Scientists Solved the Mystery of the Shark That Bites Perfect Circles

You rarely catch a cookiecutter shark in action. Instead, you notice the marks it leaves behind: neat, circular holes resembling the cut of a cookie cutter. But scientists have managed to use the few clues available to sketch out the shark’s behavioral patterns—a task that required some impressive detective work.

For the new study, published in Marine Ecology Progress Series, marine biologist detectives from the University of Hawaii at Manoa collected and studied data on cookiecutter shark bites and sighting records. The analysis revealed critical patterns in the location, timing, and frequency of cookiecutter shark bites on prized fish caught in Hawaiian waters. As the most extensive investigation on these sharks thus far, the findings present critical insights for both researchers and the fishing industry, the paper concluded.

A truly weird shark

Common cookiecutter sharks (Isistius brasiliensis) typically measure up to around 20 inches. Their most distinctive feature, of course, is their teeth: between 30 and 37 small, erect teeth in the upper jaw and 25 to 31 larger, jagged triangular teeth in the lower jaw. Using their chompers like a suction cup, the sharks bite off circular sections of larger fish, like whales, and even mammals like dolphins and seals.

Here’s where things get weirder. Unlike other sharks, the cookiecutter’s teeth are actually a single unit, so the shark sheds all its teeth at once. Then it eats the teeth (or tooth?) it just lost, which researchers believe helps it maintain calcium levels.

Generally speaking, cookiecutters—like most sharks—don’t attack humans, but their tendency to take out chunks of prized fish like tuna or swordfish has made them a nuisance to longline fisheries across Hawaii. But they’ve occasionally had run-ins with submarines and oceanographic devices.

When the cookiecutter bites

That said, these (arguably) minor inconveniences haven’t—and likely won’t—prompt population control measures. Although cookiecutter sharks are currently listed as Least Concern on the IUCN Red List, some experts project that increased fishing may lead to a population decline for the sharks in the future.

With that in mind, the new study aimed to better understand the sharks’ attack patterns so people can develop smarter strategies for avoiding, rather than controlling, their bites.

For the analysis, the researchers used reports of “cookiecutter damage” from fisheries between 2011 and 2023, building on their previous study connecting cookiecutter activity to moonless nights. They coupled the data with environmental metrics, such as sea temperature, sea surface height, and salinity, devising a statistical model for cookiecutter behavior.

According to the model, cookiecutter attacks are most likely to occur at night and closer to the new Moon, when lunar illumination is lowest. The increased frequency of cookiecutter damage may be the product of longer fishery operations, the paper noted, since the sharks are more active at night. The sharks preferred swordfish at night and bigeye tuna during the day.

A cookiecutter-shaped mark in history

The researchers also reviewed a rich volume of local Polynesian records to cross-reference fisher testimonials with the scientific data. As expected, they found multiple references to cookiecutter bites in Indigenous sources, suggesting that the sharks had long prowled the deep seas near Hawaii.

“These enigmatic creatures have resisted formal study due to their habitat, behavior, and apparent rarity,” said Donald R. Kobayashi, study co-author and an oceanographer at the Pacific Islands Fisheries Science Center, in a statement. “So it is quite gratifying to personally contribute some solid scientific knowledge towards understanding them and their ways!”