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.