Giant Oarfish Swims With a Ball-and-Socket Fin That Could Inspire Ocean Robots

The giant oarfish, a ribbon-shaped fish of the deep ocean that can reach about 26 feet (8 meters) long, moves with a surprisingly intricate set of muscles and joints in its dorsal fin, according to a new study described by EarthSky. Scientists hope the findings could point the way to quieter underwater robots that explore the sea without disturbing the animals living there.

The researchers published their peer-reviewed work on Sept. 30, 2026. The team was led by Gabriel Vinícius Felix Afonso, a Ph.D. student at the Virginia Institute of Marine Science.

A fish that swims without bending its body

Many fish swim by flexing their bodies. The oarfish does something different. It sends waves along its long dorsal fin while its body stays relatively steady. Scientists already knew about this style of swimming, EarthSky reported, but they did not know how the animal coordinates such a long fin with so much precision.

Felix Afonso said the oarfish stands out among open-ocean fish. Tunas and billfish have spindle-like shapes, and sunfish and opahs are round. The oarfish, by contrast, has an unusual body shape and an unexpected way of swimming: it holds its body rigid while only the dorsal fin undulates. That combination, he said, drew the team toward studying how the fish is built, with future biomimetic robots in mind.

What the researchers did and found

To work out how the fin operates, the team studied its anatomy in detail. They examined specimens, used imaging techniques and analyzed videos of oarfish in the wild. They looked at how the muscles, joints and fin rays are arranged and how they work together to produce movement.

The study found that a group of six muscles controls each fin ray, attaching to it at 12 points. The joint at the base of each ray works much like the ball-and-socket joint in the human shoulder, which lets it rotate through a full 360 degrees. Together, according to EarthSky, these muscles and joints help explain how the fish produces waves along the whole length of its fin.

An animal that is rarely seen

The oarfish has a silvery body with no scales, covered instead by a layer that contains guanine. Its dorsal fin is vivid red and orange and runs along almost the entire length of its back. The first few fin rays are especially long and form a crest above the head.

The fish's name does not come from that fin. It comes from its unusually long pelvic fins, each made of a single ray that resembles a small oar. These pelvic fins are not mainly responsible for the fish's distinctive motion. The dorsal fin does that job.

The oarfish lives in oceans around the world, particularly in tropical and temperate waters, mostly in open water far from shore. It is typically found hundreds of feet below the surface, with records reaching about 3,280 feet (1,000 meters). Because it lives in a place that is hard to observe, much of its life remains out of view. Sightings near the surface are unusual and sometimes involve animals that are weakened or in distress, which makes natural behavior hard to study. Stranded or near-shore specimens let scientists examine anatomy, EarthSky noted, but they reveal little about how the animal lives.

For a sense of scale, EarthSky reported that it took 15 people to hold up a specimen measuring 23 feet (7 meters).

Why it matters for ocean exploration

The research is basic science rather than a finished technology, and no robot built on these findings has been reported. But the work links the biology of the fish to the design of underwater robots. A machine that moves by sending waves along a single long fin, rather than thrashing its whole body, could in principle move efficiently and stealthily, EarthSky said.

That is the appeal for researchers who want to study the ocean's hidden environments. Scientists hope that robots inspired by the oarfish could one day let people explore the deep while leaving its inhabitants undisturbed. For now, the study offers a detailed picture of how a rarely seen giant coordinates one of the longest fins in the sea.

Around Prague, based on reporting by EarthSky