Taxonomy & naming
Gasteropelecus levis was described by Carl H. Eigenmann in 1909 on the basis of material from the Amazon. Eschmeyer's Catalog of Fishes records the valid name as Gasteropelecus levis (Eigenmann, 1909), with the parenthetical authority indicating the species was originally placed in a different genus before being assigned to Gasteropelecus.
The genus Gasteropelecus belongs to the family Gasteropelecidae, the freshwater hatchetfishes, a Neotropical family of surface-specialist characoids whose members are defined by their dramatically keeled ventral profile. Three species are typically recognised in Gasteropelecus: G. sternicla, the common hatchetfish; G. maculatus, the spotted hatchetfish; and G. levis, which is smaller than G. sternicla but larger than the marbled hatchet Carnegiella strigata, a common point of confusion in the trade. Within the hobby G. levis is most frequently sold under the name silver hatchetfish, a name it shares with G. sternicla, so size and the absence of heavy spotting or mottling are useful fieldmarks.
Morphology
The body plan of G. levis is immediately recognisable: a sharply keeled ventral surface formed by the enormously deepened sternum houses the powerful pectoral musculature, while the dorsal profile is nearly straight. The result is a fish that looks almost triangular in lateral view. Maximum standard length in the wild is around 1.5 in, though aquarium specimens may reach 2.5 in total length under good conditions.
Coloration is predominantly metallic silver, with a gold-bronze and black longitudinal stripe running along the midline from behind the gill cover toward the caudal peduncle. The fins are largely hyaline. This striping pattern, combined with the small size and the absence of the coarser spotting or marbling seen in allied species, helps distinguish G. levis from other hatchetfishes available in the trade.
Habitat
Gasteropelecus levis is endemic to the lower Amazon River basin in Brazil, where it inhabits the surface film of slow-moving to standing freshwater — forest streams, flooded margins, and blackwater or clearwater environments with surface cover such as overhanging or floating vegetation. Like all hatchetfishes it is a pelagic surface dweller, spending most of its time in the uppermost centimetres of the water column where it watches for insects and small prey falling from or sitting on the water's surface.
Reported water parameters from the hobby literature suggest a preference for soft, slightly acidic to neutral conditions: pH roughly 5.7–7.0 and temperatures in the range 73–82 °F (73–83 °F). The fish is sensitive to pH fluctuation and should be maintained only in well-established, chemically stable aquaria.
Feeding
In nature G. levis is an insectivore and micro-predator at the water surface, taking fallen insects, surface-skating invertebrates, small crustaceans, and worms. The upturned mouth and forward-set eyes are adaptations for this ambush-from-below style of surface feeding.
In the aquarium it accepts a range of small, surface-presented foods: frozen bloodworm, tubifex, and brine shrimp, as well as high-protein flake that floats long enough to be intercepted. Soft, slow-sinking pellets can work if the fish is trained onto them from above. Supplementing with live or freeze-dried fruit flies (Drosophila) and pinhead crickets encourages natural behaviour and brings the fish into good condition. Because hatchetfishes are shy and can be outcompeted at feeding time, meals should be targeted to the surface away from more assertive tankmates.
Mating
Gasteropelecus levis is an egg-scattering open spawner with no parental care, conforming to the general pattern documented for hatchetfishes. Spawning takes place just below the surface, with pairs or small groups releasing adhesive eggs among fine-leaved floating or submerged vegetation; the eggs are not guarded and the adults must be removed promptly if eggs are to be raised.
Reports from successful breeders note that the species spawns near the surface in soft, warm, slightly acidic water, and that a dimming of lights or a small cool water change can help trigger spawning activity. As with most characoids, the sexes are difficult to distinguish outside of spawning condition, though ripe females develop a noticeably rounder ventral profile.
Breeding
Breeding G. levis in captivity follows the standard hatchetfish approach: a bare or lightly planted spawning tank with very soft, acidic water (pH 5.5–6.5), warm temperatures toward the upper end of the species' range (79–82 °F / 79–83 °F), dim lighting, and clumps of fine-leaved floating plants such as water sprite or a synthetic spawning mop positioned near the surface. Conditioned adults — fed live or frozen foods for several weeks — are introduced in a ratio of one or two males per female.
The adhesive eggs are scattered among surface vegetation and hatch in roughly 24–36 hours at breeding temperatures. The tiny, transparent fry are initially very delicate and require infusoria or a commercial liquid fry food for the first few days before transitioning to newly hatched brine shrimp nauplii. Water quality must be impeccable; fry are sensitive to ammonia and nitrite. Adults show no parental care and must be removed after spawning.
In the aquarium
Gasteropelecus levis is a peaceful, shy schooling fish that should be kept in groups of six or more to reduce stress and encourage natural behaviour. Its small size makes it suitable for aquaria from around 23.5–31.5 in in length, though the tank must have a tightly fitting cover: hatchetfishes are accomplished jumpers capable of clearing gaps that would stop most other fish, and a single scare can send an entire school airborne.
The aquarium should be furnished with floating plants or overhanging leaf litter to give the fish shelter and confidence near the surface. Décor below the surface is largely irrelevant to hatchetfishes but can benefit tankmates. Water should be soft (dGH 3–17), pH 5.7–7.0, and temperature 73–82 °F (73–83 °F); the species is sensitive to ammonia spikes and bacterial infection during unstable water conditions, so it should not be added to immature tanks. It mixes well with calm mid- and lower-water species — small tetras, pencilfish, corydoras, and similar soft-water fish — that do not compete directly for surface space or food. Expected lifespan under good care exceeds three years.
Conservation
Gasteropelecus levis is assessed as Least Concern (LC) on the IUCN Red List, based on a 2018 assessment, reflecting a distribution across the lower Amazon basin that is considered sufficiently broad and stable to preclude listing under any threatened category at that time. Populations appear to be maintained across multiple river and floodplain habitats.
Nonetheless, the broader Amazon basin faces ongoing pressure from deforestation, agricultural runoff, and water-quality degradation. As a small surface dweller dependent on intact riparian vegetation for cover and for the terrestrial insect prey that falls to the water's surface, G. levis may be locally sensitive to riparian clearing even if it is not globally threatened. Wild-caught individuals occasionally reach the trade from Brazil, but most commercial stock originates from aquaculture in Southeast Asia and Eastern Europe, which reduces collection pressure on wild populations.