Gasteropelecus sternicla

The Common Hatchetfish, also known as the River Hatchetfish, is a remarkable species originating from the slow-moving rivers and tributaries of South America. Its natural environment, often characterized by dense overhanging vegetation and floating plants, has shaped its unique morphology and behavior. Its highly compressed, deep-chested body and large, wing-like pectoral fins are specialized adaptations that allow it to propel itself out of the water to catch insects or escape predation. This behavior makes a secure, gap-free aquarium lid an absolute necessity.

In an aquarium setting, it's crucial to replicate their native habitat to ensure their well-being. They exclusively occupy the top level of the water column and prefer gentle water flow. A well-planted tank with plenty of floating plants will not only mimic their natural cover but also help to dim the lighting and make them feel more secure, which can discourage excessive jumping. As a schooling fish, they must be kept in a group. A solitary hatchetfish will be stressed, timid, and susceptible to illness. A proper school promotes confidence and allows for the observation of their interesting social dynamics as they glide just below the surface.

Their diet in the wild consists mainly of terrestrial insects that fall onto the water's surface. In captivity, they are omnivores that should be offered a varied diet with a focus on floating foods. High-quality flake food can be a staple, but it must be supplemented with protein-rich options like freeze-dried bloodworms, daphnia, and flightless fruit flies to ensure proper nutrition. They are active swimmers with a moderate metabolism, requiring daily feedings. Despite their activity, they produce a relatively low amount of waste for their size, placing a manageable load on the aquarium's filtration system. They are generally peaceful community fish but are best housed with other calm species that occupy the middle and bottom regions of the tank to avoid competition for space at the surface.

Gasteropelecus sternicla is a member of the family Gasteropelecidae, a group of characins distinguished by their profound morphological adaptations for surface-dwelling life. The most prominent feature is the highly compressiform body, with a significantly enlarged and convex sternal region that houses a powerful musculature system. This muscular arrangement powers the large, high-set pectoral fins, enabling the fish to achieve short, ballistic 'flights' or powerful leaps above the water's surface. This locomotory specialization is a primary mechanism for both predation on terrestrial insects and evasion of aquatic predators. Its body plan is an excellent example of convergent evolution with marine flying fish, although the mechanism of 'flight' differs.

Ecologically, this species fills the role of a surface-level insectivore in its native South American habitats, which are typically slow-moving, acidic, and soft water bodies like creeks and floodplain lakes. Its presence contributes to the control of insect populations at the water-air interface. The species' physiology is adapted to these conditions, which is reflected in its preferences in an aquarium environment. As a schooling organism, its social structure is integral to its survival, reducing individual predation risk and stress. While its IUCN status is Least Concern, indicating a widespread and stable population, its role as a specialized surface feeder makes it sensitive to environmental changes that affect insect availability or surface water quality. In a controlled environment, its bioload factor is considered moderate, a result of its low waste production relative to its medium metabolic rate and constant activity level. Its classification within the order Characiformes places it among a diverse group of freshwater fishes, but its unique adaptations set its family, Gasteropelecidae, clearly apart.

Breeding the Common Hatchetfish in a home aquarium is considered very difficult and is a rare accomplishment, even for experienced aquarists. A primary challenge is the lack of obvious sexual dimorphism, making it nearly impossible to visually distinguish males from females. The most reliable method for attempting to breed is to acquire a large, healthy, and well-conditioned school of at least eight to ten individuals to increase the probability of having a mix of both sexes.

To induce spawning, a separate, dedicated breeding aquarium is recommended. This tank should be set up to closely simulate the specific environmental triggers of their natural habitat. The water should be very soft, with a low pH and elevated temperature within their acceptable range. Lighting should be kept very dim, and the water flow should be minimal. Providing abundant floating plants, such as water sprite or hornwort, or a synthetic spawning mop is crucial, as these fish are egg-scatterers that will deposit their adhesive eggs among the plant matter near the surface. Conditioning the potential breeding group with a diet rich in live and freeze-dried foods for several weeks can help encourage spawning readiness.

If spawning is successful, the adults will almost certainly predate on their own eggs and must be removed from the breeding tank immediately. The eggs are small and sensitive to light and fungus, so maintaining pristine water quality and dim conditions is vital during incubation. Upon hatching, the fry are exceptionally tiny and will first consume their yolk sac. Once free-swimming, they require microscopic foods, such as infusoria or commercially prepared liquid fry food. As they grow over the first week, they can be gradually transitioned to larger foods like live vinegar eels and newly hatched brine shrimp. The entire process requires meticulous attention to detail and stable conditions.