Solving the Mystery of the Disappearing Turtles
ANDREA SEABROOK, host:
You've probably seen pictures or films of the magical hatching of green sea turtles. These tiny critters break free from their shells, dig away their way out of the sand, and make a mad dash on little tiny flippers to the ocean. And then they disappear from view.
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SEABROOK: Several years later, the sea turtles show up again in shallow waters. Until recently, much of the green turtle's life remained a mystery to humans. This week in Science Out of the Box, we'll find out how researchers are solving that mystery.
Karen Bjorndal is the director of the Archie Carr Center for Sea Turtle Research at the University of Florida, one of three researchers who's looked into the question of this creature's lost years.
Dr. Bjorndal, you've described the green sea turtle as the species with a cryptic life stage. What is that?
Dr. KAREN BJORNDAL (Director, Archie Carr Center for Sea Turtle Research, University of Florida): That means a part of their life cycle that is unknown to humans. So it's not cryptic to turtles, but it's cryptic to us.
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SEABROOK: No, no, of course not. Why was it important to you to find out where sea turtles go during this cryptic, hidden part of their lives?
Dr. BJORNDAL: I suppose for two reasons. One was that it has been one of the famous mysteries in sea turtle biology for many years, and I need to make it clear that this isn't just the odd turtle running down into the sea and disappearing. We have about a half a million hatchlings entering the sea every year from Costa Rica, for example, and they disappear. So it's a large number of organisms that we've been seeking all these decades.
SEABROOK: How do you lose millions of turtles?
Dr. BJORNDAL: Oh, that's a really good question. That's one that we've been asking ourselves these many years, and so it's been an intriguing puzzle. And, of course, the other reason that we're very interested in solving this puzzle is because green turtles are endangered, and our concern is that they may be facing threats in this unknown life stage that we can't help manage because we don't know what they are.
SEABROOK: Okay, so what happens? We have - we all have the picture of the little guys, you know, going, yik(ph), yik down the beach, pulling themselves along into the water, plop, and then they're gone. What did you find out?
Dr. BJORNDAL: Well, we already knew that they undergo what's called a swim frenzy. They swim continuously for 24 hours.
Dr. BJORNDAL: And this carries them through the onshore currents out into the offshore currents. And then they're slowing rate really decreases and they are moved basically at the whim of the currents and the wind, and then that is where we lost track of them.
And so what we did was to take the green turtles when they first reappear in shallow waters. And without harming the animals, we sampled what is called the scute, which is the fingernail-like covering on a turtle's bony shell. And what we found was that this covering serves as a historical record of where the turtle has been and what it's been feeding on if you know how to decipher the code using what are called stable isotopes. And…
SEABROOK: Okay, which are a certain kind of chemical.
Dr. BJORNDAL: Yes. So these are naturally occurring chemicals in the shell. And when you interpret them, you can tell whether the turtle has been feeding on plants or animals, you can tell whether they've been living in inshore habitats or offshore habitats, out in the open ocean. And so what we discovered was that the small green turtles that we caught inshore that were feeding on plants had a history in their scute recorded as feeding on jellyfish out in the middle of the open ocean.
SEABROOK: Wow. And what does it mean for these little guys that they go be carnivores, whereas, when they're back closer to shore, they mostly eat grasses and things, right?
Dr. BJORNDAL: That's correct. At this stage, they're very vulnerable to predation. And so any bird or a large fish or shark coming past them will eat them as long as they're small enough to fit in their mouths. And so the small turtles are under a great pressure to grow very rapidly, to outgrow many of their predators. So there's a real advantage to feeding on animal matter, which provides higher levels of nutrition and supports a higher growth rate.
SEABROOK: So Dr. Bjorndal, is that it? I mean, the mystery is solved.
Dr. BJORNDAL: Well, no, I wish it were but…
Dr. BJORNDAL: …the problem now is that I can't go to a map and point and say this is where the turtles are. We know now for sure what habitats they're in and what their diet is and we've been able to estimate from that they're probably in that habitat for three to four years. So this is all really important information. But we still need to find the specific locations where they are, and that still eludes us.
SEABROOK: Dr. Bjorndal heads the Archie Carr Center for Sea Turtle Research at the University of Florida. Thanks so much.
Dr. BJORNDAL: Thank you.
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