What is a coral reef?
A: A coral reef is an underwater ecosystem characterized by reef-building
corals.
Reefs are formed of colonies of coral polyps held
together by what?
A: Calcium carbonate.
Most coral reefs are built from what?
A: Stony corals, whose polyps cluster in groups.
Coral belongs to the class Anthozoa in the
animal
phylum Cnidaria, which includes what?
A: Sea anemones and jellyfish.
Unlike sea anemones, corals secrete hard carbonate
exoskeletons that do what?
A: Support and protect the coral.
Most reefs grow best in what?
A: Warm, shallow, clear, sunny and agitated water.
Coral reefs first appeared how long ago?
A: 485 million years, at the dawn of the Early Ordovician, displacing the
microbial and sponge reefs of the Cambrian.
Sometimes called
rainforests of the sea, shallow coral
reefs form some of Earth's most what?
A: Diverse ecosystems.
They occupy less than 0.1% of the world's ocean area,
about half the area of France, yet they provide a home for what?
A: At least 25% of all marine species, including fish, mollusks, worms,
crustaceans, echinoderms, sponges, tunicates and other cnidarians.
Coral reefs flourish in ocean waters that provide what?
A: Few nutrients.
Where are they most commonly found?
A: At shallow depths in tropical waters, but deep water and cold-water coral
reefs exist on smaller scales in other areas.
Coral reefs have declined by how much since 1950?
A: 50%, partly because they are sensitive to water conditions.
They are under threat from what?
A: Excess nutrients (nitrogen and phosphorus), rising ocean heat content and
acidification, overfishing, sunscreen use, and harmful land-use practices.
Coral reefs deliver ecosystem services for what?
A: Tourism, fisheries and shoreline protection.
When were most of the coral reefs formed?
A: After the Last Glacial Period when melting ice caused sea level to rise
and flood continental shelves.
Most coral reefs are less than how old?
A: 10,000 years.
As communities established themselves, the reefs grew
upwards, pacing what?
A: Rising sea levels.
Reefs that rose too slowly could become what?
A: Drowned, without sufficient light.
Coral reefs are found in the deep sea away from
continental shelves, around what?
A: Oceanic islands and atolls.
Most of these islands are what?
A: Volcanic in origin.
Others have tectonic origins where plate movements did
what?
A: Lifted the deep ocean floor.
In The Structure and Distribution of Coral Reefs, who
set out his theory of the formation of atoll reefs, an idea he conceived
during the voyage of the Beagle?
A: Charles Darwin.
He theorized what?
A: That uplift and subsidence of Earth's crust under the oceans formed the
atolls.
Darwin's theory followed from his understanding that
coral polyps thrive in the tropics where the water is agitated but can only
live within a limited what?
A: Depth range, starting just below low tide.
Where the level of the underlying earth allows, the
corals grow around the coast to form fringing reefs and can eventually grow
to become a what?
A: A barrier reef.
A fringing reef can take ten thousand years to form,
and an atoll can take up to how long?
A: 30 million years.
What are the two main variables determining the
geomorphology, or shape, of coral reefs?
A: The nature of the substrate on which they rest, and the history of the
change in sea level relative to that substrate.
The approximately 20,000-year-old Great Barrier Reef
offers an example of how coral reefs formed on what?
A: Continental shelves.