Coming up next on Jonathan Bird’s Blue World, Jonathan explores the world of coral reefs
from the Caribbean to Australia!
Hi, I’m Jonathan Bird and welcome to my world!
Coral reefs are one of the most important inhabitants of shallow tropical oceans.
A coral reef may look like a bunch of colorful rocks, or perhaps some unusual-looking plants,
but it's actually a strange primitive animal.
Each coral animal, called a polyp, is only about the size of a pearl.
It’s little more than a mouth with some tentacles around it.
The tentacles are used to catch plankton to eat.
Coral is closely related to another tentacle-equipped animal: the jelly.
If you can imagine a small jelly stuck to the bottom that can’t swim, that’s a coral
polyp.
But unlike jellies, coral polyps grow together in colonies called a coral head.
This coral head has several hundred polyps.
A bigger one has thousands!
Beneath the living skin of the coral is a skeleton made of calcium carbonate.
A dead piece of coral is heavy like a rock, because the calcium carbonate is limestone—a
type of rock.
This heavy limestone base anchors the colony so waves and storms can’t move coral reefs.
The creation of limestone is slow.
It takes hundreds of years for a coral head to reach the size of this brain coral.
And in spite of the fact that it looks like a brain, coral has no brain.
Or eyes.
Some corals are not hard as a rock.
There are many species of soft corals.
Some look like bushes or fans.
These are often called gorgonians, sea whips or sea fans.
Others look like pastel works of art.
Soft corals grow anchored to the bottom and look to most people like plants.
But up close, you can see the coral polyps.
Instead of a hard skeleton, these soft corals have tiny splinters of calcium carbonate inside
called spicules.
They allow the coral to bend and flex with the waves.
When many coral colonies grow together, the result is a reef.
It’s a living, growing structure that provides nooks and crannies for everything from invertebrates
to fish.
A reef is like the buildings in a city, providing housing and protection for thousands of animals.
When a reef forms, it creates new habitat for other animals.
Maybe it starts out as just a small coral head with only space for a few fish.
But as it grows over many years, more and more marine life flocks to the reef until
it is a thriving metropolis.
Of course, all those small fish living around the reef attract larger fish like jacks, barracudas
and even sharks looking for a meal.
The reef provides a foothold for algae and sponges.
And that in turn attracts sea turtles to eat the sponges.
And it all starts with just one tiny coral colony.
So much life in the tropical oceans depends on coral reefs.
Coral reefs can grow quite large.
The largest one in the world is the Great Barrier Reef off northeastern Australia, which
is over 1500 miles long!
It’s so big it can be seen from space!
Coral reefs live all around the world in tropical waters.
They can’t live in cold water because the coral is unable to create calcium carbonate
if the water gets too cold.
Unfortunately, there is not much plankton in tropical water.
That’s why it’s so nice and clear.
But at night, plankton comes up from cooler, deeper water and that’s when the coral feeds.
Unfortunately, most of the time, coral still doesn’t catch enough plankton to survive.
So it has worked out a great partnership.
Many corals have a greenish or brownish color because they have tiny microscopic algae living
in their skin called zooxanthellae.
The zooxanthellae make energy from the sun’s rays.
In exchange for having a nice place to live in the skin of the coral, they share some
of their energy with the coral.
So the coral is partially solar powered.
As long as they grow in nice shallow, sunlit water, solar powered corals don’t need to
catch much plankton.
This is why reefs form in shallow water.
Sometimes I see coral that is turning white as if it’s losing its color.
This is called bleaching.
When corals are subjected to environmental stress, such as high or low water temperatures,
changes in salinity or pH, siltation, or even just too much sun from a really low tide,
they start to turn white.
This happens because the zooxanthellae starts to die.
Since corals get their color from the zooxanthellae, when the zooxanthellae goes away, so does
the color.
And if it goes on for too long, the coral dies because it needs the energy from the
zooxanthellae.
Since most coral is as hard as a rock, very few animals can actually eat it.
But a few can.
The crown of thorns sea star dines on the outer skin of the coral.
An outbreak of crown-of-thorns sea stars can wreak havoc on a reef.
All that remains of the coral after a crown-of-thorns attack is a dead white skeleton.
Parrotfish eat coral too.
They take big crunchy bites out of the reef, and grind up the calcium carbonate as they
chew.
The parrotfish have a tough beak that’s as hard as steel for biting the reef.
And the hard particles that they excrete later end up as sand on the beach.
It’s true--tropical beaches are made partly of fish poop.
So the next time you are relaxing on a tropical beach or snorkeling in shallow water looking
at the fish, you have coral to thank.
It’s a very simple animal that can’t walk or swim, can’t see, and has no brain—but
nearly all life in shallow tropical oceans depends on coral reefs.