Original subtitles

OVER RADIO: - Neptune, Neptune, Control.

- Go ahead, Control.

- Summer 2024.

- Neptune, you are all stop.

If you could move upslope to 118

and follow transect to the dock. - Roger.

- An expedition is setting off in search of a little-known fish

that lives in the deep sea.

Its fossilised skeletons have been found in rocks

that are 400 million years old

and suggest that these fish could be the ancestors

of all backboned animals, including ourselves.

But their fossils disappear at the same time

as those of the dinosaurs - 66 million years ago.

Then, in 1938, a trawler, fishing off the coast of South Africa,

made an astonishing catch.

It was undoubtedly the same kind of fish -

a coelacanth -

but an hour or so earlier, it had been alive!

It was a sensation.

A true, living fossil.

But little was known about how and where it lived.

Now, a special expedition in a state-of-the-art research ship

is setting off to try and discover more about this living fossil...

- Nadir or Neptune? - Ready for the water.

- ..and they hit lucky.

- Yeah. What's that? - Ah! That's a coelacanth! - OK. Coelacanth.

What's that? Is that a coelacanth? - Wow!

- Yeah! - Yay! - Yeah, we found a coelacanth.

- There was a whole group of these ancient fish.

- Oh, but look. Look how they're all moving.

It's so crazy!

And look at the white tummy.

- They even witnessed for the first time

what appears to be its mating behaviour.

And the video recordings they made

shed light on how primitive fish like the coelacanth

may have used their fins to walk on land.

- It's just gorgeous.

And when we look at creatures like this,

we see the evolutionary history we share with fish.

- Join us on a journey to discover more

about this mysterious survivor from the very ancient past.

The island of Sulawesi in Indonesia.

BIRDSONG

It straddles the equator...

..and the waters around it

contain some of the richest coral reefs in the world.

This is OceanXplorer,

a vessel specially equipped for deep-sea research.

The team has special permission from the Indonesian government

to spend two weeks looking for living coelacanths.

The coelacanth is called raja laut in Indonesia,

which means king of the sea.

They have a pair of powerful deep-sea submersibles.

Each has a transparent dome

that will give it almost 360-degree visibility.

And they are fitted with an 8K deep-ocean camera system

specially developed for this trip.

The ultra-sensitive cameras are able to record clear pictures

even in the darkest depths.

They will also use a large, unmanned ROV -

a remotely operated vehicle.

- Whoa!

- This ROV is equipped with additional cameras

and can film in all directions.

- Iwata. I'm a coelacanth man.

- THEY LAUGH

- I'm Kerry. I'm a coelacanth woman.

- THEY LAUGH

These are the lead scientists on the expedition.

Dr Masamitsu Iwata has been carrying out research on coelacanths

for over 20 years.

Having worked and dived in Indonesian waters for over 20 years,

Iwata knows these seas very well.

- Wow!

I'm Kerry. - Lovely to meet you. - From South Africa.

- Cool. - Yeah. - Very good.

- Dr Kerry Sink first became fascinated by coelacanths

when she was a child.

- To really see the seascape and the terrain

is going to be an incredible experience.

And I think what would be amazing

is to see a coelacanth eyeball to eyeball.

- The team are planning to explore

the waters off the northern shores of Sulawesi.

The area is renowned for its active volcanoes

and frequent eruptions have created numerous caves

and cavities on the seafloor.

An earlier survey found a steep underwater cliff here,

which the team are keen to investigate further.

As the sun rises, they're ready for their first dive.

Both subs will be used,

so they can cover as much ground as possible.

- OK.

- Just feet on there. Yep. OK.

- Welcome. - Ooh!

- OVER RADIO: - Neptune to discharge.

- MACHINERY WHIRS AND BEEPS

- Lifting now.

- Here we go. Coming up. Watch all your stuff.

- The submersible, Neptune, with Iwata on board,

is the first to be launched.

- Ready for the water.

Neptune, Nadir, full stop.

Comms are good, and you are clear to vent when ready.

- Then they launch the second submersible, Nadir,

with Kerry on board.

KERRY EXHALES DEEPLY

- Neptune venting now.

- Roger. We'll stop venting now. Have a good dive.

- 100 metres.

- Leaving the creatures of the shallows behind,

they head for deeper water.

They're now at a depth of 200m.

Deeper than this, it's pitch-black.

- Control. Control, this is Neptune. Depth - 224 metres.

- BANGING On, spotlights.

They now plan to search an area roughly half a kilometre long.

But in this darkness, that is not an easy thing to do.

They're searching for bright spots

that could be reflections from a coelacanth's eyes.

There are two bright specks in the darkness.

It's a carpet shark,

sitting motionless on the seafloor, waiting for prey.

Meanwhile, Kerry's sub is scouring

a different section of the cliff face.

- Ah! Oh! Ah! - Oh!

What's that fish? - What is it?

- Jelly nose! Jelly noses. The crazy ones.

- It's a jelly nose.

A rare kind of deep-sea fish

that, until recently, had never been seen alive.

Its long, ray-like fins are thought to be sensors

that allow the fish to feel its way around in the darkness.

- I've never seen one before.

- Thresher shark's back.

- Oh, oh, oh. Shark. - Shark. Yeah. Thresher.

- It's a thresher shark, with a distinctive, long tail fin.

It's at least three metres long.

- I'll slowly come up towards him.

I'll just see how he reacts, OK? - Yeah. - Yeah.

- They appear to have entered the shark's territory

and it's slowly circling the submersible.

- Wow.

- THEY LAUGH

Two smaller fish have sought shelter underneath the sub.

They don't seem in the least fazed by such a strange contraption

and appear to be using it as a place to hide from the shark.

- Whoa! PROPELLER WHIRS NOISILY

- But suddenly there's a problem.

- See, it's different. See it shimmering?

- Oh, I see.

- A strong underwater current has stirred up the seafloor

and the particles of sediment are obscuring the view.

It's no longer safe to continue.

They must return to the surface.

The area is known for its strong and unpredictable ocean currents,

which can make underwater exploration very dangerous.

The team have to be prepared for setbacks like this,

but it's frustrating when their time down here is so limited.

So why are scientists so fascinated by the coelacanth?

It seems that these primitive fish

may have solved a long-standing evolutionary mystery.

Coelacanths first appeared more than 400 million years ago,

long before the dinosaurs,

and they thrived in oceans around the world.

It was thought that they became extinct

at the same time as the dinosaurs, 66 million years ago.

Then, in 1938, fishermen pulled up from the depths of the Indian Ocean

a specimen of this ancient fish that was alive.

Its discovery stunned the scientific world.

The coelacanth was not extinct after all.

Then, in the 1950s, another specimen

was caught off the coast of Africa.

And I had the privilege of examining it.

Its fins have long, fleshy lobes at their base,

which make them quite unlike the fins of any other living fish.

And there seems little doubt that...

In fact, the fins were so muscular

that it was thought it might use them like simple legs

in order to walk across the seafloor.

Since those first discoveries,

scientists around the world have been searching the deep sea

for a living coelacanth.

Then, in 1997, there was another surprising discovery -

a coelacanth was caught far from Africa, in Asian waters.

The discovery re-awoke interest in this strange fish.

Three days into the expedition,

and the team have decided to try a new strategy.

- And find those...find those points.

- Er...

The target depth is about, around 160.

- They have decided to concentrate their search

to depths of around 160m,

which are rich in underwater caves

and where water currents are relatively stable.

- Today is the day we're going to find a fish.

- 50 minutes into the search

and Iwata notices some interesting-looking rocks.

Clumps of them stand on the otherwise even seafloor.

The sub approaches cautiously.

The cameraman has noticed something between the two clumps of rock.

- Yeah. What's that? - Ah! That's a coelacanth!

- OK. Coelacanth. - Yeah, maybe. - Maybe. - Maybe.

- A closer look reveals a creature hiding in the cracks.

- What's that? Is that a coelacanth? - Wow! - Oh, yeah.

- Yeah! - Yeah, we found coelacanth. - Yeah.

- The team are thrilled.

It's their first glimpse of a live coelacanth.

Unlike the dead specimens,

the living coelacanth is a steely-blue colour with light flecks.

- I can get us a little closer,

but if we just do a bit of filming here... - Uh-huh.

- ..so we don't scare it off, right? - Yeah.

- RADIO: - You found a coelacanth? Understood. Marking location.

- Coelacanths? - Yeah.

- They got there? - Yeah.

Control, Control. This is Neptune.

Confirm coelacanths. Over.

- Wait for it.

- After half an hour,

the coelacanth emerges, giving them a better view.

It's over a metre long.

- Oh, there it is in the cave. Got it. - Yeah.

- The other team have found yet another coelacanth.

- Great. That is pretty spectacular.

Thank you.

- This is the first time that anyone has recorded

such a high-quality 8K image of a coelacanth in its natural habitat.

Its large eyes are adaptations to the poor light down here.

The fish are extremely sensitive

and can see quite well, even in darkness.

Most animal species continue to evolve in some way over time,

but, astonishingly, the coelacanth has remained almost unchanged

for over 400 million years.

It has scales that shimmer in the light -

a characteristic that doesn't fossilise.

Its fleshy fins are far more versatile

than those of most fish.

They move in a coordinated way, keeping its large body steady.

One of its oddest features is its tail fin.

Like that of fossil coelacanths,

it has a little tuft at the end,

which it can move independently.

What function this could have still baffles scientists today.

Perhaps it acts as a small rudder

and helps the fish to make tight turns.

Or maybe it's an electro sensor

that helps it navigate in the eternal darkness.

Much about this ancient creature is still a mystery.

- We're back.

- APPLAUSE

- Congratulations. - Hi. Congratulations. - That was so cool!

- Yeah! - Well done. - We found it. - Yeah.

- How the coelacanth survives at great depths

still remains largely unknown,

despite efforts of scientists across the world.

Deep-sea exploration is not only challenging, but costly,

so every day is precious.

To maximise their chances of observing more behaviour,

the team have decided to deploy all of their equipment simultaneously.

- We are launching number two

before we are recovering number one? - Yes, yes.

- That was the request, yes.

- Always an overlap. - Yeah. - Yeah. - OK.

- OK. - Thank you.

- The aim is to carry out continuous observations for 72 hours.

It's an ambitious plan

and requires the crew to run the subs in shifts, day and night.

This is a first, even for the crew of OceanXplorer.

The scientists are now sure

that coelacanths stay dormant throughout the day.

They suspect that the fish is nocturnal,

so they start their daily shift in the early evening.

- Control, control, this is Neptune.

I have cave in sight.

- This coelacanth is still in exactly the same spot

as it was when they found it the previous day.

But the team must be ready to respond if and when it does move.

They know perfectly well that the coelacanth,

to sustain its massive body, must come out to feed sometime.

But it doesn't move, even at night.

As the team proves by speeding up their video recording,

it remains in exactly the same spot.

At midnight, the second sub, with Kerry on board, sets off.

She now takes over from Iwata in keeping watch.

RADIO CHATTER

But even after ten hours, the coelacanth has still not shifted

and shows no sign of wanting to look for food.

Finally, at, eight in the morning,

the team decide to bring out the unmanned ROV.

Kerry is exhausted.

They've now been watching the coelacanth continuously

for 24 hours,

but it still has shown no sign of moving.

Perhaps the sub's lights are making it nervous.

So they switch to infrared lights

that are virtually undetectable by any fish.

- OK, darkness. - Darkness.

- BANGING

And still, the coelacanth does not move.

At 7am the following day,

39 hours later,

it finally does something.

- Ah!

- Oh! We're on the move.

- Yeah. It's beautiful.

Maybe she's going out of it.

- The coelacanth, at last, is on the move.

- We're supposed to follow her. - Yeah.

- Shall we follow? - Yeah. - Yeah.

- It swims slowly,

but in a way that our scientists have not seen before.

It propels itself forward

by moving its fins rhythmically from left to right.

The two fins appear to provide forward propulsion.

- Oh, it's steep here.

- Ahead is a vertical cliff face.

- It'll be interesting to see if she goes up this crack here.

Right, she's going along the crack.

- The coelacanth makes its way purposefully along the steep cliff.

- So, this is a good...

- Then it comes to an overhang in the rocks.

- She's there. - Yeah. She's here.

- That's perfect.

Yeah. We want to leave her somewhere where she can maybe stay.

We don't want her going.

- Mark this waypoint...

..Big Mama's New House. Depth - 160 metres.

- The fish has disappeared into the crevice.

The place where the coelacanth has chosen to hide

is one the scientists had not expected.

It's a large overhang in the cliff, effectively an underwater cave.

Tracking the fish's movements has led them to a new hideout.

This is a real reward for their 72-hour vigil.

Then, Kerry spots something else.

- Oh, I think I see another one. - What? - On the left.

- Wow! - There. Is it?

Yes.

- There appear to be more coelacanths hiding in the cave.

It's an extraordinary sight.

- One, two, three, four,

five, six, seven,

eight. - Eight!

- Eight! - Eight? - Eight.

- What is happening?

- There are definitely eight individuals,

and the team are astounded.

What could be the reason for such a gathering?

- Spawning aggregation.

- It's hard to know why these coelacanths have assembled

in such a tight space,

but they seem to be resting peacefully, side-by-side.

Suddenly, the thresher shark reappears.

The coelacanth retreat further into the crevice.

The arched overhang provides good cover,

allowing even a large fish like a coelacanth to hide unnoticed.

The cave also provides protection from ocean currents,

so there is little danger of being swept away.

It's a fortress in the kingdom of the coelacanth.

To help them get a better view beneath the ledge,

the crew use another piece of equipment.

- I'm going to take a bit of...

- It's a tiny underwater camera

that they manoeuvre into position and attach to the rock face.

Despite its small size, it gives a surprisingly detailed image.

They can just distinguish a coelacanth

huddling underneath the ledge.

Within minutes, swarms of tiny fish appear.

They're lanternfish,

attracted by the camera's bright light.

Once the swarm has dissipated, the coelacanth comes into view again.

And then...

Slowed down, we can see what happened.

Faster than the blink of an eye,

it lunges for a passing morsel of food.

This is very different from the slow-moving, languid creature

the team had seen until now.

The footage clearly shows the coelacanth feeding

in a manner only known to be used by prehistoric fish,

until now.

Recent research has shown that this way of feeding

is only possible because of the unique shape of their skull.

It is, in fact, split into two halves,

which are connected by a special hinged joint.

This allows the fish to widen its mouth to more than 30 degrees,

enabling it to bite with the force needed

to feed on hard, scaly animals or to swallow particularly large prey.

Many ancient sea creatures once had this kind of a hinged joint,

but the coelacanth is the only known living fish

to still possess one.

So, at last, we're starting to unravel

some of the mysteries that surround this ancient creature.

It uses its fleshy fins to propel itself slowly through the water

and into narrow crevices,

but, when necessary, it can move surprisingly quickly.

And the unique joint in its skull allows it to open its mouth wider

than most modern-day fish are able to do.

These ancient features have clearly served the coelacanth well

for millions of years.

Recently, scientists have discovered

that coelacanths can live to be 100 years old.

In the deep,

it seems that it's the slow and the steady that wins the race.

The scientific team are keen to take a closer look

at the footage to solve a particular puzzle.

- What?!

Ah! There, there, there.

Oh, beautiful. - Yeah.

- Oh, wow!

- Its fins have a thick, fleshy base -

a characteristic feature of an ancient group

called the lobe-finned fish.

The fins of most modern-day fish are connected to the body

in a way that enables them to flap,

but little more.

The coelacanth's fins, however, are much larger

and have a row of bones attached to thick muscles,

which gives the fins much greater strength.

This arrangement is also found

in the limbs of most land animals today

and suggests they share a common ancestor.

Scientists have long puzzled over

how animals first hauled themselves out of the water and onto land.

The coelacanth may provide the answer.

Their fleshy fins may have led to limbs with an upper arm,

a forearm and fingers and toes.

And that enabled the first backboned animals

to crawl out onto land.

Whilst the evidence from the fossil record is compelling,

it can't show us exactly how these ancient creatures moved.

At the University of Chicago,

evolutionary biologist Dr Neil Shubin

has been trying to solve this puzzle for years

and is very excited by the team's discoveries.

- It's like it's a fish with arms. That's what's amazing.

So the way they swim is with movements of an arm.

They're doing this.

It's not just a little paddle.

It has bones inside that can move much like our own arm bones.

Look at that turn around.

- A closer look at the footage reveals some important details

of how the coelacanth uses its frontal - or pectoral - fins.

It appears to rotate each fin,

as if it were trying to stroke its own head.

Slowed down, we can see the movement more clearly.

By superimposing the bone structure onto the fin,

we can see that the bone bends at each joint,

allowing for this wide range of movement.

Even the scientists themselves are surprised by this revelation.

- I haven't seen it like this, no.

I can make guesses, hypotheses,

about how those bones actually moved,

but I don't really have the creature to look at.

I'm inferring it from a skeleton,

but when I see a video like this,

I begin to see a living creature and how those bones are used.

So it allows us to interpret those fossils in whole new ways.

As we study these fish,

we understand something about ourselves.

I find that very powerful and very beautiful.

And when we look at creatures like this,

we see the evolutionary history we share with fish.

- On land, these bony and flexible structures supporting the body

would have then gradually evolved into well-muscled legs.

Over time, those early limbs evolved into many different forms -

sturdy legs for walking on land...

..light wing bones for taking to the sky...

..and dextrous hands capable of grasping.

They evolved to meet the needs of different habitats

and, as a result, life on land became rich and varied.

The coelacanth's fins led to limbs suited to movement on land

and so paved the way for all terrestrial life

and our own evolutionary history.

Whilst the coelacanth has shed important light

on evolutionary questions,

we still know very little about how the species lives.

Dr Kerry Sink is hoping to answer some of these questions.

One of her main aims

is to find out more about the fish's reproductive behaviour.

Coelacanths have white speckles all over their bodies.

Each fish has its own unique pattern and looks slightly different.

Kerry has used these differences to identify individuals.

- I want to be paying attention to what's happening from fish to fish.

So I've been taking careful notes about which fish is which

and how they might be interacting.

- On this expedition,

one particular individual has attracted Kerry's interest.

It's the female that she had followed to the group under a ledge.

Kerry recognises it by a unique feature.

- So the W mark is one of the most distinctive marks

on the right flank of the big female.

- Kerry knows it's a female because of its size -

females are larger than males.

She's keen to follow this female

to see if she can discover any evidence of breeding activity.

Her sub prepares for another dive.

They have headed to the rock face

where they last saw the female coelacanth.

- I haven't seen the W. Have you seen the W?

- After 30 minutes, the female with the W mark appears.

She is over a metre long

and because she's larger than the others,

Kerry has nicknamed her Big Mama...

..and she continues to keep a close watch on her.

Five hours later, the coelacanths begin to do something

that the scientists have never seen before.

Some of the fish begin to hang vertically in the water.

- Oh, but, look - look how they're all moving.

It's so crazy!

- Kerry believes that this may be coelacanth courtship behaviour.

- I think we have mating coelacanths.

And look at the white tummy.

And they don't normally look like that, right?

Very interesting behaviour.

- The coelacanths have turned their undersides white.

Many fish change colour in order to attract a mate,

and Kerry suspects that coelacanths may also do so

to appeal to the opposite sex.

The team in the other sub

are also witnessing some interesting activity.

One of the coelacanths is heading away from the rest of the group.

It's heading straight for another fish

approaching from the other side.

The two fish begin to circle each other.

Perhaps they're rival males, competing for a female.

Back at Kerry's sub, Big Mama, with her distinctive W mark,

is also being approached by a male.

He is clearly smaller than she is.

As he sidles up to her, he appears to rub his head on her underside.

The researchers think that she might also be producing

some kind of a pheromone that attracts males.

He continues with this curious mating dance

for over three minutes.

This is the first time that anyone has seen

coelacanths interacting with one another.

It's an exciting discovery that brings us one step closer

to understanding their breeding behaviour.

For Kerry, this is a career highlight.

- Just, like, deep gratitude

and something I've wanted for a long, long time - 24 years.

I told the coelacanths, "Thank you.

"Thank you, thank you, thank you."

- Big hug first. - So cool!

- Congratulations. - Thank you. - It's so cool.

- Thanks for making this happen. - Of course. - I'm so pleased!

- Although coelacanths have managed to survive for 400 million years,

they now face a new threat.

- Oh, what's this?

- Oh!

- There is something drifting in the water.

- Trash, I guess, maybe. - Yeah, I think so.

- The team are shocked to discover plastic waste,

even at these depths.

Throughout the expedition,

they found evidence of human waste products littering the ocean floor.

- So many sandbags here. - Yeah.

- The ancient fish now face a deep-sea home full of our rubbish.

And a dead coelacanth has even been found

with plastic bags in its stomach.

The coelacanths' home is often close to human habitation.

Strong storms and typhoons

carry rubbish deep into areas where the coelacanths live.

Today, our activities threaten the home of this ancient creature

in all of the world's oceans.

The two-week expedition has come to an end.

- The coelacanth is just really a very special animal

that has this window into the past,

that has these really unique features

that inspire incredible curiosity.

For me, the coelacanth is also the ultimate survivor.

And I was just experiencing amazement and awe and gratitude.

- The expedition recorded many important aspects

of the coelacanths' life for the first time.

Its discoveries have helped us understand a creature

that looks very similar to those that lived 400 million years ago.

Hiding beside underwater cliffs,

it now has a slow and energy-efficient lifestyle...

..and breeds unobtrusively.

As a species, coelacanths have survived

all the environmental upheavals that have transformed the planet.

Watching them has brought to life

one of the most crucial episodes in the history of life -

when fish with stout, fleshy limbs

emerged onto the land for the first time

and became the ancestors of all land-living backboned animals,

including us.