Media note: Video of the oarfish can be found and downloaded here .
ITHACA, N.Y. - Cornell University researchers have revealed how the oarfish generates the wave-like motions that power its distinctive style of swimming. A new study , published in the journal Ichthyology and Herpetology , is the first to describe the anatomy and series of muscles that allow each of the oarfish’s fin rays to rotate independently in a full circle, like a joystick.
Oarfish, giant fish that are rarely spotted by humans, have hundreds of these bone-like skeletal structures, which extend from the back of the oarfish. The fin rays move in coordinated motion, creating waves along the membrane that connects the fin rays, which, in turn propel the fish.
The fish – and their fin rays – are not well studied in part because when they die they break into segments, making intact specimens rare.
“Most fishes with long bodies swim like eels,” said Willy Bemis , a retired Cornell University ichthyologist. “They move in a series of undulations, pressing the water back and forth. And oarfish do that but also, remarkably, they use the dorsal fin to propel themselves without moving their bodies laterally.” The system lets them silently stalk prey without having to swim hard to propel their whole body.
The oarfish’s unusual locomotion caught roboticist Rob Shepherd’s eye when he was researching how to make a large yet quiet swimming robot, as part of a grant he received from the Office of Naval Research. The large machine he envisioned would house a range of instrumentation for an ocean monitoring platform – which the Navy also had an interest in – that could swim silently forward and backward to monitor ocean health without scaring off fish and other aquatic creatures. To learn more, he consulted Bemis and launched this collaborative research.
In the study, the authors relied on dissections, histology, x-rays of a specimen at the Smithsonian Institution, CT scans made at Cornell, and analyses of movies of the oarfish’s dorsal fin rays. The inquiries revealed that each fin ray is attached at the top to the thin fin membrane and then below to cartilage, a series of muscles, and a ball-and-socket joint enabling the rotary movement.
The unusual anatomy allows the webbed membrane to move in waves, to propel the oarfish forward and backward. A section of the membrane may wave in a posterior direction while another part simultaneously may be waving in an anterior direction, providing subtle control. “They’re continuously able to change the pattern of those dorsal fin rays and do it very quickly,” Bemis said. “The fin rays are incredibly mobile.”
For additional information, read this Cornell Chronicle story.
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Ichthyology & Herpetology