Cities in the Sea
They may be small, but they build big things! Coral polyps (珊瑚虫), which live in the warm, shallow parts of the Earth’s oceans, are probably the biggest builders on the planet. Coral polyps turn calcium from seawater into a hard material called limestone. Slowly, they build up a hard skeleton (骨架) around their bodies. When polyps die, their skeletons remain. Young polyps attach themselves to the old skeletons and make new skeletons. Over time, weird and wonderful shapes are slowly built up into amazing coral reefs (珊瑚礁).
Scientists sometimes think of coral reefs as underwater cities. A quarter of all known marine species live in reef habitats―there are nearly a thousand coral species. Reefs are also home to millions of sea creatures, like fish, crabs, turtles, and sharks.
Humans don’t live in coral reef cities, but we benefit from them. Reefs create jobs for people in the fishing industry and other related businesses. Coral reefs are also popular for divers一many countries benefit from the tourists that they attract. Lastly, chemicals from reef creatures help scientists create new medicines, which help doctors treat different illnesses.
Coral reefs are very important, yet we don’t take good care of them. Environmental problems have already killed about twenty percent of the world’s reefs. About half of the remaining reefs are dying, and experts believe all of Earth’s coral reefs will be in danger by 2050.
Why are the reefs in such trouble? For one thing, people catch too many reef fish and often damage the reefs—divers sometimes break off pieces of coral.
Polluted water also causes problems because reef-destroying algae grows in dirty water. Even air pollution hurts coral reefs. Global warming causes warmer ocean water, which can cause polyps to lose helpful algae. Without that algae, coral turns white. This is called "bleaching”，and if it continues, the coral dies.
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