Taxonomy & naming
Thoracocharax stellatus was described by the Austrian ichthyologist Rudolf Kner in 1858, with the type locality recorded as the rio Cuiabá in Mato Grosso state, Brazil. The species was originally placed in a broader generic concept that was later reorganised; the genus Thoracocharax Fowler, 1906 now receives the largest of the true hatchetfishes, and T. stellatus is one of two species assigned to it, the other being T. securis. Eschmeyer's Catalog of Fishes (Catalog of Fishes) is the governing authority for the valid name Thoracocharax stellatus (Kner, 1858); the parenthetical authority indicates the species was described under a different generic name.
The family Gasteropelecidae — the freshwater hatchetfishes — is a small Neotropical family of characiforms comprising three genera: Gasteropelecus, Carnegiella, and Thoracocharax. All share the deep, axe-blade body profile produced by an enormously enlarged coracoidal bone and the pectoral muscle mass attached to it. Thoracocharax is distinguished from the other two genera by its larger size and relatively longer pectoral fins. GBIF records comprehensive occurrence data for T. stellatus across its wide South American distribution, and cytogenetic study has confirmed a diploid chromosome number of 2n = 48 for populations from the Paraguay River basin.
Morphology
Thoracocharax stellatus is the largest freshwater hatchetfish, reaching up to 2.5 in standard length (SL). The body is extraordinarily deep and strongly compressed laterally, with the distinctive ventral keel formed by the greatly expanded coracoid bone running from the throat to the anal fin. Viewed from the front the fish is almost knife-thin; from the side the silhouette is unmistakably axe-shaped. The large, wing-like pectoral fins attach to the massive pectoral girdle and can beat rapidly, enabling the fish to skim or even briefly fly above the surface.
The dorsal profile is relatively flat, with the dorsal fin set far back, close to the deeply forked caudal fin. The body is silvery overall, with a dark, longitudinal lateral stripe running from behind the eye to the caudal peduncle. A small but distinctive dark spot — the feature that gives the species its common name — sits at the base of the pectoral fin. The eye is large, oriented upward and forward in keeping with a fish that hunts the air–water interface. There is minimal sexual dimorphism; females may appear slightly deeper-bellied when gravid, but the sexes are otherwise difficult to distinguish externally.
Habitat
In the wild, Thoracocharax stellatus occupies the surface layer of larger rivers as well as smaller black-water and clear-water streams, tributaries, backwaters, oxbow lakes, and seasonally inundated floodplain habitats. Seriously Fish describes it as occurring in habitats with abundant surface vegetation and some degree of water current; it is fundamentally a surface and near-surface fish, rarely venturing into mid-water or toward the bottom. The species is pelagic in the sense that it holds position in open water rather than associating closely with substrate, but its ecological niche is the air–water interface rather than the water column proper.
Its range spans the Paraná, Amazon, and Orinoco river basins across a broad swathe of South America including Argentina, Bolivia, Brazil, Colombia, Ecuador, Paraguay, Peru, Uruguay, and Venezuela. Water chemistry in these habitats varies widely: Seriously Fish reports a temperature range of 68–82 °F, pH 5.0–7.5, and hardness from 18 to 215 ppm (approximately 1–12 °dH), reflecting the species' occurrence in both soft Amazonian blackwater and the harder, slightly more alkaline rivers of the Paraná drainage.
Feeding
Thoracocharax stellatus is a highly specialised aerial insectivore. FishBase records an insectivorous diet consisting primarily of ants, beetles, and mayflies; Seriously Fish notes that in analyses of wild stomach contents the diet is approximately 99.6% insects, with the overwhelming majority being terrestrial insects that have fallen or flown onto the water surface rather than aquatic invertebrates. The fish holds just below the surface film, scanning upward with its dorsally directed eyes, and strikes at prey on or just above the water's surface.
In captivity this specialisation shapes what foods are accepted. The species will take live and frozen foods presented at or near the surface — bloodworm, Daphnia, and brine shrimp are commonly used — but it is notably reluctant to take sinking foods and may refuse food that is not at the surface. Floating pellets or flake can be used to supplement live and frozen foods, but the diet should be varied and include appropriately sized live or frozen invertebrates to maintain condition.
Mating
Thoracocharax stellatus is an egg-scattering open spawner. There is no documented pair bond or lasting social pairing; individuals congregate near the surface and spawning appears to be a group or opportunistic event among shoaling fish rather than an exclusive courtship between a fixed pair. The species shows minimal external sexual dimorphism, though females may appear slightly fuller-bodied during the breeding season.
Because breeding was unrecorded in the scientific literature until relatively recently, details of mating behaviour in the wild remain sparse. What is known comes largely from aquarium observations: spawning fish scatter adhesive eggs among fine-leaved plants or over a substrate without elaborate pre-spawning ritual, and neither male nor female engages in mate guarding or territory defence around the spawn. Successful reproduction in captivity has been achieved with groups of four to six individuals rather than with isolated pairs, suggesting that the presence of multiple conspecifics facilitates spawning.
Breeding
The first recorded aquarium breeding of Thoracocharax stellatus was achieved by a Czech aquarist and documented in Tropical Fish Hobbyist magazine. Spawning was achieved in a soft, acidic, peat-darkened breeding tank with a thin gravel substrate and fine-leaved plants or a spawning grid to receive the eggs. A group of four to six adults was used. Eggs were deposited on fine-leaved plants or on the spawning grid and hatched within a few days under warm conditions. The adults offer no parental care and will consume eggs if left in the breeding tank, so eggs or adults should be removed promptly after spawning.
Fry were reported to be very small at hatching and required infusoria and freshly hatched brine shrimp as first foods. Growth in the early weeks is slow, as is typical for small surface-dwelling characiforms. Because detailed breeding accounts in the scientific literature were absent until this aquarium record, parameters such as clutch size, precise incubation time, and the timeline to sexual maturity remain incompletely documented. The soft, slightly acidic water conditions used in the successful captive spawn are consistent with the blackwater end of the species' natural range.
In the aquarium
Thoracocharax stellatus is an occasionally available species in the specialist aquarium trade, exported commercially as part of the broader South American hatchetfish trade. It requires a tank with a tight-fitting cover — its large pectoral muscles and streamlined dorsal profile make it a capable and reflexive jumper, and losses to uncovered tanks are common. Surface area matters more than tank depth: a long, shallower aquarium with plenty of horizontal swimming space at the upper water level suits the species better than a tall, deep one.
It should be kept in a group of at least six individuals; hatchetfishes are shoaling fish and solitary specimens rarely thrive. Surface cover in the form of floating plants (such as Indian fern or water lettuce) reduces stress by mimicking the overhead vegetation of natural blackwater habitats and diffuses light to comfortable levels. Tankmates should be peaceful mid- and bottom-dwelling species that will not compete at the surface — small tetras, Corydoras, and dwarf cichlids are common choices. Water chemistry should lean toward the softer, slightly acidic end of the species' tolerances for long-term maintenance, with temperatures in the low-to-mid 20s Celsius and pH 5.5–7.0. The species' surface-specific feeding behaviour must be accommodated: all foods must be offered at or just below the surface film.
Conservation
Thoracocharax stellatus is assessed as Least Concern (LC) on the IUCN Red List, with the assessment carried out in 2020. The wide distribution across the Paraná, Amazon, and Orinoco basins — one of the broadest ranges of any gasteropelecid — underpins the assessment: no species spanning that geographic and hydrological scope is thought to face an imminent range-wide threat. FishBase notes minor commercial fisheries value and participation in the commercial aquarium trade, but neither appears to constitute a significant pressure on wild populations at the current scale.
As with many Neotropical fish, the principal background concerns are habitat degradation, floodplain modification, and the fragmentation of river connectivity through damming, all of which affect the seasonal habitats and inundated floodplain areas the species uses. For a surface-dwelling, air-breathing insectivore dependent on intact riparian vegetation and floating plant cover, deforestation and agricultural encroachment on river margins are relevant pressures even where they fall below the threshold that would change the species' IUCN status. No specific conservation actions targeting T. stellatus are recorded.