Taxonomy & naming
Carnegiella marthae was described by the American ichthyologist George S. Myers in 1927. The genus Carnegiella was erected to honour the Carnegie Institution for its support of ichthyological research, and Myers selected a name reflecting elegance and grace for the taxon. The specific epithet marthae honours Martha Ruth Myers, wife of George S. Myers; the use of a spousal patronym was common practice in early twentieth-century ichthyology.
The family Gasteropelecidae — the freshwater hatchetfishes — is characterised by the enormously deepened, keeled chest formed by hypertrophied pectoral-girdle musculature, a structure unique among teleost fishes and associated with the family's ability to leap and glide above the water surface. Within the family, Carnegiella is distinguished from the larger genus Gasteropelecus by its lack of an adipose fin and by its smaller maximum size. Eschmeyer's Catalog of Fishes records the valid name as Carnegiella marthae Myers, 1927.
C. marthae is the smallest of the four recognised Carnegiella species and is sometimes confused in the trade with C. strigata (the marbled hatchetfish) and C. schereri, but the blackwing's distinctive dark pectoral-fin pigmentation and plain flank pattern set it apart from its relatives.
Morphology
Carnegiella marthae is a very small fish, reaching approximately 1 in standard length in wild specimens; aquarium individuals occasionally approach 1.5 in total length. Like all hatchetfishes the body is extremely laterally compressed and deep through the chest, with the ventral profile forming a sharp, almost blade-like keel and the dorsal profile nearly flat. The pectoral fins are large and wing-like, giving the fish its common name and its capacity for short aerial glides above the water.
The flanks are silvery with a thin, sometimes incomplete dark lateral stripe running from behind the operculum toward the caudal peduncle. The most diagnostic feature is the heavily pigmented pectoral fins: the fin membrane is darkened, appearing blackish or sooty — conspicuously darker than in related species — which gives rise to the common name blackwing hatchetfish. The dorsal fin is set far back near the tail, and there is no adipose fin, distinguishing Carnegiella from the adipose-bearing Gasteropelecus and Thoracocharax.
Sexual dimorphism is subtle. Females carrying ripe eggs are somewhat more rotund through the abdomen than males, a difference visible when the fish are viewed from above. In lean condition the sexes are difficult to distinguish.
Habitat
Carnegiella marthae is native to the Negro and upper Orinoco River basins in South America, with records from Colombia and Venezuela. It inhabits the classic Amazonian blackwater environment: slow-moving or standing water heavily stained with tannins and humic acids leached from decaying leaf litter and submerged wood, producing the characteristic dark tea colour associated with ion-poor, highly acidic tropical streams.
Within that broader habitat the species is emphatically a surface fish. It haunts the uppermost layer of the water column, typically beneath dense overhanging vegetation, submerged branches, and fallen logs. The heavy canopy shading keeps ambient light low even at the surface, and the fish are well camouflaged against the dappled reflections of the overlying forest. Reported water parameters from the type region align with the soft, acidic conditions typical of blackwater: pH in the range of 5.5–6.5 and very low hardness (0–4 dH), with temperatures of roughly 73–81 °F.
Spawning appears to be seasonal and linked to the annual flood pulse, with fish moving into inundated forest during the rainy season. This seasonal breeding ecology, common to many Amazonian small fish, is one reason captive spawning has proven so difficult to replicate in a standard aquarium environment.
Feeding
In the wild, Carnegiella marthae feeds predominantly on small invertebrates taken at and just below the water surface. Insects — both aquatic larvae drifting upward and terrestrial invertebrates falling onto the water — form the bulk of the diet, supplemented by small crustaceans and other minute aquatic invertebrates. The upturned mouth and surface-hugging lifestyle reflect this specialisation: the fish patrols just beneath the surface film, watching for prey silhouetted against the light above.
In the aquarium the species can be weaned onto prepared foods, including small flakes, micro-pellets, and freeze-dried items, but it feeds most readily and confidently on live and frozen invertebrates: mosquito larvae, Daphnia, baby brine shrimp, and cyclops nauplii are all accepted. Foods that sink quickly or that the fish must pursue into the mid-water column are less effective than those that float or hover near the surface. A varied diet supports good coloration and breeding condition; relying solely on dry foods often results in fish that feed poorly and remain thin.
Mating
Like other characoids, Carnegiella marthae is an egg-scattering open spawner with no parental care. In the wild, spawning is associated with the rainy season and the annual inundation of forest margins, which expands available habitat and food supply and appears to be a key trigger for reproductive behaviour. The fish are thought to migrate into flooded forest during high water, spawning among the roots, fine-leaved vegetation, and submerged leaf litter of the inundated zone.
Male courtship behaviour in this species has not been extensively documented in the literature, but the pattern is presumed to follow that of closely related hatchetfishes: males pursue gravid females near the surface, stimulating them to release eggs which are simultaneously fertilised. Because the spawning trigger appears to be tied to environmental cues — flooding, rain, changes in barometric pressure, and possibly temperature drops from incoming cool rainwater — replicating these stimuli in captivity is the central challenge for anyone attempting to breed the species.
Females gravid with eggs develop a noticeably rounder abdomen visible from above, the most reliable indicator of a fish in breeding condition.
Breeding
Successful captive breeding of Carnegiella marthae has not been well documented, and the species is generally considered one of the more difficult small characoids to spawn in the aquarium. The difficulty stems from the apparent dependence on seasonal environmental cues — flooding, rain simulation, water changes with softer or slightly cooler water — that are not routinely provided in typical home aquaria.
Attempts that have been reported typically involve very soft, acidic water (pH in the low 5s to 6.0, near-zero hardness), simulated rainfall via surface sprinklers or misting, and the presence of fine-leaved floating plants or spawning mops near the surface. Eggs, when obtained, are adhesive and are deposited on or among fine-leaved vegetation or among floating plant roots. As with all hatchetfishes, no parental care is provided, and adults must be removed promptly after spawning to prevent egg predation. The fry, if hatched, are extremely small and require infusoria or similar first foods before graduating to newly hatched brine shrimp. The species' small size, surface orientation, and apparent sensitivity to water quality make fry rearing technically demanding. Most fish in the hobby are wild-caught.
In the aquarium
Carnegiella marthae is a peaceful, gregarious species that should be kept in groups of six or more individuals; solitary or small groups are prone to stress and rarely feed well. A school of blackwing hatchetfish occupying the top quarter of a well-planted blackwater aquarium is a striking display: the fish hold position just below the surface in a characteristically flat, synchronised spread, and any disturbance sends them darting as a unit. The species is an accomplished jumper, and a tight-fitting lid with no gaps is non-negotiable.
Water chemistry should reflect the species' blackwater origins: soft (GH 2–8), acidic (pH 5.5–6.5), warm (73–81 °F), and filtered through peat or supplemented with Indian almond leaves to maintain low pH and provide tannins. The tank should be dimly lit with floating plants or overhanging surface cover to give the fish a sense of shelter; bright, bare tanks cause chronic stress. Filtration should provide gentle flow — a surface spray bar or sponge filter is ideal — as the natural habitat is slow-moving and the fish are not built for current.
Compatible tankmates are small, peaceful, and occupy different water levels: small corydoras at the bottom, dwarf cichlids or small tetras in the mid-water, and the hatchetfish themselves at the surface. Larger or boisterous tankmates are inappropriate. The species can reach up to seven years in ideal captive conditions, though wild-caught individuals sometimes take time to adjust to aquarium feeding.
Conservation
Carnegiella marthae is assessed as Least Concern (LC) on the IUCN Red List, with the assessment finalised on 12 August 2020. The evaluation recorded no major threats to the species at the time of assessment, reflecting its relatively broad distribution across the Negro and upper Orinoco basins and the large extent of still-intact blackwater habitat in those river systems.
However, the broader region faces pressure from deforestation, agricultural expansion, and mercury contamination from artisanal gold mining — chronic stressors that degrade the tannin-rich, leaf-litter-dependent blackwater habitats on which the species depends. The upper Orinoco and upper Negro regions are among the more remote and less disturbed parts of tropical South America, providing a degree of buffer that more accessible species do not enjoy.
In the ornamental fish trade, Carnegiella marthae is routinely collected for export. Since captive breeding remains essentially unrealised at commercial scale, the supply chain depends on wild harvest. Trade volumes are not known to be at levels that threaten the species, but the continued absence of a captive-breeding solution means that aquarist demand is met entirely from wild populations.