Pod Insights Short Wave The Giants Lurking In The Deep Sea About this episode The bathypelagic zone of the ocean is 1,000 to 4,000 meters below the surface. Sometimes it's called the midnight zone, because it's too deep for sunlight to reach. Most animals here are much smaller than their shallow-water counterparts. But occasionally, researchers find the rare deep sea giant: giant isopods, giant squids, colossal squids, sea spiders.
While these giants sound like the subjects of some people's nightmares, deep sea biologist Craig McClain https://craigmcclain.com/ dreams about them. And today on the show, he helps unravel the mystery and research behind these creatures.
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0:29 Speaker 02 You're listening to Short Wave from NPR.
0:35 Regina Barber? Hey, Short Wavers. Regina Barber here with another installment of Sea Camp, our special summer series all about the ocean and the depths within it. And this week, we're diving into the Ba'athi Pelagic Zone, which is 1,000 to 4,000 meters below the surface, also called the midnight zone because it's too deep for sunlight to reach.
0:56 Speaker 00 This is the first zone we get to from the surface that has no light.
0:59 Regina Barber? That's Noel Bolin, Short Wave's go-to marine biologist at the National Oceanic and Atmospheric Administration. And she says sometimes this is where we hit the bottom.
1:09 Speaker 00 If we take all of the oceans around the world, 4,000 meters is the average depth.
1:14 Regina Barber? Joining me for this journey to the midnight zone is producer Hannah Chin, who's going to tell us about the giants of the deep.
1:21 Speaker 02 Yeah, Gina.
1:22 Regina Barber? I'm so excited.
1:23 Speaker 02 And just to start, these giants are famous, not only because, well, the deep ocean is a very dark and very hostile place, but because most animals get smaller, not larger, in these conditions.
1:36 Speaker 03 Overwhelmingly, most deep-sea animals are much smaller than their shallow-water counterparts. In fact, deep-sea miniaturization is by far the more normal thing, and gigantism is a very rare thing that occurs in the deep ocean.
1:51 Speaker 02 So this is Craig McLean. He's a professor at the University of Louisiana-Lafayette and a deep sea biologist.
1:57 Speaker 03 Examples of deep sea gigantism, of course, include like the giant isopod, giant squids, colossal squids. The other good example are what we call pyknegonids, 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.
2:16 Regina Barber? Oh, my gosh. That's so scary. Giant spiders. I do like the name pyknegonids, though.
2:21 Speaker 02 Craig loves them. He told me, for as long as he can remember, he's been fascinated by giant things.
2:28 Speaker 03 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 Blue Sky accounts, you'll find this whole hashtag called Craig with big things. And it's just like pictures of me over and over and over again with giant versions of things, like the world's largest fire hydrant, et cetera, et cetera.
2:51 Speaker 02 Now, this is what Craig specializes in. The bulk of his work on deep sea organisms really focuses on these organisms' body size. So he studies everything from small animals to big animals to really big animals. He told me it's really fascinating because these creatures are so rare. They're so hard to find and they're so mysterious.
3:12 Speaker 03 We really don't know why deep sea gigantism occurs. And then there's a bunch of like running ideas, but in actuality, we don't know.
3:21 Regina Barber? So today on the show, how scientists are trying to unravel the mystery of deep sea size.
3:28 Regina Barber? What makes the giant sea spider so giant and the colossal squid so colossal?
3:35 Speaker 02 What do biologists think? How do they even study those theories? And what would it take to know for sure?
3:41 Regina Barber? You're listening to Short Wave, the science podcast from NPR.
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5:15 Regina Barber? OK, producer Hannah Chen, we're here to investigate the mystery of the deep-sea giants. Like, where are we even going to start?
5:22 Speaker 02 So there are multiple theories, and I promise we'll get to all of them. But the first one starts on land with one of your favorite things, Gina. Can you guess what it is?
5:31 Regina Barber? Uh, robots? Food? Batman? Movies?
5:36 Speaker 02 Movies, yes. Movies. And the early days of Hollywood.
5:39 Regina Barber? Ooh.
5:40 Speaker 02 In the heyday of Western silent movies.
5:43 Speaker 03 Hollywood was filming a lot of Westerns out in the Channel Islands because they had this rough landscape. And so to have like a more authentic filming environment, they introduced Bison out to the Channel Islands.
5:54 Regina Barber? What?
5:55 Speaker 02 So these movie crews bring in Bison. They film on Catalina Island, which is one of the Channel Islands. It's off the coast of Southern California, so real close to Hollywood.
6:03 Regina Barber? Yeah.
6:03 Speaker 02 And then when they're done filming... They just kind of leave the bison there.
6:08 Regina Barber? Wait, so what happens to the bison?
6:10 Speaker 02 So bison aren't native to Catalina Island, right?
6:13 Regina Barber? No, they are not.
6:14 Speaker 02 But they do OK. They don't die out. Instead, they form a herd. Over the years, they've become this tourist attraction. And they all get notably smaller.
6:24 Regina Barber? What? They get smaller?
6:25 Speaker 02 Yeah. And Gina, it isn't just them. Craig told me this type of thing is very common.
6:31 Speaker 03 The body size of animals on islands is wonky, for lack of a better term. Like, it's really, it's like, very noticeably off. And the tendency is, is that animals that are small on the mainland, on islands, get bigger. And things that are big on the mainland get smaller on islands.
6:47 Regina Barber? Wow. I love this. I love that it's true, but why would this pattern exist?
6:52 Speaker 02 Well, a lot of it has to do with lack of food.
6:55 Regina Barber? OK.
6:55 Speaker 02 Like, obviously, some islands are tropical, but a lot of them are not particularly green or lush environments. They're not built to sustain a lot of life. So if you're a small animal on the mainland, you might evolve to be bigger because it'll help you roam the entire island and really make the most of the territory that's available to you.
7:15 Speaker 03 And then, conversely, like, if you're a very big animal on the mainland and, you know, you colonize an island, then there's this pressure for you to become smaller because the island can't meet all your food needs.
7:25 Regina Barber? OK, this does make a lot of sense, but this is happening on land. This is the story you're telling me. And maybe you're going to tell me this is happening in the ocean, too?
7:34 Speaker 02 Yes.
7:35 Regina Barber? OK.
7:35 Speaker 02 So Craig published a whole paper on this. He plotted a bunch of data on body size and ocean depth. And he and his colleague started to notice this pattern that looked really similar to what was happening on the island. Basically, in the deep sea, smaller animals were getting bigger, and bigger ones were getting smaller.
7:55 Speaker 03 And so we hypothesized at this time that the reason why we see these patterns on islands and in the deep sea is because they're constrained by the same thing, which is lower food availability.
8:04 Speaker 02 And this would make sense, right? Because there's no plant life in the deep sea. There's very little prey availability. All of these animals are like scavengers. So they have to be big enough to travel far enough to scavenge for food.
8:16 Regina Barber? Right, because they're like reliant on the food that's produced on the surface. Like whether it's plankton or even like giant whales. Like that stuff is dying and then falling to the ocean floor.
8:27 Speaker 02 Yeah, exactly.
8:27 Regina Barber? Okay.
8:28 Speaker 02 And we should say there are hydrothermal vents and methane seeps, which produce a lot of nutrients and kind of foster their own thriving ecosystems.
8:36 Regina Barber? These are my favorite.
8:37 Speaker 02 I know, and they may be the focus of an upcoming Sea Camp episode. But Gina, those are like the equivalent of oases in the desert. They're relatively few and far between.
8:47 Regina Barber? Got it. So is this island theory like the best explanation of why some animals in the ocean are bigger and some are smaller? It feels like this might not account for all the animals we're talking about here.
8:58 Speaker 02 Yeah, that's a good point. And there are other theories. One is called Bergman's Rule. It's the idea that size increases when temperature decreases.
9:06 Regina Barber? Okay, so that would mean like animals are getting bigger when it's cold.
9:09 Speaker 02 Yes, because colder temperatures slow metabolism and other life processes. They allow animals to live longer and thus become bigger.
9:19 Speaker 03 Colder temperatures sort of slow growth rates, increase lifespans. We know that that is true. We have lots of data to show that that happens in the deep sea and it's temperature related.
9:32 Speaker 03 But the issue with that is that once you reach about like 600 meters or so in depth, the oceans are largely about four degrees Celsius. So temperature doesn't change after that. But in most groups, size continues to change after that.
9:51 Regina Barber? So there's like a lower limit to the temperature of the ocean. But we continue to see like the size variation even after like reaching that lower limit.
9:59 Speaker 02 Yeah, that's what he's saying.
10:00 Regina Barber? OK.
10:01 Speaker 02 And there's also another theory. It's called the oxygen temperature hypothesis. It combines the fact that low temperatures slow metabolism with the fact that colder water can hold more oxygen, right?
10:11 Regina Barber? Oh, OK.
10:12 Speaker 02 And it suggests that because there's more oxygen available, those animals have a larger maximum size than they would in warmer and less oxygen-rich waters.
10:22 Regina Barber? Huh.
10:22 Speaker 02 Again, that's still being studied. Scientists have different ideas about how it works, depending on who you talk to, and we don't understand it completely. So just another theory.
10:32 Regina Barber? So which of these theories is kind of like winning this race? Like, what's the solution to this like deep-sea gigantism mystery?
10:39 Speaker 02 Craig says, we don't know yet, right? It could be one of these theories. It could be a combination of them. Like, the giant isopod might be giant for a different evolutionary reason than the colossal squid or the sea spider.
10:51 Regina Barber? Right.
10:51 Speaker 02 And the reality is also that we may never know for sure, because our knowledge of these animals is still so limited.
10:58 Speaker 03 We can kind of come up with stories or sort of what we call post hoc explanations to explain things, but actually testing those may just not be possible.
11:10 Speaker 02 So that said, one key thing that scientists are trying to do is collect more data. So this summer, Craig and several of his colleagues launched the Marine Organismal Body Size Database. It's an online compilation of body size data for over 85,000 marine species, as tiny as a sea slug or as huge as a whale shark. And their goal is eventually to have information about size ranges for all the half million animals we know of in the ocean.
11:36 Regina Barber? Wow. That's really cool.
11:39 Speaker 02 You're right.
11:39 Regina Barber? OK, Han, I do have one lingering question, though.
11:42 Speaker 02 Yeah.
11:42 Regina Barber? Like, you mentioned that temperatures could have something to do with like why animals have the body sizes they do, but I'm wondering, like, what does that mean? In a changing climate because the deep sea ocean is probably like slowly going to get warmer.
11:55 Speaker 02 Yes, it is. So I talked to Camilo Mora about this. He's a researcher in the Department of Geography and Environment at the University of Hawaii. And he's also part of a team of scientists that published research almost 10 years ago, indicating that, yes, the deep sea would be impacted significantly by climate change.
12:13 Speaker 05 Unfortunately, the deepest part of the oceans are probably going to be affected, if not as much, probably even higher than what we have seen in the upper surface.
12:23 Speaker 02 And their projections indicate that the bathypalagic zone's temperature could increase by 1 degree Celsius or 2.5 degrees Fahrenheit before the end of the century. Even though that doesn't necessarily sound like a lot, Camilo said it could have really disastrous impacts.
12:39 Speaker 05 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 then on the upper part of the ocean. So these species have lived and adapted to survive in environments that are not used to change.
12:58 Speaker 05 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.
13:07 Speaker 02 And again, Gina, this is still an active area of debate. Some scientists have suggested that these deep sea animals will adjust. They'll become smaller again. They'll adapt in reverse to these shifting conditions. But others don't think that'll happen. Climate change is just happening too quickly for animals to keep up. And Craig says that as the oceans warm up, they're also going to produce less carbon.
13:32 Speaker 03 And so that means that the deep sea is actually going to get less food. And so for a system that is already very food limited, they may be at the edge of what they can actually survive at.
13:43 Speaker 02 Which is why both Camilo and Craig told me it's really important to study these deep sea animals now because they're already so rare. And if climate change continues at its current rate, they're going to become even rarer.
13:56 Regina Barber? Or if it's possible to slow climate change and buy these like giant sea creatures more time, maybe, you know, scientists will be able to study them. That would be great, too.
14:07 Speaker 02 That would be wonderful.
14:08 Regina Barber? Thanks for bringing us this reporting hand.
14:10 Speaker 02 No problem, Gina. Thank you for having me.
14:16 Regina Barber? This episode was produced by Hannah Chin and Rachel Carlson. It was edited by Brody McCoy and our showrunner Rebecca Ramirez. Tyler Jones checked the facts. Jimmy Keeley was the audio engineer.
14:26 Speaker 02 Beth Donovan is our senior director and Colin Campbell is our senior vice president of podcasting strategy. I'm Hannah Chin.
14:32 Regina Barber? And I'm Regina Barber. Thank you for listening to Short Wave from NPR.
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