooded margins, or confluences — have not been detailed in the available literature.
Feeding
As a member of the subfamily Characinae and the genus Acestrocephalus, A. stigmatus is an active predator. The elongated body, terminal mouth, and toothed jaw architecture of Acestrocephalus are consistent with pursuit of small fish and invertebrates in open water — a foraging strategy widely documented across Characinae.
Specific dietary data for A. stigmatus are not available in the accessible literature. By analogy with studied congeners and the broader Characinae guild in large Amazonian and tributary rivers, the diet likely centres on small fishes and aquatic invertebrates taken in the water column, with the balance between fish and invertebrate prey shifting with size and local prey availability. The pronounced seasonal hydrology of the Tocantins and Xingu systems — with high-water periods flooding riparian zones and concentrating prey — probably drives seasonal shifts in feeding opportunity.
Mating
Reproductive behaviour of Acestrocephalus stigmatus has not been described in the published literature accessible to this account. As a characid in the order Characiformes, it almost certainly employs the ancestral egg-scattering open-spawning strategy typical of the group: males pursue and court females, eggs are broadcast without adhesion to a fixed substrate, and no parental care follows.
In the large river systems it inhabits, spawning in characids is commonly synchronised with the onset of the rainy season, when rising waters flood marginal vegetation and provide both cover and food resources for larvae and juveniles. Fine-leaved aquatic plants, flooded grasses, and submerged woody debris in the littoral zones of the Tocantins, Araguaia, and Xingu systems offer likely egg-deposition environments, though this is inferred from the clade's general biology rather than from direct observation of A. stigmatus.
Breeding
No captive breeding records or detailed in-situ observations for Acestrocephalus stigmatus are available in the accessible literature. Breeding parameters — egg size, fecundity, incubation period, and larval development — remain undescribed for this species. Based on the open-spawning pattern general to Characiformes, eggs would likely be non-adhesive and scattered among vegetation or in open water, hatching within a few days at tropical water temperatures, with larvae relying on yolk reserves before beginning exogenous feeding.
This species is not currently part of the ornamental trade and is unlikely to be bred deliberately in captivity. Documentation of its reproductive biology in the wild awaits dedicated ichthyological field study in the Tocantins–Araguaia and Xingu basins.
In the aquarium
Acestrocephalus stigmatus is not an established aquarium species and is absent from the ornamental trade. At 5.5 in total length and as a swift, predatory characid, any individual encountered would require a large, well-filtered aquarium with ample open swimming space, and tankmates small enough to be eaten should be avoided entirely. Water chemistry matching the soft, warm conditions of the central Brazilian river systems it inhabits would be appropriate.
Practical husbandry data are absent from the literature, and the species is unlikely to become available to hobbyists under normal circumstances. Its value to the aquarium world lies more in context — as part of the rich characid fauna of the Tocantins and Xingu, ecosystems of exceptional biodiversity that underlie many of the species that do reach the trade.
Conservation
Acestrocephalus stigmatus is assessed as Least Concern (LC) on the IUCN Red List, with the assessment completed on 7 November 2018. The Least Concern classification reflects the species' occurrence across three distinct large river systems — the Tocantins, Araguaia (via the Rio das Mortes), and Xingu — and a presumed stable population within that range. Wide distribution across multiple, hydrologically separate basins in central Brazil provides a degree of resilience against localised disturbance.
Nonetheless, the Tocantins and Xingu river systems face significant anthropogenic pressures. Large hydroelectric dam projects have substantially altered the flow regime, sediment dynamics, and thermal profiles of the Tocantins and its tributaries, and major dam construction on the Xingu — including the Belo Monte complex — has affected fish passage and floodplain connectivity. Agricultural expansion and associated sedimentation across the cerrado landscape add further cumulative stress. These pressures affect the broader fish communities of these rivers, and while A. stigmatus's current LC status indicates no immediate extinction risk, ongoing monitoring of its populations within these modified systems remains important.