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Ocean Watch

Friday, December 14, 2001

Mention cookie cutters, and most people think of pressing tin shapes into sweet dough. For some of us though, a cookie cutter calls to mind a different image: sharp teeth and bloody wounds. That's because to marine biologists and other ocean lovers, a cookie cutter is a shark.

Cookie-cutter sharks get their name from the neat, Oreo-cookie-size holes they leave in the bodies of tunas, mahimahi, marlins, dolphins, seals and submarines.

But these sharks are puny little things, growing to only 18 inches long with small fins and flabby muscles. How then do these small, poor swimmers get close to such large, fast-swimming prey?

For years scientists knew that the undersides of cookie-cutter sharks glow in the dark. This was no surprise. Cookie cutters are deep-water fish, migrating up and down from the dark depths to the surface like squid and other deep-water fish, which also make their own light.

Researchers believe this self-made light disguises the creatures' shapes and thus protects them from getting eaten. Also, the light may attract potential prey.

But no one knew why fish, dolphins or seals would be attracted to the light made by cookie-cutter sharks. Then in 1998 a Florida biologist made an observation.

When viewed from beneath, the soft glow of the shark's belly blends in with the light from the sky, disguising the shark's outline almost perfectly.

But this researcher also noticed that a band of skin below the fish's jaw has no light-emitting cells and from beneath looks like a dark, oblong object. In fact, its shape resembles the kinds of fish tunas and other fast-swimming marine animals hunt.

And so, the theory goes, a big predator spots this fake fish, shoots up for the kill and instead gets a cookie-size bite in the side.

How do these sharks carve out such uniform balls of flesh?

Like some sharks, the cookie cutter can jut its jaws forward, only remarkably so. After extending its mouth, the shark clamps onto its prey with suction-cup lips and bites down with its sharp teeth. Once attached, the shark turns its body in a circle and cuts a "cookie."

Such bites usually don't kill the prey, but they nourish the shark, making cookie cutters parasites-in-passing.

It wasn't long ago that people didn't know what caused the round craters sometimes seen in big fish. Then in 1971 a Hawaii researcher who suspected the cigar shark (as it was then called) performed an experiment. He stuck a peach into the open mouth of a dead cigar shark and rotated it. Sure enough, the piece cut from the fruit matched the plugs of flesh found in the stomachs of dead cookie-cutter sharks.

Cookie-cutter sharks are found in tropical waters, and Hawaii is no exception. I have seen these sharks' signatures on the sides of monk seals, and a friend tells me she has seen them on tunas at fish auctions.

Round holes also have been found in underwater cables and at least once in the rubber of a submarine's sonar dome.

These little sharks apparently bite almost anything in sight, but don't worry about losing any tissue from your tush. Only once in Hawaii's recorded history have cookie-cutter bites been seen on a human, and in that case the victim was thought to have drowned first.

Cookie is an American word; the English use the term biscuits. I'm glad these plucky little sharks got the American name. Biscuit-cutter shark just doesn't have the ring.