Original subtitles

Deep in the ocean's dark abyss,

20,000 feet below the surface in the freezing depths,

lurks a strange, unexplored world.

Where alien-like creatures...

...grow to terrifying sizes

and are the ruthless predators...

of the abyss.

Many look like something from our darkest imaginations.

While some create the myths and legends of the deep.

Weird creatures with alien ways to hunt...

CRUNCH ..and kill.

It's said we know more about Mars than we do our deep seas

and the strange life forms within it.

This series will uncover the great unknown,

come face to face with some of the sea's most elusive creatures,

and use cutting-edge technology...

...to reveal behaviour that has never been captured.

These are the unknown and unseen monsters of the deep.

In this program me, we meet the killers.

From strange and mysterious predators

with remarkable abilities to attract...

...and hunt prey...

They are able to produce red light like a searchlight.

Looking for prey items. It's so cool.

...to super sized marine reptiles...

...and the original big biters.

MECHANICAL CLANGING, CRUNCHING

That did it. CH UCKLES

But why does the deep sea create these killer monsters?

CRUNCH

Out there is a cast of our planet's most terrifying killers.

But many of them lurk so deep below the surface

that finding them is notoriously difficult.

Exploring the endless expanse of the deep, dark ocean

is incredibly challenging.

And the conditions are so extreme

that the creatures which survive down there

are unlike anything we see at the surface.

For some of these deep sea dwellers,

there's one fish that coming face to face with

would be their worst nightmare.

This is the anglerfish.

With an enormous mouth...

...and razor sharp teeth,

it's one of THE most iconic deep sea killers.

The anglerfish lives at about two kilometres below the surface

in the eternal pitch darkness.

The chances of just happening upon prey are very slim.

So how does the anglerfish manage to find a meal?

The answer is by harnessing the remarkable ability

to create organic light.

It's called bioluminescence,

and it's produced by a chemical reaction

inside a creature's body.

On land, it's rare,

but in the oceans it's used by an astonishing three quarters

of all deep sea creatures.

Forget TikTok and Instagram,

this dazzling light show is by far

the world's most common form of communication.

But the crafty anglerfish has found a way

to take advantage of bioluminescence...

THUNDER RUMBLES

...and weaponize it.

They're not exactly built for speed...

...but what they lack in hydrodynamics,

they make up for with patience and ingenuity.

Sticking out of their forehead is a narrow dorsal spine

with a glowing tip.

Which they can wave around like a fishing rod

to attract prey in the dark.

But as they live at such depth, they've never been seen hunting.

This is because most of life in our oceans...

...exists at a depth of less than 200 metres

in what's known as the sunlight zone.

Here, the sun's rays create a warm and bright environment.

Below that is the twilight zone,

where little to no light filters down from above.

Deeper still, between 1,000 and 4,000 metres

is the freezing cold, pitch black dark abyss of the midnight zone.

So what exactly might be lurking

in this desolate abyss for anglerfish to prey on?

The unique tank room at London's Natural History Museum

may be able to shed some light.

Curator of fishes isjames MacLaine.

This is a spectacular specimen of a very rare anglerfish.

Ancl you can see several of the typical anglerfish features.

So we have the little tiny eyes.

They don't have very big eyes at all.

Ancl this is the lure.

This is where anglerfish keep colonies

of bioluminescent bacteria which attracts the prey

towards this incredibly fearsome mouth

full of vicious, backwards-curved teeth.

It also has little batteries of teeth on the roof of its mouth

and right in the back of its throat as well.

So anything it grabs has very, very little chance of escape.

Ancl this specimen is particularly interesting

because it has grabbed something quite significant.

It has this very big, stretchy, elasticated stomach,

and this has swallowed something quite considerable.

I can feel there's quite a large, lumpy object in here.

Ancl I'm very curious to find out what this is.

Not wanting to slice this unique specimen open,

James is taking it to the museum's forensic lab...

...to conduct a CT scan with manager Agnese Lanzetti.

Similar to brain scanning machines used in hospitals,

it takes thousands of x-rays to build a detailed 3D image

of both the anglerfish

and the skeleton of whatever's inside it.

This is great. It's so clear. So this is the anglerfish.

This is what it's got in its stomach.

If we rotate a bit,

we should be able to see not just the teeth in its jaw,

but it's got teeth on the roof of its mouth as well.

Yeah, there they are. There.

So it's got these quite nasty backwards-facing fangs.

Ancl what usually happens with predatory fish

who are attacking other fish,

they like to get them by the head and they sort of clamp down on it

and stop the fish from breathing.

And that will kill the fish,

that makes it a lot easier to swallow it.

So these massive eyes,

that's what would have led it to its doom.

It would have seen the lure of the anglerfish in the darkness

and gone over to investigate.

Ancl that would be the last thing it saw

before it was swallowed.

In the freezing cold isolation of the midnight zone,

two kilometres below the surface,

a hungry anglerfish is playing a waiting game.

As an ambush predator,

it has to be ready to strike at any moment.

In the deep ocean... CRUNCH

...when there's an anglerfish around,

it's wise not to be afraid of the dark,

but to be afraid of the light.

Thanks to the beautiful results we got from the CT scan,

I think I've worked out what the anglerfish has eaten.

The first thing I noticed is the fish inside the anglerfish

has got these two large fins near the back of its body

that are roughly the same size, and they're above each other.

Ancl that is a feature of a group of fishes

called the smooth-heads.

Ancl I believe it's rouleina attrita,

the soft skin smooth-head, that is inside the anglerfish.

Ancl because we have such an amazing collection here

at the Natural History Museum, if you think of a fish,

chances are we've probably got it in one of our cupboards.

And sure enough,

we have some lovely specimens of rouleina attrita here.

Ancl get one out. Let's get this one.

This is probably not too far off the size of the fish

that's actually inside there.

So the anglerfish has eaten something

that's almost twice as long as itself.

It's probably like the equivalent of us eating something like,

I don't know, a Saint Bernard or something,

something pretty hefty.

So it's got this amazing stretchy stomach

so it can fit really, really quite large prey items inside.

Food doesn't come along very often in the deep sea,

so you have to make the most of every opportunity.

Ancl that probably would have kept it going for a long time,

several weeks, if not longer.

So it's a supremely well-adapted predator for its environment.

Lethal jaws are crucial for any killer in the deep.

From a great white, to a killer whale, to an anglerfish.

But it hasn't always been that way.

The ability to bite had to evolve like any other predatory strategy.

And it all began with one of the most ferocious-looking fish

ever to swim in our oceans.

Our seas are home to countless killers.

While some, like the anglerfish,

have developed sophisticated methods to hunt...

...others use highly-potent venom.

Higher up the food chain, you can't beat a ferocious set...

of jaws.

The infamous bite of the great white shark

is the most feared weapon in the water.

But all of today's biting predators owe their killer instincts...

...to a prehistoric pioneer.

419 million years ago,

our planet was a very different place.

On land, the plants were relative newcomers.

And the dinosaurs wouldn't be around for another 200 million years,

but the oceans were teeming with life.

This was the start of the Devonian Period,

also known as the Age of Fishes.

Back then, there weren't any big predators...

...certainly not as we know them today.

But evolution was about to go into overdrive.

Spawning a new generation of aquatic killers, the placoderms.

What set them apart

was that they were the first fish to develop hard, bony jaws.

Our oceans would never be the same again.

The development of jaws was an evolutionary game-changer,

one of the most important

in the history of life on our planet.

All of a sudden, fish didn't just have to be passive filter feeders

on the sea bed, they could be active killers.

This, of course, made the sea a much more dangerous place to be.

But placoderms also developed protection

in the form of armoured bony plates around their head.

But as the arms race between teeth and arm our intensified...

...bigger, formidable, ferocious fish-slicing monsters evolved.

And around 382 million years ago,

the biggest and baddest of the lot, Dunkleosteus.

Measuring up to ten metres in length,

Dunkleosteus was not only a huge placoderm,

but also one of the world's first top predators.

Only a few complete skulls of this revolutionary big biter

have ever been discovered...

...near the Great Lakes in North America.

At the Royal Belgian Institute of Natural Sciences in Brussels...

...palaeontologist Sebastien Olive

has a cast of one of these very rare fossilised skulls.

Here we have Dunkleosteus.

The size is quite impressive because you have the skull there,

until there, and after that you have the trunk.

And the head and the trunk

are connected by very strong and powerful muscles.

When they were contracted,

it permitted a very huge opening of the mouth.

You can see here on the upper jaws, there are two fangs.

Ancl on the lower jaw, you have also two fangs,

and you have a very sharp blade.

So we had an animal with a very powerful crushing bite

to allow it to catch prey.

As the dominant predator,

Dunkleosteus would have preyed upon other placoderms.

Which means its jaws would have to be able to bite through

their bony arm our.

Similar behaviour has been observed

with modern day tiger sharks hunting large marine turtles...

...crunching through their thick, heavy shells.

Our seas have long been home to some seriously powerful biters,

but what were those first sets of apex predator jaws like?

This precise 3D scan of a rare, complete Dunkleosteus skull...

...created by the Cleveland Museum of Natural History,

could reveal clues about how they killed their prey.

Placoderm expert Martin Brazeau from Imperial College London

wants to use this model

to put Dunkleosteus' primitive bite to the test.

What we have here is a very nice three dimensional model

of the skull and shoulder of Dunkleosteus.

All these bones can really tell us a story

about how the jaws of this animal functioned.

The biting force of this type of jaw was about 4,500 newtons.

Now, to put that into perspective, the strongest bite forces today

would be something like a saltwater crocodile,

which is about 16,000 newtons,

or a great white shark, which would be about 18,000 newtons.

So it's quite a bit less in terms of force

compared to some of these modern animals.

But that very high bite force in a saltwater crocodile

is going to be distributed over a much wider area.

Whereas in the case of Dunkleosteus,

the initial force would be concentrated

on the very sharp points at the front of the jaw.

The question is, would that have been enough

to get through the tough arm our plating of its prey?

The 3D model provides another clue.

At the top and bottom of the skull,

large muscles would have worked together

to pull the upper and lower jaws apart...

...creating a massive gape...

...before interlocking cheek muscles snapped it shut in a flash.

BANG

So what we're really interested in finding out today

is what are the impacts of this very different mode of biting

on something like a piece of bone.

To bring the jaws back to life,

Martin is enlisting the help of Drjim Watson

at the Cranfield Impact Centre.

He's reconstructed the fangs of Dunkleosteus

and attached them to a mechanical press

matched to its bite force.

With a cow bone acting as the arm our plating of its prey.

Arm in.

Three, two, one.

CRACK

That did it. Yeah. CH UCKLES

Definitely got that fracture.

Yeah, it looks like the bone is broken

in a kind of long fracture,

and the fangs have gone clean through.

Obviously, the speed of closure is quite critical

because it really tells us something

about what Dunkleosteus could do with its jaws.

What we've seen is that Dunkleosteus

had these pointed fangs at the front,

and they would have initiated the puncture into the prey.

So that would have created the weak spot

for the jaw to continue travelling through the flesh of the prey.

So they were shearing jaws,

so they would have operated much like scissors in a way.

A reasonable analogy might be something like

a modern-day tiger shark,

where they gouge out these big gobbets of flesh

from the side of their prey items.

Since the time of Dunkleosteus,

marine killers have evolved and diversified

into weird and lethal biters we see today.

Like the rarely seen and eel-like frilled shark.

It usually lives 1000 metres down in the midnight zone,

but every night it rises to hunt in shallower waters.

Rather than just the two bony fangs of Dunkleosteus,

it has more than 300 teeth.

And its jaws might be weak, but it can swallow prey whole.

Its teeth act like vicious barbs

to prevent prey escaping once they're snagged.

Its cousin, the goblin shark,

uses its jaws and teeth very differently.

Its elongate snout, packed with electrical receptors,

help it detect its prey

to lock on in the darkness of the twilight zone,

hundreds of metres below the surface.

But it means its jaws are under its large snout,

so it attacks by catapulting its jaws forward, alien-like,

in the blink of an eye.

But just as predators of the deep

have become increasingly specialised,

so too have the survival strategies of the creatures they prey upon.

The transparent skin of the mesmerising glass octopus...

...acts like an invisibility cloak.

Predators will literally see straight through it.

While some deep sea squid squirt luminous ink

to create a smokescreen.

And the a toll a jellyfish

flashes like a burglar alarm when attacked.

Down here, getting your defensive game right...

...could mean the difference between life and death.

But there's one unique killer that uses light to hunt and kill

like no other in the darkness of the midnight zone,

over 1,000 metres down in the abyss.

Life in our oceans is a delicate balance

between finding food to kill and eat,

while avoiding being killed and eaten yourself.

For the very top predators, it's all about getting more deadly.

Whilst things further down the food chain

go to extraordinary lengths to avoid detection.

But there's one killer in our seas

that manages to be both a predator and as good as invisible,

the marvellously named stoplight loose jaw.

The stoplight loose jaw

is one of the most cunning and stealthy killers

in our ocean today.

A streamlined hunter which prowls the darkness

of the midnight zone,

over 1,000 metres down, in search of prey.

But how does it find its next meal in the pitch black?

The answer is to do with light.

More specifically, the colour of light

and how it behaves in the deep, dark ocean.

Sunlight is made up of all the colours of the rainbow.

When it shines down through the sea,

the colours lose their intensity and eventually disappear entirely.

But because each colour has a different energy level,

they disappear at different depths.

With the lowest energy red light being the first to go.

While at the opposite end of the rainbow,

blue light has the highest energy and penetrates the deepest.

So blue is the best colour to be seen in the deep sea,

and red is the best colour if you want to disappear.

That's why so many creatures that live in the inky depths

are bright red.

At the surface, they would stand out like a sore thumb,

but in the deep, red makes them almost invisible.

To explore how this works,

I've come to a lab at the UK's home

of deep sea exploration, the National Oceanography Centre.

So how does light function for animals in the deep sea?

Well, this box represents the infinite inky blackness

of our deep oceans, and at the back is a red fish.

Now, if I add some blue light...

...which is quite common from bioluminescence,

it actually doesn't make that much difference.

In fact, the red fish still blends in to its background.

However, if I switch that to a red light,

it's a whole different story.

All of a sudden, that fish pops.

It really stands out against its background

and would be hugely vulnerable to a predator.

It's this unique quality of red light

that the stoplight exploits.

Marine researcher Jethro Reading

has been studying these elusive fish for the last two years.

Jethro, tell me about how

the magnificently named stoplight loose jaw

uses light to its advantage.

So, it's got that name because the stoplight loose jaws

are among a very few species of fish

that are capable of producing red bioluminescence,

they are able to produce red light.

It's thought to use this red bioluminescence

like a searchlight, so it scans the water around it,

looking for prey items, in particular fish,

and these small crustaceans,

these small red shrimps called copepods.

So where is that light-producing organ?

So these shores are covered in light organs

all along the underside that provide camouflage.

And then under these huge eyes

there's this teardrop-shaped light organ,

and this is actually what produces this red light.

Ancl what's really clever about it

is that a lot of the prey items at these depths,

because there's so little red light around,

have actually lost the ability to see red light.

Which means that they can be snuck up on by loose jaw

without them having any idea.

It is like some kind of Marvel superhero, isn't it?

With lasers under its eyes,

just shining on stuff around to find it.

It's amazing. It's so cool.

Being able to both see and produce red light

gives the stoplight loose jaw

a phenomenal advantage over its prey.

It's like a sniper with night vision goggles,

illuminating its target like a beacon,

while its prey remains completely oblivious

to the danger it's in.

So that's the stoplight part of things. Mm.

What about the loose jaw area of its name?

How does that function?

So, I mean, we've got a model of a stoplight loose jaw here.

If we animate this model,

and you can see that this jaw is massive,

it's approximately 30% of the fish's total length.

Ancl it can open to a huge extent.

And it does this by some very clever articulations.

So there's an extra hinge up here behind the head.

Ancl this is actually a really, like, quite freaky adaptation.

This is where it gets this name Loose jaw.

The jaw is phenomenal and it's vast.

But there's one thing it's missing.

And you look at the underside of the jaw, it's just a gaping hole.

It's missing this bit, isn't it? Yeah, it's missing all the membrane.

Missing all the flesh underneath the jaw. Yeah.

So members of the loose jaw subfamily completely lack the membrane,

the skin on the underside of their mouth.

Instead, they just have this one elastic exposed muscle

connecting the jaw to the back,

and all the other soft tissue is absent.

This enables the jaw to move pretty rapidly and very efficiently.

It can close in about a tenth of a second.

It's got these huge, huge fangs

that basically are able to just pierce

any prey item they come across.

Ancl more importantly,

these teeth actually have a crystalline structure

which makes them translucent, makes them transparent,

makes them just completely disappear into the water.

Camouflaged teeth. Yeah.

That is going the extra step, isn't it?

You've got to just try and stay hidden

for as long as possible,

until you are close enough to get your jaw out

and strike your prey.

Very cool.

As food is such a precious commodity in the deep,

when going in for the kill,

it pays not just to have a loose jaw,

but also a fast jaw.

The stoplight can open and close its mouth quickly,

because its unique hollow lower jaw reduces drag through the water.

And many other killers in the ocean

use the fast opening of their jaws as a potent weapon.

So opening the mouth really wide allows you to take in big prey,

and closing the mouth fast

can apply greater force and pressure.

But why is it also important to open the mouth fast?

Well, let's imagine

that this little bit of chewing gum here is a fish,

and my hands are going to act as the jaws of a predatory fish.

It's all well and good closing them fast.

But if that's all you do, then this is the result.

The fish gets driven further away

because the jaws closing creates a wake or turbulence,

driving water out of the mouth

and pushing the fish away from the teeth and the jaws.

Whereas if you can open that mouth really quickly,

then it drags water and the fish in, like this.

So I guess I am opening my hands there

at about half a second.

There are fish alive today that can do it in a millisecond.

Highly specialised suction feeders

can open their mouths at such lightning speeds

that we need super slo-mo cameras to see it properly.

As they rapidly increase the size of their mouths to feed,

a vacuum is created inside.

Water immediately gets sucked in

along with any unlucky fish caught in the firing line.

It's such an effective and efficient feeding method

that almost all fish use suction, from minnows to monsters.

But many of the ocean's most successful killers

aren't fish at all.

The ultimate predator in our oceans today is the orca.

The killer whale.

And that is a mammal which came from the land.

They weren't the first air breathers to return to the sea.

So how did air-breathing animals

not only adapt to an ocean environment,

but become the ultimate killers of killers?

When we think of killers in the seas of today,

we generally think of sharks.

Yet today's most formidable ocean killers are not sharks,

not even the great white shark...

...but are orcas, the killer whale.

This is an animal that is efficient...

...ruthless...

...intelligent and deadly.

They hunt and kill on their own or as a group.

And on occasion, kill other whales.

And even great white sharks.

They are unrivalled as killers in the seas of today.

Yet these air breathing assassins evolved over millions of years

from a land-living mammal.

Just what is it that draws killers back to the seas?

The surprising answer can be found at the National Oceanography Centre.

This lab holds a part of the answer

because these jars are full of plankton.

These here are phytoplankton.

That's essentially plants.

When it's blooming, you get this pea-soupy green colour.

The abundance of this extraordinary plankton

is the greatest source of food on Earth.

Phytoplankton is food for this...

...zooplankton, which is microscopic animals.

We can get a look at those under the microscope.

You should see lots of tiny little critters swimming around.

And these zooplankton are eaten by slightly bigger fish,

and they are eaten by bigger fish, and bigger fish, and bigger fish.

This plankton provides the foundations

for the most dynamic and productive food chain on the planet.

And this is what draws air-breathing animals

back to the sea.

A process that takes evolution millions of years.

But the first air-breathing killers to return to the sea...

...were reptiles.

And they did it millions of years earlier.

A recent fossil discovery reveals they evolved into killing machines

every bit as deadly as today's killer whale.

But what is it about the evolutionary process

that turns a land predator into a killer of the deep?

THESE 8T3 mosasaurs.

And the most recent mosasaur, discovered in 2022,

is called Thalassotitan, which means giant of the seas.

It's one of the all-time killers of the deep.

Marc Jones of the Natural History Museum

is a leading expert on mosasaurs.

Mosasaurs are a diverse group of reptiles

and have been found all over the world.

The largest mosasaurs could be up to 17 metres long.

They have all the attributes of efficient killers.

You can see that this skull is triangular in shape,

which gives it a streamlined aspect,

so it would be able to move through water more easily,

but also allowed the animal to attack and eat large prey.

They were so successful they dominated the deep,

much like killer whales do today.

Here you can see the jaws of a large mosasaur.

So these are the sort of jaws you'd expect for an animal

that was attacking other large marine reptiles.

So you have these large teeth.

The length of the tooth

would have helped penetrate large prey.

Ancl this increased the likelihood

that they would have reached internal organs,

causing huge amounts of damage, huge amounts of blood loss,

separating flesh from bone, even bone from bone.

The mosasaur teeth are formidable and have a familiar look to them.

So this is a tooth of a killer whale,

and if I hold it to the mosasaur tooth,

you can see that there are broad similarities.

So you have this robust base and then this stout crown

and this subtle curvature.

Teeth are key to understanding how and what animals hunt.

And evolution shapes teeth to do the same job.

The fact that killer whales today

hunt other whales and other vertebrate animals

indicates that mosasaurs

also attacked and ate other vertebrate prey.

An adult mosasaur could eat pretty much anything.

It would have been able to tackle the turtles it lived with,

the sharks, the large fish,

and the other marine reptiles that were in its environment.

Alongside the fossil bones of Thalassotitan,

discovered in 2022,

were semi-digested bones of another mosasaur.

Thalassotitan is a killer of killers...

...and it's smart.

Bite marks found on mosasaurs skulls reveal it went for the head,

preventing its prey from biting back and drowning it.

CRACKING AND CRUNCHING

Smart, ruthless, and deadly.

We know that mosasaurs evolved from land-living lizards.

But how do you get from something like that

to a colossal monster of the deep?

Today's marine iguana is unique to the Galapagos Islands.

It's a lizard that spends much of its time in the sea.

They can spend up to an hour underwater.

Over time,

they've evolved some adaptations to an aquatic lifestyle.

Their tails are flattened to help them swim,

their feet are partially webbed,

and their bones are more dense to help them dive.

While the marine iguana has some characteristics

that allow it to spend time in the sea, it's not fully aquatic.

Thalassotitan is more closely related

to the Komodo dragon, a predator.

So how could a land-based reptile like the Komodo evolve

to become a ferocious killer of the deep,

but unlike the marine iguana,

is entirely adapted to life in the ocean?

Evolutionary biologist Dr Jennifer Hoyal Cuthill

from the University of Essex

can shed light on how the Komodo dragon

could evolve to become an ocean killer.

If Komodo dragons were to re-enter the ocean,

they may have to evolve in different ways.

We can think about how their body might change

to improve their swimming ability in the ocean.

Komodo dragons are already good swimmers.

But to be efficient ocean killers,

they would need to adapt their swimming style.

They use an undulating motion of the body,

so we would see a stiffening of the body,

and then the evolution of webbing between the toes.

Then progressively producing flippers for swimming,

almost like flying through the water.

The tail increasing elongation to form a tail fin,

allowing them to move through the water

with tail-driven propulsion, similar to mosasaurs.

The Komodo dragon has an amazing sense of smell

that might potentially be helpful, as it is for sharks, for example,

to find prey from a distance.

That might be something that would be kept by evolution

on a re-entry into the oceans.

Evolution refines and reshapes an animal's body

to enable it to live in the sea,

adapting it over millions of years

to make it more efficient in its new world.

The main reason animals are drawn back to the sea

time and time again

is because the oceans are the richest food sources

on the planet.

And when mosasaurs became extinct,

that paved the way for a new generation

of air-breathing animals to rise to the top of the food chain.

Culminating in the orca...

...to claim the title

of the ultimate killer in the seas of today.

Our oceans are rich with opportunities

to thrive and survive,

both the creatures that were drawn back

and for those that never left.

They may be vast, dark, and hostile, but they're also an engine,

driving the evolution of our most formidable killers.

Unlike on land, the ocean is home to few large herbivores.

Virtually the whole ecosystem is made up of killers.

Of animals eating other animals.

From the earliest killers with primitive, but powerful jaws,

and those that use light not just as a lure...

...but to seek and kill prey.

And land dwellers that became giant ocean predators.

It's an unforgiving place where only the strongest survive.

Because the rule of life is eat or be eaten.

Next time, we go in search

of the ocean's weird and wonderful giants.

From the biggest monsters lurking in the deep

to giants that live longer than any other animal.

From ferocious super predators

to enigmatic monsters that haunt our subconscious.

Oceans continually create the most immense giants

the world has ever seen.