Why these ancient, air-breathing fish are masters of swimming both forward and backward

Nature · Published 3 October 2026 Swipe more like this →

In short: The Notopteridae family, known as featherbacks or knifefishes, are masters of movement. By keeping their bodies rigid and rippling their long anal fins, these freshwater predators navigate quiet waters in Africa and Asia with ease. They even possess specialized swim bladders that allow them to breathe air and produce sound.

The Notopteridae family consists of 11 species of bony-tongued fishes that have inhabited freshwater and brackish environments across Africa and Asia for millions of years. Their evolutionary history is deep, with the earliest known fossil—an otolith—dating back to the Late Cretaceous period in India, roughly 70.6 to 66 million years ago. Despite their common name, these fish are entirely distinct from the electric knifefishes of the Americas, known as Gymnotiformes; the two groups share a superficial resemblance and swimming style but are not closely related.

Physically, these fish are defined by their elongated, knife-like bodies and a unique fin structure. Their caudal fin is small and fused with a long anal fin that extends along most of the body's length. This arrangement allows them to move forward or backward with precision. When a dorsal fin is present, it is characteristically small and narrow, which inspired the name 'featherback.' Their internal anatomy is equally specialized, featuring a swim bladder that extends throughout the body and into the fins. This organ is not only used for buoyancy but also serves as an auxiliary respiratory system to absorb oxygen from the air and a vocal device for sound production by forcing air into the pharynx.

Notopterids are generally nocturnal predators. While some species, such as Chitala ornata, are harvested as food or kept in aquariums, others like Xenomystus nigri are also popular in the pet trade. Their reproductive behavior is notable, as some species are known to construct nests and guard their eggs until they hatch. The largest members of the family, such as Notopterus chitala, can reach lengths of approximately 80 centimeters. Their name is derived from the Greek words for 'back' and 'fin,' a fitting descriptor for these ancient, air-breathing survivors.

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