Original subtitles

The coral reefs of Arabia are teeming with large predators.

Small residents need strong defensive strategies to survive.

Some are equipped with body armor...

..or chemical weaponry.

Neighbors help neighbors in times of need.

And often attack can be the best line of defense.

These are the defenders of the reef.

(Birds call and hoot)

It's summer in the Gulf of Oman.

The scorching hot Arabian sun heats the shallow waters

to around 86 degrees.

The warmth boosts the growth of algae

across a vast coral reef...

..attracting thousands of hungry herbivores.

It is the setting of a fish-versus-fish turf war.

Many of these competing fish are spectacular,

sporting bright colors and bold designs.

But not so the humble domino fish,

silver-gray, with a white spot on either side.

With his drab appearance,

this male is easy to overlook.

But what he lacks in color and size...

..he makes up for in attitude.

This little guy will stand up

to any of the big bullies on the reef.

Even a parrotfish many times his own size.

He'll need his confidence in the coming weeks,

because this little fish is on a mission.

He's about to breed for the first time.

Step one: he needs to establish his territory

and lay claim to a small patch of algae

to cultivate and call his own.

This is not a food store.

It's a giant advertisement, like a billboard,

that says to female domino fish

"eligible bachelor in residence".

A large, well-maintained algal lawn will demonstrate

his abilities to protect and nurture a nursery,

something the females will find irresistible.

But on his first attempt at selecting real estate,

he makes a rookie error.

His new territory is too exposed.

Large numbers of hungry algae-feeders,

including other domino fish,

can decimate new green growth.

Non-breeding dominos are darker in color.

Despite his best defensive efforts,

the lighter-colored male is outnumbered.

The hungry domino fish strip his new lawn clean.

The setback means he has to start again,

find a safer spot to defend

away from the hungry masses.

He sets off in search of somewhere new.

The reef is part of a marine paradise lying nine miles

off the coast of Arabia

called the Damaniyat Islands.

The archipelago consists of nine main islands

and a string of rocky outcrops in the Gulf of Oman.

The bare, jagged cliffs are in stark contrast

to the treasures that lie beneath.

The area lies at the heart

of one of the world's five largest upwellings.

Here, cold nutrient waters well up in the Arabian Sea,

triggering plankton blooms

that fuel the region's productivity.

They're exceptionally diverse

and promote dramatic coral growth.

A coral reef is like an underwater fortress,

providing protection, food and family homes

to a stunning array of marine creatures.

It is also an underwater battleground

of predators and prey...

..of attack and defense,

as each plays its critical part

in protecting the delicate balance of life.

This is not a safe place for little fish.

The fusilier has been bitten by a predator.

The nasty wound's a reminder

of the many dangers of the reef.

The odds seem stacked against the domino fish.

A number of large, deadly hunters patrol.

At just over a foot long, the yellowfin hind hunts fish.

Active by day, this ambush predator

sneaks around the gullies and escarpments.

Skirting the fringes of the reef,

a huge bowmouth guitarfish

patrols the exposed areas of the vast seabed.

Guitarfish can reach up to nine feet

and his size provides some defense

against most larger predators like sharks.

He looks like a cross between a shark and a ray

and is actually a type of skate.

Like his relatives,

he can detect even the faintest electrical signal

from prey buried in the sand.

His back has built-in body armor, scaly thorns

to dissuade anything larger from biting him.

He's not alone in having bio-armor.

On this dangerous reef,

many species have physical defenses to keep them safe.

Trunkfish have only a few large scales

fused together to form a solid box-like body.

This armament makes them harder

for other fish to swallow.

Some go further, secreting toxic mucous,

making them quite unpalatable.

Their body shape limits their movements,

leaving just a hinge-like joint in the tail

to help them get around.

But despite appearances,

they're streamlined and good at cornering.

Porcupinefish, too, are a tough act to swallow.

If threatened, they can fill their bodies with seawater,

making themselves three times bigger

and erecting sharp spines that lie flat on their skin.

It's also poisonous.

As it consumes shellfish, it recycles their toxins

and secretes them from its skin.

This gives off a scent warning to predators

not to eat it.

Not all species have built-in defenses.

Some must find other ways to protect themselves.

A tiger pistol shrimp hides within a burrow,

using his broad, stout pincers

as shovels to excavate.

He's an excellent digger, but has very poor eyesight.

While he's at work,

he could easily miss an approaching predator.

So, to help him, he enlists a goby as lookout.

This triple-spot goby is also low on the food chain

and needs refuge.

Unlike the shrimp, it has excellent eyesight

and it can dash away at a moment's notice.

But it lacks the digging skills to make a safe home.

So the two join forces.

While the shrimp plows away,

it always keeps one of its long antennae

in contact with its watch-dog.

An important defense,

for there's predators lurking in every nook and cranny.

This huge honeycomb moray eel

can reach six feet long.

He's a master of camouflage, thanks to his mottled skin

that resembles the surrounding coral.

He rarely leaves his sentry post during the day,

but, under the cover of darkness,

he stalks the smaller fish as they sleep.

In addition to the fish,

he'd pounce on an unwary crustacean given the chance.

Luckily, pistol shrimps have goby lookouts.

This co-operative behavior is known as "mutualism".

Both species benefit from the relationship.

And predators go hungry.

Away from the prying eyes of the moray eels,

the little domino fish has found a new patch

on which to cultivate his algae.

This time, he's chosen a less-obvious spot,

closer to the sea floor,

under the protection of a steep coral cliff.

This patch should be easier to defend.

It will serve as a nest site - if he can attract a female.

Females prefer a neat lawn.

It shows that the male is fit and strong

and capable of defending his patch.

His lawn is coming on nicely.

By protecting the area from grazers,

he's given the algae a chance to take hold and flourish.

Now he's ready to start his advertising campaign

and lure in a potential mate.

His circular dance draws in the first female.

But she doesn't stick around.

He's going to have to try harder to impress.

He'll have to improve his dance moves

and grow more algae if he's to succeed.

The domino isn't the only fish

to tend and defend a garden.

His flashier neighbor, a sohal surgeonfish,

is one of the most territorial gardeners on the reef.

Unlike the domino's little lawn,

this algae isn't for breeding.

It's for him to eat.

The surgeonfish's vast lawn of algae

can stretch out over 40 feet.

It's a large area of food to defend, but he's well-armed.

The surgeonfish gets his name

from the yellow, scalpel-like blade

on either side of its tail.

The blades are usually tucked into a groove in the skin

but, like a switch-blade,

they can be unleashed to slash at trespassers.

Apart from occasionally stopping

to sample his algal crop,

the surgeon is endlessly on border patrol.

While he's very aggressive to some species...

..he ignores others.

He needs to keep out other algae-feeders,

especially the large and numerous parrotfish,

with their powerful and destructive parrot-like jaws.

The surgeonfish uses attack to defend his territory,

from hungry fish, but also from coral,

which could take up space,

where he'd rather be growing algae.

The coral itself is made up of tiny creatures.

They're colonizers, always looking to expand.

Individually, they are known as "polyps",

but they are never alone.

They live in vast colonies,

their own defensive strategy against predator attack.

Some create hard calcium-carbonate shells

to defend their soft bodies.

New generations of these hard corals

build on top of the last,

creating vast structures that can be over 3,000 feet high.

Black-spotted butterfly fish feed mostly on coral.

Eye-catching Arabian butterflyfish

use their tiny, tweezer-like jaws to gently nip out

the soft, living body parts of the coral.

Butterflyfish are believed to consume

around 6% of live corals each year.

They play a key role in the reef's size,

shape and distribution.

By clearing the coral,

they help the surgeonfish increase his crop.

It's not just the fish that threaten the coral.

Even the algae can fight back.

Here in Oman's tropical shallow waters,

photosynthesizing algae reproduce rapidly.

They can slowly smother the coral,

making it harder for the tiny animals to filter food.

To fight back, corals have their own protectors.

Large herbivores, like parrotfish, constantly graze

on the algae, cutting back the growth.

And on a smaller scale,

individual corals hire their own mini army.

A host of little, live-in bodyguards.

Tiny coral crabs weed out the algae.

They look heavy-handed,

but can pluck the algae with precision

and use their pincers

to pass it to their grinding mouth parts.

They're assisted by citron gobies,

so-called for their bright yellow color.

The goby only eats small amounts of algae,

preferring to nibble on the coral itself.

But his occasional nip is a small sacrifice,

as he guards the coral

from larger voracious predators,

like the butterflyfish.

The goby, unlike most fish, doesn't have any scales.

Just smooth skin,

from which he secretes a slimy, toxic mucus.

He's only two inches long,

so relies on this chemical defense for his own safety.

Like the domino fish,

the goby is ready to start a family.

The coral branches provide a safe haven and a nest site.

Both parents look identical.

They will be faithful to each other for life

and will stay loyal to this one patch of coral.

Other species of fish come and go,

but only one alpha pair of citrons

will breed on the coral.

Young animals have a queuing system and wait in line.

They will only mature

if either of the alpha pair dies.

When that happens, one of the smaller, darker juveniles

will quickly develop to take their place.

All the youngsters are female,

but if the adult male is lost,

the largest will undergo a sex change to replace him.

This female has already laid around 100 eggs,

tiny baubles of jelly,

almost invisible,

which hang under the arch of coral in neat rows.

The male rolls upside down

to deposit sperm over the eggs to fertilize them.

He will stand guard over his microscopic family

and do his best to protect them from hungry mouths.

Fish eggs are a favorite food for many reef species.

The sneaky yellowfin hind is a constant threat.

It will certainly vacuum up the eggs,

if it can get to them.

Too big to take on,

the goby relies on the coral's structure

to protect his eggs.

The citron goby has chosen well.

The nest site is just out of reach.

The hind doesn't stick around.

He heads on, arriving at the domino's nest site.

But the male domino fish has yet to impress a female

and gain some eggs.

He practices his best defensive move

and scares off the hungry hind.

The commotion attracts attention.

A light gray female investigates his lawn.

Her color indicates she's also ready to breed.

Using a white ovipositor, a mobile tube,

she precisely deposits up to 20,000 eggs.

Each is sticky and will adhere to the algal lawn.

He follows, releasing sperm over the clutch.

The whole courtship and laying lasts 20 minutes.

Then she saunters off,

leaving the male to defend the eggs.

The female plays no further role in raising their young.

Instead, she will visit

other males tending lawns in the neighborhood.

The male has to stand guard 24 hours a day for a week,

until the eggs hatch.

It's a long time for a little fish

on a big and dangerous reef,

where monstrous predators lurk.

The wide gape of a honeycomb moray eel reveals a glimpse

of its powerful weaponry.

Its teeth are needle-sharp and there are lots of them,

up to 38, with not two,

but four pairs of large canines at the front.

A third row lines the roof of its mouth.

It has a second set of jaws in its throat,

which help drag any victim in, crushing it as it goes.

If disturbed or challenged in its den,

the eel can strike with devastating effect.

Defense may be futile against this monster of the reef.

Its jaws never rest.

Unlike other fish, it doesn't have structured gill flaps

to pump water through,

so it must continuously force water over its gills.

Many species of moray eel

also secrete a toxic mucous on their skin

that has the ability to destroy red blood cells.

It may be a defense against their own predators.

Some species might also transfer the toxins

with their serrated teeth when they bite their prey.

It looks like this fish

is taking its life in its own hands.

But it's a cleaner wrasse,

one of the few small fish the moray won't eat.

The wrasse is a defender of health,

a marine doctor and dentist

that gives the reef inhabitants

a thorough check-up and eats their pesky parasites.

Observations suggest that a single cleaner wrasse

will tend to over 2,000 fish

and remove up to 1,200 parasites a day.

They're a vital part of a healthy reef.

Beyond the defensive walls of coral lies a no-man's land.

A vast open seabed has little cover

for animals to escape danger.

This is where the big guys prowl.

Heavily-armored predators, like the guitarfish.

Those who brave the open need ingenious adaptations

to better their chances of survival.

Kangaroo nudibranchs are one of 3,000 known species

of colorful, shell-less sea slugs.

The word "nudibranch" means "naked gill"

and this one's are on full display.

With an unprotected soft body,

it should be very vulnerable to predators.

But this sea slug

has a defensive trick up its sleeve.

It mostly eats toxic sponges.

It recycles the dangerous parts of its dinner...

..storing the toxins in a sheath over its gills.

This makes it repellent

to anything that might try to eat it.

Its bright colors act as a warning not to nibble.

Armed with these chemical weapons,

the sea slug is even able to flirt out in the open.

This behavior is known as "tailgating".

One slug follows the slime trail of another

to make sure it doesn't lose it on the vast seabed.

It's a rare sight

and is thought to be a precursor to mating.

Long-spined sea urchins appear equally unappetizing

with their intimidating weaponry.

Moving on tiny, suckered arms,

they patrol the open sea floor grazing on algae.

It's been recently discovered that their tiny legs

are also eyes, capable of detecting light,

and guiding the urchins across the reef.

As added protection, their long spines are covered

in microscopic barbs and toxic mucus.

Despite this heavy defense, they're not immune to attack.

This foot-long Picasso triggerfish,

so-called for the bold abstract markings on its face,

will eat just about anything.

Including sea urchins.

But it requires some shooting practice.

With its big, strong jaws,

it can squirt a jet of water that'll flip an urchin over,

enabling its powerful teeth to break through

their more vulnerable undersides.

But it's not a done deal.

When caught in the open,

the urchins form big, prickly herds...

..making it even harder to reach their weak spot.

The urchins are a magnet for some species

and for good reason.

Glass shrimps are delicate creatures.

They survive with help from their prickly neighbors.

By using the urchin's ten-inch spines as cover,

the shrimp hitchhike to travel to areas of the reef

normally out of their reach.

Their very own mobile safety cage.

The long, tubular snout of a trumpetfish

is built to suck up small creatures.

Glass shrimp make a good snack.

But whilst they're protected by the urchin,

the hungry trumpetfish stands no chance of getting one.

The glass shrimp aren't freeloaders, though.

As the shrimp forage around the urchin,

they remove smaller parasites that could attack their host.

Yellow-striped cardinalfish

also use the urchins for defense.

These tiny nervous creatures

hide amongst the spines during the day,

only venturing out to feed under the cover of darkness.

The plucky male domino fish has been guarding

his microscopic eggs for three days.

So far, so good.

He's not let anyone take them from under him.

But there's egg-thieving trouble on the horizon.

A pack of hungry crescent wrasse.

Across the reef,

his domino neighbor's eggs are under attack.

Domino fish are fierce fighters.

They can drive away one or two attackers.

But the wrasse use their numbers

to overwhelm the guards.

Once one family is destroyed...

..the greedy gang moves on to the next.

With each raid, they're moving closer and closer

to the male and his eggs.

He's chosen a good spot for the lawn,

low-down and out of sight.

But little escapes the attention

of a hungry mob of wrasse.

The wrasse home in on the male and his exposed family.

Suddenly, an unlikely hero

comes to the male domino's aid.

A surgeonfish.

His size, weapons and assertive defense

are enough to send the wrasse packing.

The surgeonfish isn't deliberately helping the domino,

he just doesn't want the trespassers

on his patch of reef.

The domino's lawn is on the fringes

of the larger fish's territory.

Far enough to be tolerated,

but close enough to reap the benefits

of the security guard.

The surgeonfish sees off the intruders

and the domino's eggs survive the attack.

The male just needs to protect them

for three more days.

The surgeonfish's large lawn

is close to the steep slopes of the shore.

Twice a day, the changing tide churns the water.

The movement in the water column

often triggers a movement of creatures.

Crabs make the most of their ability

to move freely between reef and shore.

When the low tide exposes the rocks,

they tuck into the algae and other food

that's now out of reach of the solely aquatic species.

Below the waves, the churning waters stir up nutrients,

attracting large shoals of hungry fish.

Among them are foraging parrotfish.

They mostly eat algae

and will munch through anything to get to it.

The parrotfish get their name from their strong beak,

which enables them to power through whole chunks of coral.

Everything goes in.

The edible is processed and whatever they can't digest

comes out the other end.

In such great numbers, this attack looks destructive,

but parrotfish play a curious role in defending the reef.

By clearing an area like this,

parrotfish level the playing field

and increase diversity on the reef.

By removing the dominating algae

and fast-growing corals,

they give slow-growing species, like hard corals,

the chance to develop.

Beneficial or not,

they're still trespassing on the surgeonfish's algal lawn.

He turns on his side,

flashing his yellow fin markings

as a reminder of his secret-blade weapons.

The surgeon keeps close tabs on who's on his patch.

He's mostly solitary,

but does allow female surgeonfish

to share parts of his territory.

It's times like this when roomies are a good idea.

They work together to clear the hungry intruders.

But there are some grazers that just won't budge.

The green turtles have found his stash of algae

that's too good to miss.

And they won't be moved.

Try as he might,

the surgeonfish just can't persuade them to leave.

(Waves roar and crash)

The fish are constantly

waging war with their neighbors

and defending their resources.

But one individual's efforts have paid off.

Our fearless father's vigil is at an end.

After a week on guard duty, his eggs have hatched.

The tiny baby domino fish drift out into the current.

There are too many dangers on the reef itself

for the microscopic fry.

Their chances of survival are increased

by floating amongst the plankton

out in the vast tracts of ocean.

After a few weeks,

they will smell their way back to the reef.

But the male's parental duties are done.

He makes one final sweep of his lawn,

then swims off to re-join the other domino fish...

..and the protection of the shoal.

A multitude of fish use this technique

of schooling over the reef for defense.

When fish school, they seem to turn with a sixth sense,

without bumping into each other.

The fluidity of their movements

is mostly based on sight.

They watch their neighbors closely

and when their neighbors move, they do, too.

Their ability to school might also be helped

by the lateral line along their sides.

This fluid-filled canal in their skin detects vibrations

and changes in water pressure,

which might also help them to space evenly

from one another.

Moving as one vast mass makes it hard for a predator

to pick out an individual.

Hidden among the coral, the moray eel watches and waits.

The sneaky yellowfin hind waits with him.

Then they set out to hunt.

Together.

It's seldom-seen behavior,

combining the eel's ability to squeeze into tight spaces

with the hind's lightning-fast reactions.

For potential prey, this is double trouble.

Anything trying to escape the hind risks swimming

straight into the jaws of the eel.

Anything flushed from the reef by the eel

may get picked off by the hind.

They could be competitors,

but teaming up creates greater success for both.

Observations show each performs better

with their partner by their side.

This level of co-operative hunting

between two different species

is extremely rare.

With neighbors like this,

the pistol shrimp's going to need his protective bunker...

..and the sharp eyes of the goby.

Hidden among the coral branches, a family reunites.

The goby's eggs hatched long ago and the babies dispersed.

But now they've returned from their time in the plankton

as fish fry by using their sense of smell

to guide them back to the coral.

The young citron gobies can settle into their new homes.

While the gobies look for sanctuary in the coral,

the baby domino fish also return from the plankton

and seek protection among the long spines of a sea urchin.

It's a perfect nursery.

The tiny juveniles live up to their name,

looking like little dominoes.

It will take several years

for them to be sexually mature

and fade to the dull gray color of the breeding adults.

As they get older, they will become more aggressive and territorial

and ready to tend a lawn of their own.

Just like their father.

They will learn to keep an eye out for predators,

like the moray eel, the guitarfish...

..and the hungry gangs of wrasse.

These hunters often seem like the bad guys,

but they're vital

for keeping populations of reef fish in check.

Creatures of the reef walk a fine line

between success and failure,

using adaptations for attack and defense.

Their relationships forge cunning tricks

and powerful alliances.

Here, the brave and hardworking thrive,

defending themselves, their family

and even protecting the health of the reef itself.