Taxonomy & naming
Thoracocharax securis was originally described by the Italian naturalist Filippo De Filippi in 1853. As with many mid-19th-century descriptions from South American material, the type locality is broadly stated as the Amazon basin region. The valid name, as maintained in Eschmeyer's Catalog of Fishes, is Thoracocharax securis (De Filippi, 1853); the parenthetical authority reflects its original placement in a different genus before subsequent revision moved it into Thoracocharax.
Thoracocharax is one of three genera in the family Gasteropelecidae, the freshwater hatchetfishes — a group entirely unrelated to the marine hatchetfishes (Sternoptychidae) despite sharing the same common name and similar body silhouette. Gasteropelecidae sits within the order Characiformes; molecular work places it close to the subfamily Serrasalminae (piranhas and pacus) and allied groups rather than near the slender tetras it superficially resembles. Thoracocharax securis is distinguished from its genus-mate T. stellatus (the spotfin hatchetfish) primarily by size and the absence or reduction of the dark spots on the dorsal fin that are diagnostic for T. stellatus.
Morphology
The giant hatchetfish is the largest of the freshwater hatchetfishes, reaching approximately 3.5 in in total length, though most individuals in the aquarium hobby are 2.5–3 in. The body plan is unmistakable: extreme lateral compression combined with a massively deepened ventral keel, formed by an enlarged pectoral girdle and the oversized muscles that drive the pectoral fins. Viewed head-on, the fish is almost paper-thin; in profile, the deep, axe-blade body outline gives the family its name.
The coloration is essentially metallic silver across the flanks, with a faint iridescent sheen that shifts from gold to greenish depending on the angle of light. A thin, dark horizontal stripe may run from behind the eye to the base of the tail, though it is often faint or broken. The dorsal fin is set far back, almost over the tail; the pectoral fins are long, wing-like, and held outstretched. The caudal fin is forked. Unlike some close relatives, T. securis lacks conspicuous dark spot markings on the dorsal-fin rays — a useful field character separating it from T. stellatus.
Habitat
Thoracocharax securis occupies a wide range of lowland South American river systems, with confirmed records from the Amazon and Orinoco basins, the Río de la Plata drainage, and various coastal drainages. It is found in a range of habitat types — main river channels, flooded forest (igapó and várzea), still backwaters, and slow-moving streams — but almost invariably at or near the surface, where the surface film is its ecological domain.
Water conditions across its range vary considerably, but the species most commonly inhabits warm, soft, acidic to neutral blackwater and clearwater environments typical of Amazonian lowlands: temperatures in the range of 73–86 °F, pH broadly from 5.5 to 7.5, and low to moderate conductivity. It shows a strong preference for areas with overhanging vegetation, floating plants, or other surface cover that provides shade and concentrates fallen insects. Open, brightly lit stretches of river with no overhead cover are avoided. The species is often found in loose aggregations close to the surface, rarely descending into deeper water.
Feeding
The giant hatchetfish is a specialized surface predator. Its permanently upturned mouth opens directly into the surface film, and the fish feeds primarily on terrestrial and semi-aquatic insects, small invertebrates, and other items that land on or drift across the water surface. This is a highly efficient ambush position: the fish hangs motionless just beneath the meniscus and strikes upward the moment a prey item registers movement.
In the aquarium, T. securis accepts floating foods readily — floating pellets, freeze-dried insects, and surface-delivered live or frozen foods such as fruit flies (Drosophila), mosquito larvae, and small crickets are all taken with enthusiasm. The fish is less inclined to pursue food that sinks below the surface; uneaten items that fall away are typically ignored. Because the upturned mouth is adapted for surface feeding rather than bottom-grazing, care should be taken that the fish actually receives its food rather than allowing faster, mid-water tankmates to intercept it at depth.
Mating
Thoracocharax securis is an egg-scattering open spawner with no parental care, as is typical throughout the family Gasteropelecidae. Reliable sexing outside of spawning condition is difficult; ripe females may appear noticeably deeper-bodied and more rounded in the ventral keel region when gravid, but this difference is subtle and not always apparent on non-breeding individuals.
In the wild, spawning is believed to be seasonal and linked to the flood cycle — rising water inundating the surrounding forest provides surface cover, floating plant material among which eggs are deposited, and a sudden increase in floating invertebrate prey that brings fish into breeding condition. Courtship behaviour has not been described in detail in the literature, and the species is not frequently bred in captivity, so the finer points of mate choice and pre-spawning behaviour remain sparsely documented.
Breeding
Breeding Thoracocharax securis in captivity is uncommon and considered challenging; successful accounts in the hobby literature are scarce. When attempted, conditioning pairs or small groups with live and frozen surface foods over several weeks, then performing a large (30–50%) water change with slightly cooler, very soft acidic water, has been reported to trigger spawning in the more tractable hatchetfish species and may stimulate T. securis similarly.
Spawning appears to take place near the surface among floating plants or surface vegetation, with eggs scattered non-adhesively and allowed to fall or remain suspended in the water column. Eggs and fry receive no parental care, and adults should be removed after spawning. The fry are very small and require microscopic first foods — infusoria, rotifers, and paramecia — before graduating to newly hatched brine shrimp. Growth is slow and rearing success in hobbyist accounts is modest. Most T. securis in the trade is wild-caught rather than captive-bred.
In the aquarium
The giant hatchetfish is a rewarding but somewhat specialized aquarium fish that demands respect for its surface-dwelling biology. It must be kept in a covered aquarium: these are powerful jumpers capable of launching themselves well clear of the water, and an uncovered tank is a death sentence. Even the narrowest gap around filter pipes and heater cables is enough for a determined hatchetfish to escape; secure, well-fitted covers are non-negotiable.
A group of six or more individuals is strongly preferred over solitary or pairs, as T. securis is a schooling species that is visibly more settled and active in company. The tank should be spacious enough for the group — a 47 in or longer aquarium for an adult school — and decorated with floating plants (Amazon frogbit, water lettuce) or overhanging terrestrial leaves to replicate the shaded surface environment the fish favours. Subdued lighting, dark substrate, and driftwood or leaf litter typical of Amazonian blackwater biotopes suit it well. Water should be warm (75–82 °F), soft, and slightly acidic to neutral. It is peaceful toward similarly sized tankmates and does well in a biotope community with calm mid-water and bottom-dwelling Amazonian species such as smaller tetras, corydoras, and apistogrammas. Avoid vigorous or nippy species that may stress fish living perpetually at the surface.
Conservation
Thoracocharax securis has not been formally assessed on the IUCN Red List as of this writing, and its status is therefore Not Evaluated (NE). Its wide distribution across the Amazon and Orinoco basins — among the largest and most intact river systems remaining on Earth — means that range-wide population collapse is not a current concern, and the species is not considered threatened.
Wild collection for the ornamental-fish trade does occur, and T. securis is regularly exported from the Amazon. As with many large-basin Amazonian species, local pressures such as deforestation of riparian and floodplain forest, increased sedimentation, and changes to the flood cycle may affect surface-feeding specialists more acutely than deep-water species. Captive breeding is rare, so most aquarium specimens are wild-caught; purchasing from importers with responsible sourcing practices supports sustainable collection from range-state fisheries.