The cookiecutter shark is barely the length of a human forearm, but it hunts animals many times its size by latching onto them with suction-cup lips, spinning its body, and twisting out a perfectly circular plug of flesh — a bite mark distinctive enough that scientists have found it on whales, dolphins, and even the rubber sonar domes of submarines.

A cookiecutter shark is a 16- to 20-inch-long shark, about the length of an adult’s forearm and it routinely takes bites out of animals up to ten times that size. Whales. Dolphins. Tuna. Seals. Even great white sharks. The forearm-sized fish is not the prey in these encounters. It’s the one doing the biting.

So how does something that small feed on animals it could never fight, chase down, or overpower?

It doesn’t overpower anything

The trick is that a cookiecutter never tries to win a struggle. It doesn’t kill what it bites. It takes a mouthful and leaves, and the host usually swims off wounded but alive.

That makes it closer to a parasite than a hunter, and that’s the key to its strange diet. A predator that has to subdue its meal is limited by what it can beat. A shark that only needs one clean bite is limited by almost nothing.

Yannis Papastamatiou, a marine biologist at the Florida Museum of Natural History, put the oddity plainly when his team documented a bite on a great white. He described “this unique situation of a small shark that will target animals much, much larger than itself, up to 10 times their own size.”

His co-author, California Academy of Sciences scientist John McCosker, added sharper irony: even white sharks, he said, “are preyed upon in this case, by a shark no longer than your forearm.” “Preyed upon” may oversell it a little, since the white shark usually survives. But the reversal is real. The apex predator gets a scar. The small fish gets dinner.

Suction, a spin, and a saw for a jaw

A cookiecutter has thick, fleshy lips that work like a suction cup. It presses them against a passing animal and forms a seal, the way a plunger grips a wall. Then it sets its hooked upper teeth in as an anchor.

What happens next is the part that gave the shark its name. Its lower teeth are fused into a single saw-edged blade. With the seal holding and the upper teeth locked, the shark rotates its whole body against the anchored jaw and carves out a round crater. The biggest craters on record were about 5 centimeters wide and 7 centimeters deep, a near-perfect cylinder of flesh, as if someone had gone at the animal with an apple corer.

Those lower teeth are unusual in another way. Back in 1963, biologist Donald Strasburg noticed the cookiecutter sheds them not one at a time, as most sharks do, but as a whole connected row. The blade stays a blade.

The bite is the calling card

For a long time nobody knew what was making these wounds. They kept showing up on big open-ocean fish and on whales, smooth-edged and circular, matching nothing anyone recognized.

Papastamatiou described the early confusion well: when biologists first noticed them, “they thought it might be a viral infection because they didn’t know any animal that could bite and leave such a smooth wound.” The puzzle wasn’t solved until 1971, when Everet Jones found neat conical plugs of flesh sitting in cookiecutter stomachs and linked the fish to the craters.

Since then, that scar has become the main way we study an animal we almost never see alive. Because the shark lives deep and stays hidden, researchers mostly read it backward, off the bodies of the things it has bitten.

When Papastamatiou’s group looked at fish landed at the Honolulu Fish Auction, they found that swordfish carried the most scars: 87.9 percent had healed cookiecutter wounds. A 2025 study off Hawaii went further, cataloging 9,281 bite marks across 396 of 399 short-finned pilot whales, meaning 99.2 percent of the population marked by a fish most of them probably never got a good look at.

The submarine problem

The bites don’t stop at flesh. In the 1970s, US Navy nuclear submarines started coming back with damage to the soft rubber domes that housed their sonar. The round gouges were baffling. The culprit turned out to be the same small shark, mistaking the soft rubber for something edible. The Navy reported cookiecutter incidents on around 30 submarines and eventually shielded the domes with fiberglass to stop the leaks.

The shark’s indifference to what it’s biting extends further than that. As Papastamatiou noted, “they’ve taken chunks of plastic out of submarines and underwater oceanographic equipment, so it’s pretty amazing what they can do.” Rubber, whale, or research instrument, the mechanism is the same.

Why a slow deep-water fish ended up built this way

Step back and the design starts to make sense. A small shark drifting in deep water has no realistic path to becoming a chase predator. It can’t outswim a tuna or wrestle a seal. What it can do is get close, latch on for a second, and leave with a piece. The suction lips, the anchoring hooks, the rotating saw of a jaw are all built for that single quick transaction. Evolution didn’t build a hunter here so much as a very efficient taker of samples.

The odd consequence is how little we know about the animal itself. Papastamatiou called it “a very mysterious little fish”, mostly because “it’s so difficult to study — you hardly ever see them alive.” Researchers, he said, have largely had to “study cookiecutter sharks based on either dead specimens or by their bite wounds on prey.”

We keep coming back to that inversion. For most animals, the creature is the thing we know and the traces it leaves are the footnote. With the cookiecutter it runs the other way. We have counted its bites on thousands of whales and swordfish, mapped them by season, found them on submarine hulls and ocean instruments, and built almost everything we understand about the fish out of the holes it leaves behind.

The shark stays in the dark. Its signature is everywhere.