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Let's talk about sea creatures. One of the most majestic and mysterious is the whale shark. It's the biggest shark there is - 30 feet or more in length, weighing in around 10 tons. Among the mysteries is where the whale shark migrates and also, where it gives birth. Now, scientists have completed the biggest-ever whale shark study and as NPR's Christopher Joyce reports, they think they've come up with some answers.

CHRISTOPHER JOYCE, BYLINE: Marine biologist Robert Hueter swims with sharks. He's been doing it for 40 years, but he has a special fondness for whale sharks.

ROBERT HUETER: This is the largest fish, as far as we know, that's ever existed. There's nothing bigger in the fossil record. But it's a very unusual kind of shark, in that it's not a top predator; it feeds on plankton.

JOYCE: Plankton are the flotsam of tiny plants and shrimplike animals that float in the ocean. The whale shark just opens its mouth - about twice the size of a manhole cover - and sucks them in. Hueter was studying some regular, meat-eating sharks in the Yucatan Peninsula in Mexico when he came across whale sharks feeding. It turned out to be the world's largest concentration of whale sharks - over 400 of them. Hueter was smitten.

HUETER: It's polka-dotted, and it lets people swim with it. And when you go down and see them at these feeding aggregation sites, it's a spectacle of nature. It's unlike anything else that you'll ever experience.

JOYCE: Hueter directs shark research at Mote Marine Laboratory in Sarasota, Fla. He organized a project to tag whale sharks. Many of these tags record where a shark goes, how deep it dives and the water temperature; and relay the data back to his lab via satellite. Hueter attached many of these tags himself, by hand.

HUETER: Your heart starts racing every single time because you're just next to something that's just so huge.

JOYCE: After tagging more than 800 whale sharks over nine years, the team discovered that after feeding, the sharks head off in seemingly random directions. Some travel thousands of miles, and they can dive a mile deep. One female in particular - they called her Rio Lady - swam to the middle of the Atlantic between Brazil and Africa, and just hung out.

So what was she doing there? they wondered. Hueter suspects that she was giving birth to pups - in the shark vernacular, out in the open ocean, perhaps a safer place for young pups.

HUETER: We think that Rio Lady has led us to the place where this particular species gives birth.

DEMIAN CHAPMAN: For most sharks, that's oftentimes the million-dollar question - is where they give birth.

JOYCE: Demian Campbell studies sharks at Stony Brook University in New York. [POST-BROADCAST CORRECTION: The professor's last name is Chapman.]

CHAPMAN: Nursery areas are places usually close to the coast, so giving birth in the middle of the ocean is sort of a fairly unusual thing for sharks to do.

JOYCE: Hueter describes the research in the journal PLOS One. He says the whale sharks' far-flung travels demonstrate that protecting this rare species will require international collaboration, not unlike the protection given migrating bluefin tuna. Already, the research has led Mexico's government to protect the sharks' big feeding grounds there.

Christopher Joyce, NPR News.

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