Taxonomy & naming
Gasteropelecus sternicla was described by Carl Linnaeus in 1758, making it one of the earliest hatchetfishes to receive a formal Linnaean name. Eschmeyer's Catalog of Fishes (the authority for valid names) records it as Gasteropelecus sternicla (Linnaeus, 1758); the parenthetical authority indicates that Linnaeus originally placed the species in a different genus. The type locality is Suriname.
The family Gasteropelecidae — the freshwater hatchetfishes — comprises around nine species distributed across tropical South America, grouped into three genera: Gasteropelecus, Carnegiella, and Thoracocharax. The group is defined by the extraordinary development of the sternal keel and the associated flight musculature that allows true powered flight above the water surface, distinguishing freshwater hatchetfishes from any superficially similar tetra. Within Gasteropelecus, the species are separated partly by size, partly by colour-pattern details, and partly by distribution; G. sternicla is the largest and most widespread species in the genus.
The family has historically been placed within the order Characiformes and treated as the sister group of various other characid assemblages; molecular analyses broadly support its monophyly as a distinct family. Common synonyms in older aquarium literature include Gasteropelecus faciatus, but G. sternicla is the valid name recognized by Catalog of Fishes.
Morphology
The body plan of Gasteropelecus sternicla is immediately recognizable: the ventral profile forms a deep, strongly convex keel from chin to vent, while the dorsal profile is nearly straight or gently arched, giving the fish a hatchet shape when viewed from the side. The chest is massively enlarged relative to total body length, packed with the flight muscles that power the rapid wingbeat of the pectoral fins. Standard length reaches approximately 1.5 in, though some individuals approach 2 in.
The flanks are silvery, often with a bluish or pewter sheen, and a dark horizontal stripe runs from behind the pectoral fin base toward the tail — a feature shared with several hatchetfish species but variable in intensity. The belly is pale to white, and the fins are largely transparent with faint pigmentation. The mouth is small and distinctly upturned, directed almost vertically at the surface — a structural adaptation for surface feeding. Pectoral fins are long and narrow, inserted high on the body; in undisturbed fish they are held folded back along the flanks, but they are extended and beaten rapidly during surface skimming or jumping.
Sexual dimorphism is subtle. Females are typically deeper-bodied and somewhat rounder in the belly than males, especially when gravid, but both sexes share the same basic colour pattern and there is no striking colour difference. Distinguishing males from females at a glance requires some experience and is easiest in well-conditioned fish prior to spawning.
Habitat
Gasteropelecus sternicla occupies slow-moving and still surface waters across a broad range: the Peruvian Amazon, middle Amazon basin of Brazil, the Guianas (Guyana, Suriname, French Guiana), and Venezuela. FishBase also lists the Paraguay basin as part of its distribution. The type locality is Suriname. The species is essentially restricted to the top few centimetres of the water column, where it patrols along the surface film of creeks, backwaters, flooded forest margins, and swamps.
Preferred habitats are well-shaded, slow-flowing or still environments with surface vegetation, overhanging bank vegetation, or floating plant cover — conditions that provide both insect fall-in and protection from aerial predators such as kingfishers and herons. Water conditions in these habitats tend toward the soft and acidic end of the spectrum: temperatures around 73–82 °F are typical, with pH in the range 5.0–7.5 across the range. Darker, tannin-stained forest streams are characteristic haunts. The fish is gregarious and usually found in loose groups or schools at the surface rather than singly.
Its surface specialization makes it unusually vulnerable to overhead threats and to surface disturbance. In the wild, any sudden shadow or splash sends the fish skimming or jumping clear of the water on their wing-like pectorals — an escape behaviour that is spectacular and that aquarists must plan for when housing the species.
Feeding
Gasteropelecus sternicla is primarily an insectivore and surface-feeder. In its natural habitat the upturned mouth and surface position are adaptations for intercepting insects, small crustaceans, and worms at or just beneath the water surface — including terrestrial insects that fall or fly onto the water, which form a significant component of the natural diet. FishBase records the diet as worms, crustaceans, and insects. The fish relies more on surface-bound prey items than on hunting in the water column.
In captivity the species is not difficult to feed but does best on foods delivered at the surface. Floating flake food and small floating pellets are accepted, but live and frozen invertebrates — bloodworm, Daphnia, brine shrimp, small mosquito larvae — are taken with far more enthusiasm and do more for condition and colour. Freeze-dried foods float readily and are convenient. The species will not generally pursue food that sinks; competition from mid-water or bottom-feeding tankmates for food falling through the column is largely irrelevant, but it also means hatchetfishes can be underfed in a community where food disperses before it reaches the surface. Feeding should be observed to confirm they are getting enough.
Mating
Gasteropelecus sternicla is a seasonal breeder in the wild, with spawning apparently triggered by the onset of the rainy season — a pattern consistent with the broader reproductive ecology of small Amazonian surface-fish that use flooded forest margins and rising water levels as spawning cues. Animal Diversity Web records a rainy-season breeding season for the species. Males and females come into spawning condition as water levels rise and temperatures are moderated by rainfall.
Broadly, the species is believed to spawn in the manner of other small Gasteropelecidae: eggs are scattered among fine-leaved surface or near-surface vegetation, released by the female and fertilized by the male with no pair bond and no parental interest in the outcome. Reproductive output may be delivered all at once or in intervals across a spawning period, according to records for the genus. The upturned, surface-oriented mouth suggests spawning occurs in very shallow or surface-associated water, and vegetated, shallow, calm backwater margins of forest streams likely serve as spawning sites in nature.
Breeding
FishBase notes that Gasteropelecus sternicla has not been bred in captivity with reliable regularity, and detailed captive-breeding records are sparse compared with the more commonly bred Carnegiella strigata (marbled hatchetfish). Seriously Fish notes that breeding is likely similar to that of C. strigata: eggs scattered among fine-leaved floating plants, adhesive and very small, with no parental care from either parent. The adults should be removed after spawning or the eggs separated to a rearing vessel to prevent predation.
Animal Diversity Web records that eggs are laid by the female and fertilized externally by the male, and may be released all at once or in intervals. If spawning is achieved in captivity, conditioning should involve excellent live or frozen foods and a gradual simulation of the rainy-season trigger — soft, slightly acidic water with a modest temperature drop followed by a gradual warm-up has been used with related species to bring fish into condition. Any fry that hatch are very small and require tiny first foods such as infusoria or rotifers at the water surface, as they will not readily descend to feed. Their surface orientation is present from the earliest free-swimming stage.
In the aquarium
Common hatchetfish are rewarding aquarium residents for the experienced community keeper. They require a secure, tight-fitting lid without gaps — the species jumps readily and powerfully, and losses to uncovered tanks are common. The tank should have a long surface dimension rather than being especially deep, as the fish uses only the top few centimetres of the water column. Surface agitation should be gentle to moderate; heavy turbulence at the surface stresses the fish and interferes with feeding. Floating plants are appreciated and reduce surface disturbance while providing security.
Water should be soft to moderately hard and acidic to neutral (pH 5.0–7.5; 68–82 °F), reflecting the forest-stream habitats the species inhabits naturally. Dim to moderate lighting with plenty of plant cover suits them well. The species is peaceful and gregarious; a group of at least six is strongly preferred, as isolated individuals are shy and prone to stress. Suitable tankmates are small, peaceful species that occupy the mid- and lower water column — small tetras, cories, pencilfish, and dwarf cichlids are compatible; boisterous or fast-swimming surface fish that compete for space at the top are not. The species is sensitive to poor water quality and will not tolerate elevated ammonia or nitrite; stable, well-filtered water is essential.
Live and frozen foods — bloodworm, Daphnia, mosquito larvae — should form a regular part of the diet alongside quality floating flake or micro-pellets. With attentive husbandry the common hatchetfish is a long-lived species and one of the most fascinating surface-dwellers available in the hobby.
Conservation
Gasteropelecus sternicla has not received a formal species-specific IUCN Red List assessment; FishBase references an LC (Least Concern) status but no dedicated IUCN assessment page for this species was located at the time of writing. Its wide range across multiple countries and river systems — the Amazon, Orinoco, and Guiana drainages — suggests it is not under the same acute threat as narrowly endemic species, and the species remains available in the aquarium trade from wild-caught and occasionally tank-bred sources.
The principal long-term concern for any Amazonian species of limited monitoring is ongoing deforestation and watershed degradation, which alter the shaded, slow-flowing forest-creek habitats that hatchetfishes depend on. Loss of overhanging vegetation removes both food sources (terrestrial insect fall) and cover from aerial predators. The species' sensitivity to water quality and its surface-specialist ecology make it a reasonable indicator of forest-stream integrity. Aquarists sourcing the species should prefer tank-bred stock where available, though in practice most common hatchetfish in the trade are wild-collected.