The answer may surprise you.

Nope, it's not Megalodon. SIT DOWN, DISCOVERY CHANNEL, AND HUSH YOUR FACE. It's NOT Megalodon. But Mythbri, I hear you saying through the power of the Internet, We already learned about the largest living shark yesterday, and it's a filter feeder! Whatever kind of shark eats whales, it's got to be pretty big, right?

Wrong.

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It's this little guy. This is the Cookiecutter Shark, Isistius brasiliensis. It's a species of small dogfish, which are a group of sharks that belong to the order Squaliformes. Squaliformes has two dorsal fins, five pairs of gill slits, and usually some kind of sharp spine. They also lack the nictitating membranes that act as the transparent third eyelid for other species of sharks, but other than that there is significant variation between different species of dogfish. And the Cookiecutter Shark is pretty unique. They're called "Cookiecutters" because of their teeth:

Its rounded jaw and extremely sharp teeth allow it to feed on animals much bigger than itself - Cookiecutter Sharks are only 22 inches (56 cm) long at most, and females are larger than males. Its upper jaw contains 30-37 tooth rows, which are small, narrow and tapered. Its lower jaw contains 25-31 tooth rows, and the lower teeth are larger, broader and very, very sharp.

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Various peoples have known about the feeding habits of the Cookiecutter Shark for centuries, but in more modern times there were other explanations put forth about the strange, circular wounds observed on whales, seals, sharks, and other large marine life. These explanations included bacterial infections, lampreys or other kinds of parasites, but in 1971 it was confirmed that the cigar shark, as the Cookiecutter was then commonly known, was responsible. These types of wounds have been described in old folk tales, like the ancient Samoan legend of how the atu (skipjack tuna) entering and then leaving Palauli Bay, would leave behind chunks of their flesh as offerings for their chief. The term "cookiecutter shark" was popularized by a noted shark expert called Stewart Springer, who had also called them "demon whale-biters."

You can kind of see his point. Cookiecutter Sharks can be found worldwide in tropical and temperate waters between 20 degrees north and south latitudes. During the day it dwells in the open water at depths of around 2.3 miles (3.7 km), but at night it rises to around 279 feet (85 meters) below the surface. Usually it will stay around this depth, but they have been observed venturing to the surface as well. The Cookiecutter Shark will take bites out of pretty much any medium/large ocean-dwelling animal, and the tell-tale round scars have been recorded on an impressive variety of animals:

  • Cetaceans (including porpoises, dolphins and whales)
  • Pinnipeds (seals and sea lions)
  • Sharks
  • Rays
  • Dugongs
  • Bony fishes
  • Squid

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The wounds left by Cookiecutter Sharks are generally 2 inches (5 cm) across and 2.8 inches (7 cm) deep. Animals that are sickly or weakened in some way are more vulnerable to the Cookiecutter's attack, as studies have shown that healthy animals bear few scars, while emaciated or diseased animals have many. The Cookiecutter Shark will latch onto the hide of its prey and create some suction by closing its spiracles (small holes behind the eye that open to the mouth) and retracting its tongue. Its lips form a tight seal, which is when its jaws come into play, carving out a chunk of flesh as the shark twists and rotates its body to rip its meal free. Its possible that the struggles of its prey also aid its sharp teeth in cutting out a bite.

If that weren't intriguing enough, the Cookiecutter Shark is actually the most bioluminescent of any shark species, so bright and strong that it has been known to continue to glow for up to three hours after being removed from the water. Its specific luminescent strategy is called counter-illumination, which is when the photophores are positioned so that its silhouette is disrupted when viewed from below. This is a common form of luminescence in other species that inhabit the same oceanic zone.

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But wait, there's more! During the 1970s, there were multiple cases of U.S. Navy submarines having to make their way back to port for repairs to their navigation systems. Cookiecutter Sharks were taking bites out of the neoprene boots covering the sonar domes of the submarines, and before they figured out that the culprit behind all of of their navigation troubles was a tiny dogfish, they feared that there was some kind of new enemy weapon that was targeting their systems. They solved this problem by sheathing the domes in fiberglass, but only a decade later the U.S. Navy was again attacked by Cookiecutters. The sounding probes used to navigate safely while coming to the surface in shipping zones were damaged when the electric cables powering them were severed. Again, the U.S. Navy fought cookiecutter teeth with fiberglass.

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Cookiecutter Sharks are considered to be a species of Least Concern by the IUCN, due to their healthy population and lack of significant threats. In general humans don't have much to fear from them, although attack on humans by Cookiecutters have been documented. I'll spare you those pictures, but they are out there.