ADRIAN MA, HOST:
We turn now to the ocean with NPR's Short Wave podcast. This story plays out in the midnight zone, which is 1,000 to 4,000 meters below the surface...
(SOUNDBITE OF SUBMARINE MOVING)
MA: ...Even beyond the reach of sunlight. Noelle Bowlin is a marine biologist who works at the National Oceanic and Atmospheric Administration, and she says the midnight zone is where we hit the bottom in most of the ocean.
NOELLE BOWLIN: If we take all of the oceans around the world, 4,000 meters is the average depth.
MA: Down here, some sea creatures are spectacularly large, and Short Wave host Regina Barber explores the many possibilities of why.
REGINA BARBER, BYLINE: The midnight zone is a pretty extreme place - dark, cold, under enormous pressure. Most creatures down here are smaller than their shallow water counterparts, but there are some enormous exceptions.
CRAIG MCCLAIN: Examples of deep sea gigantism of course include, like, the giant isopod, giant squids, colossal squids.
BARBER: Craig McClain is a professor at the University of Louisiana, Lafayette, and a deep-sea biologist. And for as long as he can remember, he's been enthralled by giant things.
MCCLAIN: My entire life, even as a kid, like, been really, really fascinated by, like, roadside attractions - the world's largest ball of yarn, the world's largest rocking chair. And in fact, if you go to, like, my Instagram or my, like, Bluesky accounts, you'll find this whole hashtag called CraigWithBigThings.
BARBER: And now that's what he specializes in - the big things of the deep sea, like pycnogonids.
MCCLAIN: Which are sea spiders - typically in shallow water, they're like the size of a quarter. But in the deep ocean, they can get up to the size of a dinner plate.
BARBER: So why do some species in the deep sea get bigger while some species gets smaller? One leading theory has to do with the dearth of food in the deep sea. Because there's no plant life in the deep sea and very little prey availability, animals in the deep sea are basically scavengers, big enough to travel far enough to find scarce food. But McClain says that there are other possible explanations that may contribute to these massive swings in body size, like temperature.
MCCLAIN: Colder temperatures sort of slow growth rates, increase lifespans.
BARBER: This is known as Bergmann's rule - the idea that size increases when temperature decreases. So when it's cold, animals are inclined to get bigger, at least up to a certain point.
MCCLAIN: Once you reach about, like, 600 meters or so in depth, the oceans are largely about 4 degrees Celsius. So temperature doesn't change after that, but in most groups, size continues to change after that.
BARBER: There is a third theory they may help explain deep sea gigantism - the oxygen temperature hypothesis. It combines the fact that low temperatures slow metabolism with the fact that colder water can hold more oxygen and suggests that because there's more oxygen available in the deep sea, those animals have a larger maximum size than they would in warmer, less oxygen-rich waters. So which is it? Or is it a combination of these hypotheses? Well, there may be limited time to find out. Some researchers like Camilo Mora at the University of Hawaii think that these creatures may not be able to survive the ocean warming from climate change.
CAMILO MORA: The species that are living on the deepest part of the ocean - they are used to stability. The temperature changes are very small. They might not even have seasons as strong as you see them on the upper part of the ocean. So when you change the conditions for just a tiny bit, that could be enough for you to wipe out a lot of species down there.
BARBER: But this is still an active area of debate. Some scientists have suggested that these deep sea animals may become smaller again, adjust.
Regina Barber, NPR News.
MA: This story is part of Short Wave's Sea Camp series. To learn more, follow the podcast and subscribe to their newsletter at npr.org/seacamp. Transcript provided by NPR, Copyright NPR.
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