The black ghost knifefish, found in the Amazon Basin, moves in a unique way that sets it apart from other aquatic life; gliding forward and backward, and even treading in place, all without bending its body. Instead, this unique fish uses a long, ribbon-like fin located underneath its body to perform these maneuvers.
Now, researchers studying the mechanics behind that motion say it could help shape the next generation of underwater robots. Researchers at Northwestern Polytechnical University’s Ocean Institute in China studied 18 live specimens of Apteronotus albifrons, better known as the black ghost knifefish, and recorded almost 2,000 instantaneous movements with high-speed cameras.
Their results, published in the journal Ocean, provide a detailed look into how the fish‘s long anal fin generates thrust and why this method outperforms traditional propellers.
Wave Control, Not Body Movement
While most fish swim by bending their bodies, the knifefish takes a different approach. By generating waves along its anal fin, the knifefish can move without shifting its body. This reduces drag and lets the fish concentrate on sensing its environment through weak electric fields. Traditional swimming motions would interfere with this sensitive navigation system.
The knifefish’s fin is effective for this type of movement because it can control the direction of the waves it creates. The fish can send waves forward, backward, or even both directions at the same time. When two waves meet, they create a node that cancels each other out, giving the fish the ability to hover or turn in place without moving its body.
“What’s striking is that the fish can maintain a rigid body posture while swimming, which reduces drag and simplifies the engineering challenge of building robotic systems,” said co-author of the study Yi-Wei Fan.
Frequency As the Throttle
The team used a specialized process, known as spatiotemporal Fourier transform analysis, to determine if factors such as wave frequency, speed, wavelength, or amplitude contribute to controlling the fin’s movement. They discovered that wave frequency is the main factor that determines how fast the fish swims.
“Wave frequency emerged as the most reliable predictor of speed, while amplitude and wave number remained relatively stable across different swimming conditions,” said study co-author Dong-Yang Chen. “This suggests that the fish modulates its motion by fine-tuning frequency, much like a musician adjusting tempo.”
The researchers noticed that the wave’s amplitude along the fin isn’t uniform. It’s wider in the middle and narrower at the ends, forming an arch. Most existing robotic prototypes use designs that stay uniform in size, which could explain why they don’t show the same level of control as a black ghost knifefish.
“Most bio-inspired undulating fin designs to date have relied on idealized rectangular fins and constant-amplitude undulations,” said lead author Ze-Jun Liang. “Our findings show that real biological systems are far more sophisticated. By incorporating the morphological and kinematic synergies we’ve identified, we believe robotic propulsion efficiency and maneuverability can be significantly improved.”
Nature As an Engineer
This research contributes to the broader field of biomimicry, where engineers seek to improve designs by studying biological systems in nature that have been optimized over time through evolution. Examples of this practice in application range from the hooked structures of burs that inspired Velcro to the textured skin of humpback whales that improves wind turbine efficiency.
In aquatic robotics, animals like stingrays and squid have already inspired new designs. The black ghost knifefish stands out for using thrust, stability, hovering, and steering with just one fin. This could help solve a problem that engineers are still trying to overcome.
“Traditional propeller-based systems struggle with low-speed maneuverability and stability in complex environments,” said corresponding author Peng Xu. “By contrast, the knifefish achieves precise control using undulations of its anal fin—a flexible, elongated structure that generates traveling waves to produce thrust without body bending.”
A Blueprint for the Next Generation
The team now aims to use their data to develop control algorithms and test prototypes in real aquatic environments. Their goal is to create an underwater vehicle that matches the efficiency of the black ghost knifefish.
“Our ultimate goal is to create underwater vehicles that can operate with the same efficiency and agility as the knifefish,” Xu said. “This would open up new possibilities for inspection, exploration, and search-and-rescue missions in complex underwater environments where traditional propellers fall short.”
Austin Burgess is a writer and researcher with a background in sales, marketing, and data analytics. He holds an MBA, a Bachelor of Science in Business Administration, and a data analytics certification. His work focuses on breaking scientific developments, with an emphasis on emerging biology, cognitive neuroscience, and archaeological discoveries.