Oarfish Fins Inspire a Quieter Kind of Underwater Robot
When roboticist Rob Shepherd began working on a large underwater robot intended to swim silently so it would not startle sea creatures, he looked for design cues in an unexpected place: the oarfish. This meters-long fish, rarely encountered by humans, moves through the water with a ribbon-like dorsal fin that undulates in waves. That wave-driven propulsion system, Shepherd suspected, might translate into engineering principles for a machine that glides rather than thrashes.
To understand the fish better, Shepherd contacted Willy Bemis, a retired Cornell ichthyologist. Bemis, professor emeritus of ecology and evolutionary biology in the College of Agriculture and Life Sciences, recalled that Shepherd called him expressing interest in oarfish — and Bemis told him the interest was mutual. That brief exchange launched a collaboration to investigate the oarfish's unusual dorsal fin rays.
An Anatomy Built for Wave Propulsion
Oarfish are striking animals, with ribbon-shaped bodies and a global distribution. Only two species are recognized, and they inhabit oceans around the world at depths down to roughly 1,000 feet (305 meters). They are enormous: the largest individuals are estimated to reach about 8 meters, or nearly 26 feet, which makes the giant oarfish the longest bony fish alive. Even so, these giants feed on tiny crustaceans rather than large prey.
Their dorsal fin sets them apart. It contains hundreds of bone-like skeletal structures called fin rays that project from the fish's back. These rays do not simply stiffen the fin; they move in a coordinated sequence, creating waves along the membrane that links them. The traveling waves push the fish forward. The result is a smooth, continuous form of propulsion quite different from the oscillating tails or flapping appendages used by many other fishes.
- Two recognized oarfish species are distributed across the world's oceans.
- They can live at depths of up to about 305 meters (1,000 feet).
- Giant oarfish may reach around 8 meters in length, making them the longest bony fish.
- Their diet is made up of small crustaceans.
- Hundreds of dorsal fin rays work together to create propulsive waves.
A First Look at Joystick-Like Fin Rays
A new study published Sept. 30 in the journal Ichthyology and Herpetology provides the first description of the anatomy and the series of muscles that allow each of the oarfish's fin rays to rotate independently in a full circle. The movement has been likened to a joystick, and it gives the fish precise, multidirectional control over individual rays — a capability that helps explain how the fin can generate such complex waves.
Bemis is the senior author of the paper. Shepherd, the John F. Carr Professor of Mechanical Engineering in the Cornell Duffield College of Engineering, is a co-author. Gabriel Afonso, a former visiting researcher in Bemis's lab who is now a doctoral student at the Virginia Institute of Marine Science, is the first author.
Independent control at the level of a single ray
Each ray's ability to pivot through a full circle means the fin is not a passive sheet. Instead, it is an actively controlled array of hundreds of moving elements. By adjusting the orientation of individual rays, the fish can shape the wave that travels along the fin membrane, tuning the thrust it produces. For engineers, that kind of distributed, fine-grained control is appealing: it suggests a propulsion system that can be modulated without large, noisy moving parts.
Why Oarfish Are So Hard to Study
Much about oarfish remains mysterious, and the difficulty of studying them is a major reason. When the fish die, their bodies tend to break into segments, so intact specimens are rare. That makes anatomical work challenging, and it helps explain why basic questions about their fin rays went unanswered for so long. The new paper's detailed account of the fin ray muscles therefore fills a notable gap.
The animals' deep habitat adds another obstacle. Living hundreds of feet below the surface, they are seldom observed alive, and their size and fragility complicate efforts to collect or examine them. The collaboration between a roboticist and an ichthyologist offered a way forward: engineering questions about propulsion highlighted anatomical features that had not been fully described, while the fish's biology provided a natural model for the robot.
From Deep-Sea Fish to Engineering Design
The appeal of the oarfish for robotics lies in its quietness. A large underwater robot that moves with traveling waves along a flexible fin could operate with fewer abrupt, high-energy motions, reducing the disturbance it creates. That matters for missions in which the goal is to observe marine life without altering its behavior. A vehicle that slips through the water with a continuous, wave-like motion might come closer to blending into the environment than one driven by propellers or oscillating foils.
The oarfish also demonstrates that such a system can work at a large scale. The fish's body can reach several meters in length, and its fin still produces effective propulsion through the coordinated action of hundreds of rays. That suggests the underlying principle can be scaled up, which is relevant for a robot described as large.
What the Research Could Mean for Marine Robotics
The study does not present a finished robot; it establishes the anatomical basis that could inform one. By documenting how each fin ray is muscled and controlled, the researchers give engineers a template to draw from. A future vehicle might mimic the fin rays with individually actuated elements that rotate to generate a traveling wave. Such a design could offer both stealth and maneuverability, allowing a robot to hover, turn, or accelerate by reconfiguring the wave rather than by changing the speed of a propeller.
There are practical questions still to address. Building hundreds of independently rotating elements into a marine vehicle would demand robust, compact actuators and a control system capable of coordinating them. The ocean environment is corrosive and unpredictable, and a flexible fin membrane must endure repeated bending. Yet the biological system shows that these challenges are solvable in principle: the oarfish has been doing it for millions of years.
A Model Organism for Silent Propulsion
The research also underscores the value of interdisciplinary work. Shepherd brought an engineering problem — how to swim quietly at scale — to Bemis, who brought decades of ichthyological expertise. Their joint effort, with Afonso as first author, produced a description of fin ray anatomy that had been missing. It is a reminder that some of the most useful design ideas may be hiding in organisms that are rarely seen and hard to collect.
For now, the oarfish's fin remains both a biological puzzle and an engineering inspiration. As roboticists look for ways to explore the ocean with minimal disruption, the fish's ribbon-like fin and its hundreds of joystick-like rays offer a compelling model — one that could eventually help large underwater robots swim as silently as the animals they are built to observe.
This article is based on reporting by Phys.org. Read the original article.
Originally published on phys.org
