Ball-and-socket joints let each oarfish fin ray swivel in a full circle

Oarfish are hard to study because they come apart. Their bodies break into segments after death, so intact specimens are rare ^[8]. The two recognized species live in oceans worldwide at depths down to about 305 meters. The largest individuals are estimated at 8 meters, and the giant oarfish is the longest bony fish alive ^[9].

The new paper, published Sept. 30 in Ichthyology and Herpetology, is the first to describe the anatomy and muscles that let each fin ray rotate independently in a full circle, according to the Phys.org report ^[1]. Gabriel Afonso, now a doctoral student at the Virginia Institute of Marine Science, is first author, and Willy Bemis of Cornell is senior author ^[3]. With so little intact material, the team combined dissection, histology, CT scans made at Cornell, X-rays of a specimen at the Smithsonian Institution and analysis of film of the rays moving ^[4]. Imaging shows the skeleton without a scalpel. The film ties that structure to motion. The report does not say how many fish were dissected or which species they came from ^[4].

An oarfish carries hundreds of these bone-like rays along its back, joined by a membrane they drive in waves ^[2]. At its tip each ray meets the thin membrane. At its base it meets cartilage, a series of muscles and a ball-and-socket joint that allows the rotation ^[5]. Because the rays turn independently, one stretch of membrane can wave toward the tail while another waves toward the head, so the fish can move forward or backward ^[6]. "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." ^[7]

For an engineer, the appeal is thrust from the fin while the body stays straight. "Most fishes with long bodies swim like eels," Bemis said. "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." ^[10] According to the report, this lets oarfish stalk prey silently without hard swimming ^[11]. The fin also helps them hold the vertical posture they take while eating tiny crustaceans ^[17].

Rob Shepherd, a Cornell mechanical engineer and co-author ^[3], came to the fish from the robotics side. He received an Office of Naval Research grant for a large swimming machine that would carry instruments for an ocean-monitoring platform the Navy was also interested in ^[12]. The robot is meant to move forward and backward without scaring fish away ^[12]. He took the question to Bemis. "He called me and said, 'I'm interested in oarfish,' and I said, 'Me too,'" Bemis said ^[16].

Shepherd's case for quiet starts from boat noise. "If you swim in Cayuga Lake in the summer and a boat goes by, it's really loud," he said. "We think that a more biomimetic approach would make it better for playing along with fish." ^[13] "We think" is the honest verb there. I'd call the anatomy the firm result and the quiet robot a reasonable engineering bet. The thing this doesn't tell you is how loud rotating rays would be at machine scale, or how fish would react to them. Those answers need a prototype in the water with a hydrophone beside it.

Ribbon-fin swimming has evolved about 10 separate times in bony fishes, Bemis said, and no one yet knows whether the other lineages also evolved spinning rays ^[14]. "We're describing it here," Bemis said. "The next steps would be to go and see whether that's the case for other fishes." ^[15]