This is one of the most ancient and chaotic settings on the planet.
There are about 650 known species of fish living here.
And animal life outnumbers plant life as it does nowhere else on earth.
The place is dominated by predators.
It is densely packed with living things.
Even the rocks are alive.
St. Croix, the U.S. Virgin Islands, 1500 miles southeast of Florida in the Caribbean Sea.
The shoreline, protected by spectacular coral reefs.
This is their story.
Below us, the reef itself.
Next to it, ribbons of white sand.
And beyond that, to the left, meadows of seagrass.
Where the reef protects the shore, mangrove trees can grow.
Further inland, there's a tidal salt pond.
These varied habitats form a united and intricate system whose keystone is the reef and whose
sustenance is the sea.
The water is clear, with few of the nutrients and minerals that make cold northern waters
so turbid.
Here creatures live in the highest density of almost any known environment.
Under these unique conditions, life has had to solve the universal problems of concealment,
survival, and reproduction.
The reef is a city at the core of a system of habitats.
Nearest the reef are the outskirts, a no-man's land of sand flats.
Beyond the sand flats, outlying meadows of seagrass.
Each of these three zones, the reef, the sand flats, and the grass beds, has its roster
of inhabitants, each playing a part in a drama of coexistence.
We begin with the reef and its predators.
The great barracuda cruises the midwater above the reef.
He hunts at dawn and dusk, a gray shadow against gray water.
The moray eel is generally nocturnal and spends its days lurking in caves.
It breathes by swallowing water through powerful jaws lined with needle-sharp teeth.
The yellowtail snapper is the journeyman hunter of the reef by day.
They may not look as ferocious as barracuda and moray, but this kind of fish is the most
numerous and ever-present of the carnivores.
The system is tyrannized by predators of all kinds and sizes, and their prey has evolved
in genius patterns of escape and defense.
These dwarf herring travel in schools.
As individuals, they'd be exposed, but there's safety in numbers.
This school gives even the most agile predator, only a phantom target.
Blue tangs are larger and less vulnerable.
Each one is armed with sharp spines, but even they school for protection against the cruising
barracuda in the distance.
Herring also intimidates other fish in whose territories the tangs trespass.
This spiny blenny survives by living in a tiny hole.
It will leave only to settle territorial disputes.
Even within the safety of the hole, they are alert for trouble.
A half an inch long, it watches for plankton that may drift close enough to be snatched
and eaten.
Some creatures specialize in excavation.
There may be no other way to hide.
The mantis shrimp can dig a burrow in the sand at the base of the reef in a matter of
seconds.
It has ten legs for digging and eight basket-like arms for carrying.
With the obvious importance of keeping out of sight, why are so many reef fish brightly
colored?
These ostentatious four-eye butterfly fish are not what they seem.
Their true eyes are disguised by black lines.
They have false eye spots at the base of their tail fins.
It's a subterfuge that could confuse a predator.
The gray angelfish, like the butterfly fish, has a large profile.
When he turns, the change in silhouette might confound an attacker.
The blunt, rudder-like body gives them a short turning radius and great maneuverability.
The bold patterns on these Atlantic spadefish work as camouflage as they swim in the dappled
light.
They also help them identify each other.
The few fish that survive to reach full size are then too big to be eaten by all but the
largest predators.
The habits of this juvenile angelfish are part of a pattern of cooperative behavior
that would seem as surprising as bright coloration.
It usually feeds by plucking small organisms from the surfaces of coral rock, and it also
feeds by helpfully plucking parasites from the skins of other fish.
This blue-striped grunt encourages cleaning, but for that he must remain immobile and exposed
to danger in open water.
Blue chromis and creoles are groomed by the small yellow juvenile wrasse.
Such exposure is a gamble, but parasites could carry a greater risk of death than predators.
Inland from the reef are sand flats that seem as barren of life as they are of shelter.
But certain creatures can turn the drawbacks of the place into advantage.
Goatfish feed together in small schools, but not without sometimes challenging each other
over feeding rights.
Goatfish have stiff barbels below their jaws for churning the sand and flushing small creatures
from it.
Working in a group gives them a better chance of finding hidden patches of prey.
Following them are opportunists, waiting to snatch whatever the goatfish stir from the
sand.
A couple of inches long, this crab is too big for the goatfish in company.
But now it has to worry about the powerful, crushing jaws of this porcupine fish.
Creatures not only dig into the bottom, they sometimes even look like it.
But the sand can be littered with rubble or seaweed.
As the flounder moves, it alters its color.
Through its central nervous system, it controls the color of pigment cells in its skin.
So the patterns of the bottom are automatically reproduced as the flounder sees them.
The sand is crowded with buried creatures.
Eyeset eyes allow this one to see while concealed.
The lancer dragonet is only half an inch long, a bottom dweller, a poor swimmer, but perfectly
camouflaged until it flashes a brilliant dorsal fin.
Why they do it is a mystery.
It may frighten or confuse predators.
It may give them a fake target, like the butterflyfish's false eye, showing its colors, then vanishing
into the sand.
The sand that hides the prey also hides the hunters.
The lizardfish is motionless for hours, then rockets from the sand after unwary foragers.
If it misses, it returns to its ambush with a shudder.
A scorpionfish, another hazard.
Under it, a juvenile puffer, a poor swimmer without protective coloration or a hole to
hide in, unable to dive into the sand and without the safety of a school, vulnerable.
There is a constantly shifting balance of advantage between predator and prey, maintaining
an uneasy standoff.
Instead of being the sort of creature that makes life easy for hunting fish, the puffer
is a creature this scorpionfish will regret it ever saw.
A useful defense has to be good enough to protect against a slow learner that might
try the same thing again.
In slow motion, he lunges, sucking in the puffer.
The puffer inflates inside its attacker's mouth.
The scorpionfish almost certainly learns from this experience.
He got an unforgettable mouthful.
For when the puffer inflates, it does the same thing as this closely related, much bigger
porcupinefish.
Spikes that normally lie flat are erected when the fish inflates.
But even this defense is not foolproof.
A big fish like a shark will sometimes eat them spikes and all.
Hermit crabs have evolved with a hard body shell that is incomplete, but they have solved
their lack of armor through behavior.
They live in cast-off shells.
When this hermit crab outgrows its current shell, he finds a suitable new home, investigates
it and moves in.
He'll stay here for a few months till he outgrows it.
There is a family of fish with bony plates that form a rigid box armed with spikes above
the eyes and near the tail fin.
The cowfish is another impossible mouthful.
Like the puffers, this fish has substituted armor for speed.
Hardly able to move its jaws, the cowfish and its close relative, the trunkfish, feed
by squirting a jet of water into the sand, uncovering small hidden creatures.
They feed over the sand by day and move into the reef at night.
They are well armored and produce poisons from lip glands.
They have little to fear from most predators.
A sea cucumber writhes along the edge of the flats.
It has a double protection.
It is camouflaged and toxic.
It gleans algae and organic debris from grains of sand.
There are legions of creatures constantly processing the material of the bottom.
The sea cucumber's sticky tentacles carry grains into its central mouth.
If it weren't for this continuous flow of sand through various digestive tracts, the
bottom might be covered with a thick mat of algae.
Beyond the inner city of the reef and next to the sand are outlying grass beds.
The grass is safe only for the small, the well-concealed and the well-protected.
Wild fish like the dwarf wrasse and well-covered ones like this blenny.
Caribbean lobsters have only small pincers.
This one gropes through the roots and probes the rubble, looking for small crustaceans
or mollusks like this quarter-inch snail.
So small its shell would do it little good against the lobster's jaws.
This is as good a place to hunt as it is to hide.
The grass scorpion fish is well camouflaged like its cousin on the sand flats, and it
too hunts from ambush.
A
survival strategy that some vulnerable creatures have is living inside a well-defended animal.
One of the most impregnable of these is the common urchin of the grass bed, an animal
that bristles with short, stout spines.
Among the defenses of these urchins lives a shrimp.
It mimics both the shape and coloring of the one-inch spikes.
An apparent specialist, this shrimp is found virtually nowhere else.
There appears to be nothing in it for the urchin, but the shrimp finds shelter here
as well as food.
Debris gets caught in the spines and the quarter-inch shrimp picks it off with its tiny claws.
Somehow the shrimp knows it belongs with this black and white urchin and probably spends
its entire life here.
There are also white urchins.
Their shrimp are white too.
Neither kind of shrimp is ever found with the host urchin of the other.
The conch is the largest and best protected of the snails here, growing to ten inches
or more in length.
They have among the best of all invertebrate eyes.
Each eye is at the end of a separately controlled flexible stalk.
At the end of its trunk-like proboscis is a mouth.
It has an abrasive tongue that rasps the algae and other organisms from the blades of grass.
This small one-inch cardinal fish has a special relationship with the conch.
By day it shares the security of the mollusk's home.
This fish seems to live nowhere else.
By night it leaves the shelter of the shell to feed.
If the conch moves during the night and the fish loses it, another conch will do just
as well.
The fish will defend its borrowed home against all others.
Some of the island's shores are not protected by the reef.
This environment is so harsh that very little can survive here.
But a few members of some familiar families thrive in spite of the continuous pounding.
The urchins living here are closely related to those from the calm waters of the grass
beds, but they are only an inch or two in diameter.
Like their cousins in the grass beds, they provide some of the only shelter around.
The nine-lined goby lives under these urchins and almost nowhere else.
A thankless parasite, it eats the tender tube-like feet of the urchin.
The urchin's spines provide shelter from predators, but more important from the environment itself.
With a set of five chisel-like teeth, the urchin excavates a depression in the rock.
It settles in and holds on.
Underneath the nine-lined goby makes its home.
It isn't always alone here.
Occasionally, the green-banded goby will take up residence under the same patron, but without
devouring its feet.
It wriggles to safety unhurt by the spines.
When the waves recede, there's always a small reservoir for the tenants under the urchin.
This rocky shore is demanding because it is exposed.
It's on a point of land jutting beyond the reef.
Where the waves are breaking is a massive barrier against the power of the sea.
And here is the story of how it grew.
The coral reef, the metropolis at the heart of this system, is more than just inert rock.
The reef itself is alive, constructed by a minute master builder, the organism coral.
Most coral grows in colonies made of many individuals, each one a small circle called
a polyp.
This is how a colony begins.
This minute organism called a planula is one of many offspring produced by an individual
coral polyp.
As it grows, six sections become discernible.
It will test different surfaces for the best place to start a new colony.
It often settles on a piece of coral rock, cleaned by grazing fish.
When the planula settles, it is smaller than a pinhead, about two weeks old and changes
shape as it grows.
Coral is both plant and animal, and that's the secret of its success.
Inside each coral individual live plant cells.
They are the yellow-brown spots seen in this top view.
They photosynthesize by day, and their byproduct is used as food by the coral for rapid growth.
In turn, the plant cells, seen here in close-up, thrive on the coral's nitrogen-rich output.
So both recycle each other's waste products.
It's mutualism, an arrangement from which each benefits.
It's a month old now, and this rare time-lapse view shows us the young polyp over a period
of several hours.
This is how an individual polyp begins its life as a result of sexual reproduction.
Once established, it spreads through asexual reproduction, cloning itself to form a colony
of thousands of genetically identical individuals.
Each one is a tiny predator an eighth of an inch across, consisting of stinging tentacles
surrounding a central mouth.
As each one grows, it forms its own compartment by manufacturing the calcium carbonate mineral
aragonite.
A honeycomb of rock is built up, giving shelter to each polyp and structure to the colony.
During the day, the tentacles withdraw, and the coral's algae cells photosynthesize.
By night, the tentacles extend to feed on plankton.
Some kinds of coral, like this sea fan, are flexible, don't manufacture rock, and look
more like plants, but these, too, are colonies of individual polyps.
All corals begin the same way, whether they are soft or rock-producing, reef-building
ones, like brain coral, pillar coral, and many others.
This reef is an immense structure reaching all the way to the surface.
On its crest grow organisms best adapted for intense sunlight and exposure to the air and
the surf.
Just below the surface, fast-growing elkhorn coral.
It needs bright light and rapid water circulation.
Its branches grow several inches a year.
Each jagged tree rises six or seven feet from its base, and may be a hundred years old.
Forty feet down, moving seabird.
Below the elkhorn is the reef terrace, dominated by slow-growing mound corals, and divided
by parallel channels leading toward deep water.
Fifty feet.
The channel drops, approaching the vertical wall.
Seventy feet.
Know the depth where fast-growing corals do well.
Sponges grow thick on the drop-off wall.
Ninety feet.
The few kinds of coral that grow here are the deepest of the reef-building corals.
They spread wide plates to collect the dim light that filters down.
One hundred feet.
The mouth of a cave that will lead upward within the reef itself and back to the reef
terrace.
This entire structure has been built by countless generations of primitive masons, converting
seawater and sunlight to rock at the rate of a few grams a year for each individual,
but eight tons a day for any given square mile of coral reef.
Back on the reef terrace in forty feet of water.
More than sixty-five species of coral live side by side, constructing an edifice larger
than any that man has ever built.
But there is also intense competition for space among corals, and they will attack each
other.
The slow-growing brain coral on the left, behind the little mushroom coral, is more
aggressive than the fast-growing star coral on the right.
Watch the place where the two mounds touch.
The coral on the left senses that the other has grown too close.
The slow one reacts, sending out white filaments to attack the trespasser, which is powerless
to prevent the assault or counterattack.
This is a time-lapse sequence taken over a period of forty-eight hours.
This small coral, the same as the aggressive one we just saw, has left a scar on its large,
less aggressive neighbor.
How corals identify each other is still a puzzle.
They observe a rigid pecking order.
Each kind will only attack less aggressive ones.
Within forty-eight hours, the filaments have devoured the living tissue of the intruder,
exposing the bare rock skeleton.
Corals may wage this slow-motion warfare on each other, but together they face a common
threat in the form of ever-present plant life.
Growing fast with intense light, this algae can quickly cover any surface.
The entire reef could be overgrown and killed, and could eventually look like this if it
weren't for these herbivores, the vegetarians of the reef.
They nip at exposed surfaces and keep the algae under control.
They have to feed constantly.
Their digestive systems lack certain enzymes and are inefficient, so they must overeat
in order to survive.
Parrotfish have front teeth fused into a beak.
Their jaws are so powerful and their teeth so hard they can gouge the rock itself.
This one is covered with white scars.
Parrotfish are one of the many creatures that manufacture the sand that covers the bottom.
They take bites of rock and algae and grind it with an extra set of internal jaws.
They digest the food, and what they excrete is carbonate sand made of pulverized coral
and shell.
The long-spined sea urchin, cousin to those in the grass beds and the surf zone, is a
plant eater too.
It stays close to the coral by day, gripping it tightly.
Its bristling defense is almost impregnable.
The only vulnerable spot is below, a soft underbelly around a mouth with five teeth.
The ruffled slug, a snail without a shell, is another plant eater.
It has a double use for its diet.
First, it draws nourishment from the algal cells it eats as a juvenile.
Then the slug strips them of their green chloroplasts, storing them in the leaf-like convolutions
on its back.
The chloroplasts do for the adult slug what symbiotic algae does for coral.
The byproducts of photosynthesis contribute to the growth of the slug.
It can spend less time foraging because it gets free energy from a built-in solar collector.
Turbivores are nomadic grazers and keep the reef terrace cropped.
But scattered throughout the reef, there are small farms, patches of luxuriant plant life
maintained and guarded by an energetic agriculturalist, the three-spot damselfish.
Within its territory, it encourages the rampant growth of algae.
It will even strip living coral from its rock skeleton, cultivating more space for its crops.
The damselfish never has to travel far to feed.
It eats not only the algae, but also the minute crustaceans, an eighth of an inch long, that
find shelter within it.
All this plant life is attractive to other herbivores.
To protect its farm, the three-inch damselfish has a combative temperament.
It knows the boundaries of its territory and patrols them relentlessly.
It won't even tolerate neighboring damselfish.
Even if a sea urchin gets into the garden, the damselfish is undaunted.
It rushes to the attack, biting off the urchin's spines one by one, forcing as fast a retreat
as any urchin ever has to make.
Sometimes the damselfish are briefly outnumbered.
A school of juvenile wrasse makes a raid on the egg nest of this yellowtail damsel.
Earlier, these were the same wrasse that so helpfully removed parasites from other fish.
The eggs are laid in a mass at the base of the Elkhorn coral and are a favorite of these
marauders.
They withstand the damsel only a few minutes and soon leave for opportunities elsewhere.
As long as the reef is there, so are the lagoon, the grass beds, and the mangroves along the
shore.
The trees prevent erosion of the coastline and provide a complex habitat like that of
the reef itself.
A sluggish tidal creek leads into the swamp.
Mangrove trees grow in salt water, which kills most plants.
But the mangrove absorbs sea water, purifies it internally, and harmlessly gets rid of
the salt through pores in its leaves.
The roots are host to colonies of mussels.
Below the roots, the Cassiopeia jellyfish.
Like its distant relative coral, the Cassiopeia has symbiotic food-producing algae in its
tentacles.
This animal carries its own source of food when there may be no other, or when the competition
is too stiff.
The jellyfish often lies on the bottom, tentacles and algae facing the sun.
The tentacles also have stinging cells.
When not behaving like a plant, the predatory jellyfish can comb the water for plankton.
The thicket of roots along the tidal creek provides the same sort of shelter as the coral
reef.
The place attracts small fish like this cloud of dwarf herring.
It also attracts the predators, like barjacks that hunt here as they do on the reef.
Beyond the mangroves is a shallow salt pond, created long ago by the barrier of trees.
It's still connected with the sea by the tidal creek, so there's a daily replenishment of
this important feeding ground for the island's birds.
The pelicans are there every day, working the schools of baitfish.
Specialized predators, they are dependent on this system.
Other birds like these stilts are migratory, spending winters on this island and summers
in the north on the mainland.
They are only one among many species of wading birds here.
Another year-round resident, a royal tern, dives for fish.
It's as dependent on the water as the pelicans.
Late in the afternoon, the pelicans leave the pond and head for another fishing spot
in the lagoon beyond the mangroves.
Pelicans often feed in shallow water over grass beds where they can scoop up schooling
fish close to the surface.
Adult fish from the reef itself stay away from the perils of the open water of the lagoon
and don't figure in the pelican diet.
The reef creates the pelicans' hunting ground, so they nest nearby where they can watch the
water.
These pelicans live in a rookery among the cactus and thorn trees on a hill above the
reef.
Only two weeks old, this chick's huge beak pouch serves it as an air conditioner.
Rapid breathing cools the blood flowing through the walls of the pouch.
The chicks grow fast. In a month, they have thicked down and begin to learn about their
wings.
In another month, they'll fly like their parents.
Dusk is the time for change and movement on the reef.
There is an increase in traffic as creatures move towards shelter and as others begin to
stir toward a night of feeding.
Sea urchins are the first to move.
Beginning late in the afternoon, they migrate down from the safety of the reef towards the
grass beds, as we see here in time-lapse.
Normally the movements of urchins are almost imperceptible. In time-lapse, their travels
are 500 times faster.
Passing fish are only flickers of light.
The migration continues well past sunset and throughout the night.
Schools of grunts and squirrelfish spend the day resting deep in the recesses of the reef.
When night falls, the entire school moves down toward the nearby grass beds.
Like most fish that feed strictly at night, the squirrelfish is a carnivore.
It is so sensitive to vibration, pressure, and smell that it navigates at night as easily
as by day.
Plankton moves away from the reef where the coral is feeding to the relative safety of
the mid-water. Many are minute crustaceans.
This is a larval crab. Shrimp, a few millimeters long, are a main staple of the diet of carnivores.
An eighth of an inch long, this medusa is a tiny carnivorous jellyfish.
The newly hatched crab larvae is snared, then escapes.
The tinafore, no relation to jellyfish, is bioluminescent. It shines in the dark.
It moves slowly through the water, feeding on plankton propelled by rippling bands of
cilia.
Mute by the failing light, coral polyps emerge as soon as it's dark.
Extended tentacles soften the outlines of the coral.
This parrotfish has returned to the crevice where it usually sleeps at night.
All herbivorous fish are now asleep.
Almost the only fish found at night are carnivores, usually drab in color. Many of them are closely
related to primitive deep-water fish that evolved in conditions of near total darkness.
The teeming, competitive daytime crowd has disappeared, leaving the place deserted by
all but a few.
This trunkfish can be as active at night as it is by day. It just changes its habits and
while it's dark moves into the grass beds to feed.
Corals cover themselves with dead grass for added protection against the night hunters.
The porcupinefish is one of the most common at night.
A thorny balloon it hovers above the grass beds, bristling and virtually immobilized.
The spotted moray is the most common of all eels in these waters. A primitive, snake-like
fish, this one is three feet long.
It has left its cave in the reef to stalk for prey by detecting and following the traces
of their chemical scent in the water.
It's midnight in 50 feet of water, the domain and natural element of the octopus. Solitary,
it hunts by touch, blanketing a coral head, groping with two-foot tentacles for the sleeping
populace of the daytime reef. Its changing color can be a sign of mood and intent. Camouflage
is irrelevant at night, but with its surroundings lit, this one's complexion is kaleidoscopic.
It may even be able to feel and mimic the texture of a surface.
It stalks the reef all night, returning to its cave at first light.
The clock has come full circle. The heat of the sun and the length of the days are constant
in the tropics, but life is complex and chaotic.
It's fostered by the food and shelter of a rich environment maintained by a stable climate.
The interlocking habitats extend well inland. Seaward is the reef, the metropolis, ancient
and teeming. The bewildering diversity of the place obscures its underlying order, but
patterns discerned here have a special validity in understanding all natural systems.
The city is ruled by carnivores that enforce a rigid curfew both night and day. Competitors
maintain a tense and shifting balance. Vegetarians must gorge just to survive, but certain creatures
like coral have built in resources allowing them to conserve energy.
It's a strategy found in all complex habitats, and here it gave rise to an organism that
is neither all plant nor all animal. It is a minute master builder that transforms sunlight
and sea water into a scaffold of stone crowded with life. A city of coral.
It's a city of life, and it's a city of life, and it's a city of life, and it's a city
of life, and it's a city of life, and it's a city of life, and it's a city of life.
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