Humans are naturally wary of sharks ...
... and we’re rightfully hesitant to embrace the average parasite ...
... but what happens when a shark is also a parasite?
Meet the cookiecutter shark, the only parasite of its species. It’s so named because of the slightly concave circular bites it takes from its victims’ bodies.
Not because it cuts cookies.
Illustration by Dana Song
The ancient Samoans came up with an explanation for the circular wounds they’d see in the skipjack tuna they fished out of the ocean.
Their legends say it was an offering of flesh the fish made to an island chief as the fish swam into Palauli Bay.
Western scientists identified the cookiecutter shark in 1824.
The tuna that are brought to the famous Honolulu, Hawaii, fish market every morning — the same tuna that eventually end up on your plate — often bear the cookiecutter's signature bite.
Photo by Sayzie Koldys
In fact, in Hawaii, most adult spinner dolphins show the marks of this parasite.
Photo Credit: Dr. Jan Witting
A human’s chances of being bitten by a cookiecutter are about as small as the shark.
It’s tiny, growing up to only 22 inches long, and humans rarely see them because they come to the surface only at night.
So how does this bite-sized shark manage to take big bites of the ocean’s fastest swimmers? DECEPTION.
The cookiecutter commutes (or migrates vertically, as scientists say) a couple of miles every morning and evening, traveling away from the sun.
Its Latin name is Isistius brasiliensis, after Isis, the Egyptian goddess of light, because of the way it manipulates light as part of an elaborate ruse to get dinner.
The shark's belly is covered with light-emitting photophores, except for a small patch of skin at the throat that remains dark (and rather fish-shaped).
It swims above its much larger and faster prey, stealthily matching its belly to the light level of the water around it.
To a dolphin below, it looks invisible! Or, better yet, like a small, harmless meal.
Before the dolphin can react, the cookiecutter uses its suction lips to attach to the dolphin’s flank. Then it gouges a chunk out of its flesh with its saw-like teeth and hurries off with its stolen dinner.
Although large mammals have died from multiple cookiecutter bites ...
... most cookiecutters are fans of ocean conservation. They seem to want their victims to survive to be snacked on another day.
They’re also recyclers. Like most sharks, they lose and re-grow their teeth many times to keep them sharp.
But instead of spitting them out, they swallow their old teeth to retain the calcium and therefore the energy it takes to become invisible.
If you covet an encounter with the little cookie cutter shark, take heart! Although they frequent warm ocean waters near islands, they’ve recently been seen off the coast of California.
With waters warming everywhere, the cookiecutter may be coming to an ocean near you!