Rob Shepherd, who has a grant from the Office of Naval Research, was figuring out how to build a big, quiet robot that could keep tabs on ocean health without frightening fish when the oarfish caught his eye. The fish swims by rippling a fin along its back, and he wanted to understand how. So he phoned a retired colleague who studies fish, Willy Bemis.
"He called me and said, 'I'm interested in oarfish,' and I said, 'me too,'" said Bemis, the study's senior author in a press release.
The call led to a study published in the journal Ichthyology and Herpetology that is the first to describe the anatomy that lets each of the fish's hundreds of bony fin rays turn on its own. Each sits on a ball-and-socket joint and can swing through a full circle, like a joystick. That lets the oarfish ripple its fin to move without bending its body, and it could give engineers a model for quieter underwater robots.
“If you swim in Cayuga Lake in the summer and a boat goes by, it’s really loud,” Shepherd said. “We think that a more biomimetic approach would make it better for playing along with fish.”
Explore All OptionsHow the Oarfish Swims With a Rippling Fin
No bony fish alive grows longer than the oarfish, which reaches at least 26 feet. A dead oarfish tends to come apart, so intact specimens are hard to come by.
Many long, thin fish swim like eels, bending the whole body in waves. Oarfish can do that, but they also run ripples down the fin on their back, which moves them forward or backward while the body stays fairly straight. The paper cites an earlier underwater film of one oarfish that swam on its fin alone for about nine minutes, then bent its body to speed away from a research vehicle.
Ripples can travel in both directions at once, with one stretch of fin moving toward the tail while another moves toward the head, and Bemis said the fish can change that pattern very quickly.
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Inside the Oarfish's Joystick-Like Fin Rays
The team, led by the Virginia Institute of Marine Science doctoral student Gabriel Afonso, studied eight preserved fish. They dissected them, cut thin tissue slices for the microscope, and took X-rays and CT scans. They also studied a 2.4-second clip of a wild oarfish filmed off Louisiana in 2011, slowed to a tenth of its speed to trace waves moving in both directions along the fin.
The top of each ray holds up the thin skin of the fin. The bottom sits in a ball-and-socket joint, the same kind found in a human shoulder. Three pairs of muscles attach at 12 points across the supporting bones, the joint, and the base of the ray, which the researchers interpret as allowing the ray to circle at its joint. The joint and its muscles stand less than an inch tall, and one fish the team examined had 401 of them.
How Oarfish are Inspiring Quiet Underwater Robots
Engineers have already built robotic fins with rotating rays, the authors note, so engineering reached the idea before anyone had described how the real fish does it.
One challenge has been building parts small enough to copy the oarfish, since a robot would need hundreds of them. The authors point to newer soft, muscle-like motors as a sign that may now be possible. The study describes the fish, but it does not test a robot or measure noise.
Bemis said ribbon-fin swimming has evolved independently about 10 times in different groups of bony fish. It is not known whether the others also spin their fin rays, and he said checking is the next step.
For now, the robot is still an idea that began with one colleague telling another he was interested in a fish and hearing the same back.
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Article Sources
Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:
- This article references information from a study published in the journal Ichthyology and Herpetology: Dorsal-Fin Musculoskeletal System of Oarfishes (Regalecidae): Fin Rays as Rotating Elements in an Undulatory System
