These are not visitors from an alien planet.
Nor are they science fiction.
They are real.
Creatures of our own world.
And their destiny...
...is linked to ours.
Coral mountains rise more than half a mile from the floor of the Caribbean Sea.
They were built, inch by inch, over centuries...
...by billions of coral animals.
But the animals didn't do it alone.
Tiny plants living inside the coral...
...capture energy from the sun to make its food.
Neither plant nor coral...
...can survive without the other.
And that's the key:
The whole reef community is built on relationships.
Little fish live here.
And big fish eat little fish.
He's a 100-pound black grouper.
And he's always hungry.
But for the community to remain in balance...
...the prey needs a fair chance at survival too.
And nature provides some ingenious ways to even the odds.
Take the frogfish.
He hides from predators by looking like a sponge.
In his disguise...
...the frogfish can sit back and attract lunch...
...with a fishing pole mounted on his forehead.
Glassy minnows.
Like quicksilver.
That black grouper on the ledge is hoping to catch one.
The shimmering school dances for good reason.
It's very hard for the grouper to isolate a single minnow.
And he often fails.
There's more diversity here than anywhere else on Earth.
And every single citizen, from the tiniest fish...
...to the ferocious tiger shark, must depend upon others to survive.
The balance between predator and prey is always shifting.
But the community stays healthy and whole...
...because there are so many different relationships between so many species.
Sometimes...
...even the coral itself is prey.
The crown-of-thorns sea star.
He eats coral.
Too many of these could wipe out an entire reef.
The triton trumpet snail hunts sea stars.
Although the snail is almost blind...
...it can smell the sea star's trail.
Those nasty thorns are full of venom.
But the snail is immune.
- What's that? - It's called a proboscis.
And he'll use it to drill through the stars leathery hide.
Then he'll inject a venom...
...that will dissolve the sea star from the inside out.
So the triton trumpet snail helps save the coral.
And that helps keep the reef's eco system in balance.
Even different species you'd think would be enemies...
...often help each other.
This is a cleaning station.
A sort of a dermatology clinic.
The spotted coney is the patient.
The little cleaner gobies move over his skin...
...removing and eating parasites.
Both species benefit.
It's called symbiosis.
Another cleaning station.
Away from this spot...
...the barracuda might swallow the little Spanish hogfish whole.
When he's being cleaned, the barracuda seems to call a truce.
The cleaning station is a sanctuary.
Green sea turtles love coming to this reef near the island of Hawaii.
It's sort of an undersea spa.
Amazingly, it's only the size of a living room.
But even coming from miles away...
...they somehow manage to find this special spot.
When algae accumulates on their shells, it can really slow them down.
But the reef fish give them a good scrub.
In exchange, the turtles give the reef fish a healthy vegetarian feast.
Some don't even wait their turn.
They swoop in close to other turtles to steal away the school.
With her shell beautifully polished...
...she now returns to her migrations...
...which may take her thousands of miles away...
...through the trackless open sea...
...where jellyfish drift through liquid skies.
That pulsing is how it swims.
Each pulse also forces tiny animals inward...
...where they are stunned and consumed.
Thousands of different species of jellyfish ride the currents...
...like sailors in a gale.
Most are solitary travelers.
But moon jellies sometimes swarm together in astonishing numbers.
They know not what lies in wait below.
A monster jellyfish ensnares them in a venomous web...
...that can stretch more than 30 feet across.
The stinging filaments slowly drag them in toward the translucent bell...
...where they will be dissolved and digested.
For some mysterious reason...
...it's called the "fried egg" jellyfish.
Ocean currents can sweep entire kelp forests...
...away from California's Channel Islands and out into the open sea.
The drifting kelp rafts are a favorite gathering place...
...for one of the strangest creatures in the ocean.
It's called a Mola mola.
This one's about 5 feet across.
But he can grow to more than 4000 pounds...
...mostly on a gossamer diet of jellyfish.
But Molas don't come here for food.
They come here to be cleaned by halfmoon perch.
It's a cleaning station in the open sea.
Looks like this time, the entire family came along.
Currents are as vital to life underwater as rain is to life on land.
Along the North Pacific Coast...
...they sweep nutrients up from the depths to nourish a dense growth of plankton.
The basket star has sensed the current...
...and opens serpentine arms to snare passing tidbits.
Barnacles are feasting too.
They stick out their furry little legs to catch food.
The translucent creatures clinging to the kelp...
...are called nudibranchs.
They come in a thousand varieties.
They're close relatives of the common garden slug...
...but much more beautiful.
These are hooded nudibranchs.
They use their hoods to scoop in water...
...then slowly squeeze out all but the food.
The hairy arms of feather stars are great for combing out food.
But in a pinch...
...they're even better for escaping from marauders.
The marauder is a sun star...
...and it will eat a feather star if it can catch one.
But it much prefers to dine on juicy shellfish...
...especially sea scallops.
Did that one bite back?
What an intriguing landscape.
It's home to one of the oddest relationships ever between a predator and its prey.
It's actually a forest of animals known as tube anemones.
They protect themselves with stinging cells hidden in their tentacles.
But there's a predator who covets those stingers.
It's a rainbow nudibranch.
And he's a thief.
He eats tube anemone tentacles.
But he doesn't digest the stinging cells.
Instead, he'll pass them on into the feathery gills on his back...
...where they continue to live.
So now the stolen stingers protect him.
The sun star is back...
...and he'll devour a nudibranch, stolen venom and all.
But like any good thief...
...the nudibranch has a getaway plan.
The Irish lord.
He can hide from his enemies and his prey.
It's not so easy for the crab to hide.
But he has great defenses:
Sharp pincers and a rock-hard shell.
The Irish lord may have bitten off more than he can chew.
Before he can swallow his dinner...
...the crab will have to release that death grip on his upper lip.
And now, a rare close-up look...
...at the domestic life of the California mantis shrimp.
He's only 10 inches long...
...but he's much stronger than he looks.
He's gathering mussels for dinner.
He's got incredibly powerful claws.
In fact, those claws are as fast as a 22-caliber bullet.
That makes him the most powerful animal, for his size, in the world.
He's using his claw like a hammer to smash open the mussel.
Now he's off to grab some more.
Here comes trouble.
He won't be safe in his burrow.
If he's cornered there...
...the octopus can inject a poison that will kill him.
Something tells me he won't go down without a fight.
His display is a warning:
"Don't mess with me."
If the octopus still doesn't get the message...
...maybe a shot to the chops will set him straight.
A thousand miles north, in the frigid waters of British Columbia...
...lives an octopus that is truly a sea monster.
The giant Pacific octopus.
This one weighs about 50 pounds...
...but he may grow up to weigh more than 200...
...with tentacles that could stretch more than 20 feet across.
He's a master of disguise.
As he moves, he changes both his color and texture...
...to match his surroundings.
This octopus is on the hunt.
And his favorite food is crab.
His eyesight is only fair...
...but the lightest brush against his prey, and the octopus attacks.
He extracts every morsel of crabmeat...
...then ejects the remains.
In the kelp forest, there is an ever-changing balance...
...between predator and prey.
But dramatic shifts can cause entire ecosystems to crumble.
For instance, sea urchins eat kelp.
But if the urchin population explodes, the kelp could disappear.
Once again, there are animals that help preserve the balance.
One of them is the wolf eel.
Certainly a face only a mother could love.
Though he looks like an ogre...
...he's important to this whole community.
Wolf eels eat sea urchins.
And there's supper.
So as the wolf eel dines on urchins, he helps protect the forest.
Just as the triton trumpet snail helps protect the coral reef.
Night falls...
...and with it, begins the largest migration on Earth.
Attracted by moonlight...
...vast swarms of plankton rise up from the deep.
And following right behind them...
...the night hunters:
Giant manta rays.
They can span 18 feet, wingtip to wingtip.
They feed only on plankton...
...scooping up millions of tiny prey in a single pass.
No ocean predator is more graceful.
In the Sea of Cortes...
...nocturnal fish come up to join the feast.
And rising to feed on them...
...the creatures of your nightmares:
Humboldt squid.
Some are more than 6 feet long and can weigh as much as 130 pounds.
They change their color several times each second.
Maybe it's excitement.
Or maybe it's a threat.
Maybe even rage.
They'll attack almost anything.
Sharks, humans...
...even each other.
A shipwreck lies on the ocean floor off North Carolina.
It's a desert here, with few natural reefs.
Now the wreck itself has become a reef...
...a shelter for these little fish.
And some not so little.
The sand tiger shark.
They like to hunt here.
But there's a surprising bond between the sharks and these small fish.
Instead of hiding from the shark, schools of small fish gather round...
...using him as protection from tuna, jacks and other predators.
So the sharks provide another kind of sanctuary for small fish...
...as they travel from wreck to wreck across this desert of open sand.
Between lemon sharks and remoras...
...it's easy to see who benefits.
By hitching a ride on the shark, the suckerfish can count on finding food.
We're not used to thinking of sharks as helpful creatures.
It may not be obvious...
...but the coral reef owes its very survival to sharks and other large predators.
They're part of the balance.
We know the balance is always shifting...
...but now it's falling apart.
In the last 50 years...
...90 percent of all the big fish have been taken from the ocean.
We are taking more than the ocean can give.
We now know that the killing of sharks is one reason why coral reefs are dying.
Overfishing is decimating one species after another.
Entire ecosystems...
...have begun to unravel.
But every year, eight nights after the full moon in August...
...something miraculous happens.
Tonight, in the Gulf of Mexico, on the flower garden banks...
...the entire coral reef will spawn.
Precisely one hour after sunset...
...coral polyps begin jetting spawn into the night sea.
Brain corals begin releasing tiny packets of eggs.
Then star corals.
How is it that millions of tiny polyps...
...from all these corals...
...choose this single moment, on this single night...
...to spawn?
How do animals that have no eyes to see...
...or brains to think...
...coordinate this event with such precision?
That remains a mystery.
These drifting galaxies are the future of the coral reef.
Most will be lost in the depths...
...but with luck, a few tiny coral larvae...
...will settle somewhere far away...
...and give birth to new reefs.
And new life.
Not so long ago...
...we knew almost nothing about the creatures living in this ocean world.
Now we are beginning to acquaint ourselves with them.
This young right whale is as interested in us as we are in him.
At last, we're beginning to learn...
...how important all species are to each other...
...above and below the surface.
And it's clear that our own destiny...
...is linked to theirs.