Short Wave - The Giants Lurking In The Deep Sea

Episode Date: August 4, 2025

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 dreams about them. And today on the show, he helps unravel the mystery and research behind these creatures. SIGN UP FOR OUR SEA CAMP NEWSLETTER! WE WORKED SO HARD ON IT!Interested in more ocean mysteries? Let us know at [e-mail] to every episode of Short Wave sponsor-free and support our work at NPR by signing up for Short Wave+ at plus.npr.org/shortwave.See pcm.adswizz.com for information about our collection and use of personal data for sponsorship and to manage your podcast sponsorship preferences.NPR Privacy Policy

Before we start, I want to encourage you to sign up for our Seacamp newsletter, where we go deeper with all the cool science we learn each week. There's extra research, animals, and even fun games. Sign up at npr.org slash Seacamp. You're listening to Shortwave from NPR. Hey, shortwaivers, Regina Barber here with another installment of Seacamp, our special summer series, all about the ocean, and the depths within it. And this week, we're diving into the bathy pelagic zone, which is 1,000 to, 2.000 to. 4,000 meters below the surface, also called the Midnight Zone, because it's too deep for sunlight to reach.

This is the first zone we get to from the surface that has no light. That's Noel Boland, shortwave's go-to marine biologists at the National Oceanic and Atmospheric Administration. And she says sometimes this is where we hit the bottom. If we take all of the oceans around the world, 4,000 meters is the average depth. 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? Yeah, Gina.

I'm so excited. 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. 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. So this is Craig McLean.

He's a professor at the University of Louisiana Lafayette and a deep sea biologist. Examples of deep sea gigantism of course include like the giant isopod, giant squids, colossal squids. The other good example are what we call picnagonids, 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. My gosh. That's so scary. Giant spiders. I do like the name Picnagonids, though. Craig loves them. He told me for as long as he can remember, he's been fascinated by giant things. 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. 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. We really don't know why deep sea gigantism are really. And I mean, there's a bunch of, like, running ideas, but in actuality, we don't know. So today on the show, how scientists are trying to unravel the mystery of deep sea size. What makes the giant sea spider so giant? And the colossal squid so colossal. What do biologists think? How do they even study those theories?

And what would it take to know for sure? You're listening to Shortwave, the science podcast from NPR. Okay. producer Hannah Chen, we're here to investigate the mystery of the deep sea giants. Like, where are we even going to start? 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? Robots, food, Batman? Movies? Movies, yes. And the early days of Hollywood in the heyday of Western silent movies. Hollywood was filming a lot of westerns out on 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 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

Yeah And then when they're done filming They just kind of leave the bison there Wait so what happens to the bison? So bison aren't native to Catalina Island No, they are not. But they do okay. They don't die out.

Instead, they form a herd. Over the years, they become this tourist attraction. And they all get notably smaller. What? They get smaller? Yeah. And, you know, it isn't just them. Craig told me, this type of thing is very common.

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 that animals that are small on the mainland, on islands get bigger, and things that are big on the mainland get smaller on islands. Wow. I love this. I love that it's true, but why would this pattern exist? Well, a lot of it has to do with lack of food. 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. 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. Okay, 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? Yes. Okay. 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. 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.

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. 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. Yeah, exactly. Okay. 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. These are my favorite. 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 dead. And desert. They're relatively few and far between. 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. 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. Okay. So that would mean like animals are getting bigger when it's cold. Yes, because colder temperatures, slow metabolism and other life processes,

They allow animals to live longer and thus become bigger. Colder temperatures, sort of slow growth rates, increased 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. But the issue with that is that 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. 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. Yeah, that's what he's saying. Okay. And there's also another theory. It's called the oxygen temperature hypothesis. It combines the fact that low temperature is low metabolism with the fact that colder water can hold more oxygen, right? Oh, okay. 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. 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. So which of these theories is kind of like winning this race? Like what's the solution to this like deep sea gigantism mystery? 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. And the reality is also that we may never know for sure, because our knowledge of these animals is still so limited. 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. 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. Wow. That's really cool. You're right. Okay, Han, I do have one lingering question, though. Yeah. 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. Yes, it is. So I talked to Camila 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. Well, fortunately, the deeper.

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. And their projections indicate that the bathapalagic 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. The species that are living in 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 and you see them on the upper part of the ocean. So these species have lived and adapted to survive with environments that are not used to change. So when you change the conditions for just a tiny bit, that would be enough for you to wipe out a lot of species down there. 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. 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. Which is why both Camillo 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. Or if it's possible to slow climate change and buy these giant sea creatures more time, maybe, you know, scientists will be able to study them. That would be great too. That would be wonderful. Thanks for bringing us this reporting hand. No problem, Gina. Thank you for having me. This episode was produced by Hannah Chin and Rachel Carlson.

It was edited by Burley McCoy and our showrunner Rebecca Ramirez. Tyler Jones checked the facts. Jimmy Keely was the audio engineer. Beth Donovan is our senior director, and Colin Campbell is our senior vice president of podcasting strategy. I'm Hannah Chen. And I'm Regina Barber. Thank you for listening to Shorewave from NPR.