The Knifefish navigates the dark using a self-generated electric field for constant sensing.

Nature · Published 3 October 2026 Swipe more like this →

In short: Gymnotiformes are freshwater fish that master the murky depths of Central and South America. By rippling an elongated anal fin, they move with precision in any direction while emitting continuous electric signals. This unique biological radar allows them to navigate, communicate, and hunt in total darkness with ease.

The order Gymnotiformes, commonly known as knifefish, consists of about 134 species of freshwater fish found in Central and South America. These animals are defined by their slender, elongated bodies and the absence of pelvic and dorsal fins. Instead, they rely on a single, exceptionally long anal fin that stretches along the underside of their body. By rippling this fin in controlled, sinusoidal waves, they achieve a rare form of locomotion that allows them to move backward as effortlessly as they move forward, all while keeping their bodies rigid.

Beyond their movement, these fish are defined by their mastery of electricity. Most species are weakly electric, possessing specialized organs derived from muscle cells—or in the case of adult apteronotids, nerve cells—that generate continuous, low-voltage fields. This electric discharge serves as a sophisticated sensory system for active electrolocation, enabling the fish to map their environment and detect prey in dark or turbid waters. While the electric eel, a member of the family Gymnotidae, is famous for its ability to deliver powerful shocks for defense and hunting, the vast majority of knifefish use their signals solely for navigation and social communication.

The evolution of these sensory capabilities is a remarkable example of biological innovation. Research suggests that the ancestors of modern Gymnotiformes and Siluriformes independently evolved ampullary receptors approximately 150 million years ago. This development allowed for passive electroreception, re-establishing a trait that had been lost in the common ancestor of the Neopterygii subclass. Today, these fish remain a subject of fascination for their complex control over fin-ray curvature and their ability to manipulate electric fields, which they maintain day and night throughout their entire lives.

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