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Chapter 1: What are the giant creatures lurking in the deep ocean?
The most hostile place on the planet doesn't destroy life, but grows it into giants. Right now, in total darkness, six kilometres below the surface of the ocean, there's a jellyfish trailing tentacles the length of a London bus, and it's not alone. There are squids with eyes the size of basketballs, crabs the size of cars, a six-metre shark that catapults its entire jaw clean out of its face.
And that's not to mention one of the largest active predators on the planet that can grow up to 500 kilograms in just two years. This might sound like the stuff of nightmares, and in many ways, the bottom of the ocean is. Down past the midnight zone, amongst near freezing temperatures and crushing pressures, resources are scarce.
Less than 1% of the food from the surface ever makes it down to the ocean floor. There simply shouldn't be enough energy to build something big. And yet there is, and not just big, gigantic. So why do the deepest, darkest, most hostile parts of our ocean produce the largest creatures on Earth?
Chapter 2: Why do the deepest parts of the ocean produce the largest creatures?
This mystery has baffled scientists for more than a century, but in 2025 that all changed and we finally found the answers we were looking for. I'm James Stewart and you're watching Astrum Earth. Join me as we descend into the darkness to reveal monsters of the deep and the astonishing science of their abyssal gigantism, each one more enormous than the last.
We'll find out not just why they exist, but how secrets written into their DNA might even affect you. Deep Sea Giants may sound like the stuff of fiction, and in many cases, they are. The Hydra, the Kraken, the Leviathan. But as with many myths and legends, they're also rooted in truth. Now, finding these deep sea giants has never really been the problem.
The history books are packed with tales of these beasts, their bodies washed up dead and degraded on beaches. But without seeing them alive and well in their natural habitats, we never truly understood how they grew so large.
Chapter 3: How do giant isopods survive in extreme deep-sea conditions?
Until now. In 2025, we finally filled in those huge gaps in our scientific knowledge. ROVs probed deeper in some places than ever before. 4K cameras captured these creatures alive for the first time. And genomic sequencing of their DNA revealed something mind-blowing. As soon as you enter the ocean and go down past 200 meters, sunlight fades. Past 1,000 meters, it's gone entirely.
Welcome to the Midnight Zone, the home of a creature that looks like it's crawled in from a galaxy far, far away. Its species name is Vadery, because when the scientists who first described it in January 2025 saw its helmet-shaped head, they thought of one person, Darth Vader.
This Sith Lord is actually a supergiant isopod, or if you prefer the nightmare version, a woodlouse the size of a soccer ball.
Chapter 4: What unique adaptations do deep-sea creatures have for survival?
Pick one up, and people have when they've caught them in nets, and you're holding 33 centimetres and nearly a kilogram of armour-plated deep-sea tank. Flip it over and there are 14 legs, two compound eyes built from roughly 4,000 lenses each, pats like honeycomb, and a mouth made of four jagged plates that close inwards.
Giant isopods can live as deep as the Barthel Zone, the deep continental slope, somewhere around 2,000 metres down. Pitch black, near freezing, crushing pressure and almost nothing to eat. So how do they survive? When a dead whale carcass or dead tuna does drift down, these isopods gorge themselves so heavily that they can't move afterwards.
Scientists politely call it post-feeding immobility, although you and I might just call it a food coma. This can keep Barthenimus vaderi full for an incredibly long time. In fact, one individual in a Japanese aquarium went five years and 43 days without eating. Not because it was sick, it just wasn't hungry.
Chapter 5: How does the Antarctic sun starfish thrive in cold waters?
But here's the paradox. In a world with almost no food, you'd expect tiny, hyper-efficient scavengers, right? As a basic principle of evolutionary biology, when food is scarce, being small is an advantage. Smaller bodies need less energy to survive. But in the deep ocean, you get an armored giant that treats half a decade without a meal as a minor inconvenience.
Down here, being big isn't excess, it's strategy. The deep ocean is cold. Below 1,000 meters, temperatures hover between 2 and 4 degrees Celsius, just above freezing. Now, Barthinomus Vaderi is cold-blooded, and you would therefore be forgiven for assuming that it doesn't generate its own heat, but it does. Even cold-blooded animals lose energy to their environment.
Chapter 6: What is the significance of the Japanese spider crab's size?
Chemical reactions in cells produce heat as a byproduct. And in cold water, a larger body retains that energy more efficiently. They might not have warm blood, but they don't want to freeze. This idea that a larger body stores more energy and burns it more slowly is called Bergmann's rule, and it dates back to 1847, when German biologist Karl Bergmann noticed something simple.
Animals in colder places are bigger. It's about the ratio of surface area to volume. Let's break this down for a second. So a cube with size of one centimetre has six centimetres squared of surface area and one centimetre cubed of volume. Make it two centimetres on each side, the volume goes up eight times, but the surface area only goes up four times.
The bigger you get, the more inside you have relative to your outside.
Chapter 7: How does the goblin shark hunt in the dark depths of the ocean?
Bartholomew's giant body holds more energy than a small one, but crucially, it burns through that energy far more slowly. Less surface area relative to its mass means less heat escapes, which means its metabolism can drop to almost nothing and stay there. It's not waiting for food, it's barely noticing food is gone. This explains why this creature has grown so large on such little food.
But a big question still remains about how they evolved to be like this. And it turns out the answer lies even deeper into the ocean with an even bigger monster. So extreme that when scientists sequenced its DNA, they had to check the results twice. You'll know already about the Mariana Trench. In fact, we made a video on it.
In this place, seven kilometers under the ocean's surface, so dark and crushing we barely visited it, scientists found a supergiant.
Chapter 8: What biological breakthroughs can we learn from deep-sea giants?
Alicella gigante is the world's largest amphipod, at 34 centimetres long. Now that might not sound enormous until you realise its shallow water equivalent is the size of a grain of rice. This deep sea beast is actually 50 times the size of its cousins. It's the equivalent of a can of beer scaled up to the size of a T-Rex.
Alicela is built to live in what's called the aphotic zone, below a thousand meters where less than one percent of sunlight makes it through. Most deep sea amphipods here are red or orange, and there's a really good reason for that. Camouflage. Red light has a longer wavelength compared to other colours, and is usually absorbed within the first hundred metres or so of the ocean.
So essentially no red light makes it to the aphotic zone. Here, red animals appear completely black, the perfect disguise. But Alicella isn't red. It's almost a translucent white. Why? Well, down here it has virtually no predators. It's too big and its armored exterior too tough to make it a worthwhile meal for other inhabitants of the deep.
The fact that it's not camouflaged also suggests that visual hunting isn't a big thing at this depth. In other words, it doesn't need to hide. Now as interesting as all of that was, it doesn't tell us how this thing became a giant. This was something scientists only discovered in 2019, when they sequenced its genome. Every living thing runs on a genome, it's like a master instruction manual.
Humans, mice, dogs, virtually all creatures have roughly the same number of genes, somewhere around 20,000. When researchers measured the nuclear genome sizes of 13 deep sea amphipods, Ali Chellas dwarfed everything else in the study by a factor of nearly nine. At 34 gigabases, it is one of the largest crustacean genomes ever recorded.
And crucially, in this case, the animals with the biggest genomes had the biggest bodies. Size is written into the blueprint before the animal even begins to grow. Then in 2021, researcher Wenhao Li and his team at Shanghai Ocean University went deeper. They compared the transcriptomes of Ali Chella against a much smaller Heidel relative and found something striking.
If you needed a quick refresh on transcriptomes by the way, here's how I think of them. So the genome in my head is sort of a library of every book ever written, and the transcriptomes are the sort of list of books currently open on the reading desk. Essentially the transcriptome tells us which instructions the body is actually using at that exact moment.
Now, Alicella carries genes that regulate how cells multiply and how bodies accumulate mass, that show clear signs of positive selection. Genes that are not just present, but actively being pushed by evolution. The signal, it seems, never fully stops. Most animals reach adult size and the instruction to grow goes quiet.
In Alicella, something keeps the dial turned up, year after year, decade after decade, with no hard ceiling. Scientists call this indeterminate growth, and Alicella's genome appears built for exactly that. This is the molecular machinery that allows gigantism to happen. At the surface, a giant genome would be a liability. It's slow and inefficient to copy and uses up lots of energy.
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