The Giants Lurking In The Deep Sea

Summary

Podcast Summary: Short Wave – "The Giants Lurking In The Deep Sea"

Release Date: August 4, 2025

Introduction

In the episode titled "The Giants Lurking In The Deep Sea," NPR's Short Wave delves into the mysterious phenomenon of deep sea gigantism. Hosted by Regina Barber and producer Hannah Chin, the episode explores why certain marine creatures in the bathypelagic zone—ranging from 1,000 to 4,000 meters below the ocean surface—exhibit unusually large sizes compared to their shallow-water counterparts. This zone, often referred to as the midnight zone due to the absence of sunlight, presents a challenging environment where gigantism is both rare and intriguing.

Deep Sea Gigantism Explained

The bathypelagic zone is a realm of perpetual darkness and extreme conditions, making it a harsh habitat for marine life. Contrary to the common trend where animals become smaller in challenging environments, the deep sea hosts some remarkably large species. Noelle Bolen, a marine biologist at the National Oceanic and Atmospheric Administration (NOAA), explains, “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.”

Theories Behind Deep Sea Gigantism

Several theories attempt to explain why gigantism occurs in the deep sea, each addressing different environmental pressures and biological factors.

1. Island Theory

   Regina Barber draws a parallel between island biogeography and deep sea ecosystems. Hannah Chin introduces an analogy involving bison on Catalina Island, where isolated environments lead to size variations in species. Noelle Bolen notes, “The body size of animals on islands is wonky... animals that are small on the mainland on islands get bigger, and things that are big on the mainland get smaller on islands” (06:36). Applying this to the deep sea, low food availability constrains growth, leading to an inverse relationship where smaller sea creatures evolve to become larger to maximize their foraging range.

2. Bergman's Rule

   Another theory discussed is Bergman's Rule, which posits that “size increases when temperature decreases” (08:55). Colder environments slow metabolism, allowing organisms to grow larger over time. However, Noelle Bolen points out a limitation: “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” (09:08). This suggests that temperature alone cannot fully account for the observed gigantism.

3. Oxygen Temperature Hypothesis

   Combining aspects of metabolism and oxygen availability, this hypothesis suggests that colder waters hold more oxygen, enabling larger body sizes. Noelle Bolen elaborates, “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” (09:40). However, like Bergman's Rule, this hypothesis does not entirely explain size variations beyond certain depths.

Current Research and Challenges

Understanding deep sea gigantism remains an ongoing scientific mystery. Hannah Chin emphasizes the rarity and difficulty in studying these colossal creatures: “They’re so rare, they’re so hard to find, and they’re so mysterious” (02:52). To address the gaps in knowledge, Craig McLean, a deep sea biologist at the University of Louisiana, Lafayette, and Noelle Bolen have initiated the Marine Organismal Body Size Database. This comprehensive online repository aims to compile body size data for over 85,000 marine species, aspiring to include information on all half a million known oceanic animals (10:59).

Impact of Climate Change

The episode also explores the potential ramifications of climate change on deep sea giants. Camilo Mora, a researcher from the University of Hawaii, warns, “The species that are living on the deepest part of the ocean, they are used to stability...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” (12:29). Increasing ocean temperatures, projected to rise by 1 degree Celsius by century's end, could disrupt the delicate balance of these ecosystems. Additionally, warmer waters may lead to reduced carbon availability, further stressing food-limited deep sea habitats (13:21).

Conclusion

Despite numerous theories, the exact causes of deep sea gigantism remain elusive. The combination of limited food resources, temperature fluctuations, and oxygen availability likely plays a role, but the complexity of deep ocean ecosystems means that multiple factors are at work. As climate change continues to alter underwater environments, understanding and preserving these gigantic marine creatures becomes increasingly urgent. Continued research and data collection, such as the efforts by McLean and Bolen, are essential to unraveling these deep-sea mysteries and protecting these fascinating giants.

Notable Quotes

• Noelle Bolen on deep sea miniaturization vs. gigantism: “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” (00:56).

• Regina Barber on the common pattern in body size changes: “Animals that are small on the mainland on islands get bigger, and things that are big on the mainland get smaller on islands” (06:36).

• Camilo Mora on climate change impact: “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” (12:29).

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This summary was crafted based on the provided transcript of the NPR Short Wave episode "The Giants Lurking In The Deep Sea."

Transcript

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[Regina Barber]

Before we start, I want to encourage you to sign up for our SeaChamp newsletter where we go deeper with all the cool science we learn each week. There's extra research, animals, and even fun games. Sign [e-mail] secamp.

[Hannah Chin]

You're listening to Shortwave from NPR.

[Regina Barber]

Hey, Shortwavers. 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 bathypelagic 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.

[Hannah Chin]

This is the first zone we get to from the surface that has no light.

[Regina Barber]

That's Noelle Bolen, Shortwave's go to marine biologist at the national oceanic and Atmospheric Administration. And she says sometimes this is where we hit the bottom.

[Craig McLean]

If we take all of the oceans.

[Hannah Chin]

Around the world, 4,000 meters is the average depth.

[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.

[Hannah Chin]

Yeah, Gina, I'm so excited. Just to start, these giants are f 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.

[Noelle Bolen]

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.

[Hannah Chin]

So this is Craig McLean. He's a professor at the University of Louisiana, Lafayette and a deep sea biologist.

[Noelle Bolen]

Examples of deep sea gigantism, of course, include, like, the giant isopod, giant squids, colossal squids. The other good example are what we call pycnagonids, 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.

[Regina Barber]

Oh, my gosh, that's so scary. Giant spiders. I do like the name pycnagonids, though.

[Hannah Chin]

Though Craig loves them. He told me for as long as he can remember, he's been fascinated by.

[Noelle Bolen]

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.

[Hannah Chin]

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.

[Noelle Bolen]

We really don't know why deep sea gigantism occurs. And, I mean, there's a bunch of, like, running ideas, but in actuality, we don't know.

[Regina Barber]

So today on the show, how scientists, 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?

[Hannah Chin]

What do biologists think? How do they even study those theories? And what would it take to know for sure?

[Regina Barber]

You're listening to shortwave, the science podcast from NPR.

[Hannah Chin]

Foreign.

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[Regina Barber]

Okay. Producer Hannah Chen. We're here to investigate the mystery of the deep sea giants. Like, where are we even gonna start?

[Hannah Chin]

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?

[Regina Barber]

Robots, Food. Batman. Movies.

[Hannah Chin]

Movies, yes. And the early days of Hollywood.

[Regina Barber]

Ooh.

[Hannah Chin]

In the heyday of western silent movies.

[Noelle Bolen]

Hollywood was filming a lot of westerns out in the channel Islands because it had this rough landscape. And so to have, like, a more authentic filming environment, they introduced bison out to the Channel Islands.

[Hannah Chin]

What? 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.

[Regina Barber]

Wait, so what happens to the bison?

[Hannah Chin]

So bison aren't native to Catalina island, right?

[Regina Barber]

No, they are not.

[Hannah Chin]

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?

[Regina Barber]

They get smaller?

[Hannah Chin]

Yeah. And, Gina, it isn't just them. Craig told me this type of thing is very common.

[Noelle Bolen]

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.

[Regina Barber]

Wow. I love this. I love that. It's true. But why would this pattern exist?

[Hannah Chin]

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.

[Noelle Bolen]

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.

[Regina Barber]

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.

[Hannah Chin]

Yes.

[Regina Barber]

Okay.

[Hannah Chin]

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.

[Noelle Bolen]

And so we hypothesize 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.

[Hannah Chin]

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.

[Regina Barber]

Right, because they're, like, reliant on the food that's pretty produced on the surface. Like, whether it's plankton or even giant whales, that stuff is dying and then falling to the ocean floor.

[Hannah Chin]

Yeah, exactly.

[Regina Barber]

Okay.

[Hannah Chin]

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.

[Regina Barber]

These are my favorite.

[Hannah Chin]

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.

[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.

[Hannah Chin]

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.

[Regina Barber]

Okay, so that would mean, like, animals are getting bigger when it's cold.

[Hannah Chin]

Yes, because colder temperatures slow metabolism and other life processes. They allow animals to live longer and thus become bigger.

[Noelle Bolen]

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.

[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.

[Hannah Chin]

Yeah, that's what he's saying.

[Regina Barber]

Okay.

[Hannah Chin]

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.

[Regina Barber]

Right.

[Hannah Chin]

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.

[Regina Barber]

Huh.

[Hannah Chin]

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.

[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?

[Hannah Chin]

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.

[Noelle Bolen]

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.

[Hannah Chin]

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.

[Regina Barber]

Wow, that's really cool.

[Hannah Chin]

You're right.

[Regina Barber]

Okay, Han. I do have one lingering question, though.

[Hannah Chin]

Yeah?

[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.

[Hannah Chin]

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.

[Camilo Mora]

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.

[Hannah Chin]

And their projections indicate that the bathypelagic 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.

[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 this species had live and adapted to survive on environments that are not used to change. 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.

[Hannah Chin]

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.

[Noelle Bolen]

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.

[Hannah Chin]

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.

[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.

[Hannah Chin]

That would be wonderful.

[Regina Barber]

Thanks for bringing us this reporting, Han.

[Hannah Chin]

No problem, Gina. Thank you for having me.

[Regina Barber]

This episode was produced by Hannah Chin and Rachel Carlson. It was edited by Burleigh McCoy and our showrunner, Rebecca Ramirez. Tyler Jones checked the facts. Jimmy Keeley was the audio engineer, Beth.

[Hannah Chin]

Donovan is our senior director and Colin Campbell is our senior vice president of podcasting strategy. I'm Hannah Chen.

[Regina Barber]

And I'm Regina Barber. Thank you for listening to Short Wave from.

[Craig McLean]

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