Cities Beneath the Waves
Written by Studio AM.
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A shallow tropical coral reef looks like a colorful rocky garden, but living animals build it. A coral colony contains tiny soft-bodied polyps. Many reef-building polyps make hard calcium-carbonate skeletons. As generations grow on older skeletons, the reef structure rises.
Inside many shallow reef-building corals live microscopic algae. The algae use sunlight to make food and share some products with the coral. In return, they receive shelter and materials they need. This partnership helps explain why these corals thrive in clear, sunlit water.
Not all corals live this way. Deep-sea corals grow far beyond sunlight and lack the same photosynthetic algae. They capture food carried by currents. Some form large habitats, although their shapes and growth differ from familiar tropical reefs.
When shallow tropical water stays too warm, a coral may expel its algae and turn pale, an event called bleaching. A bleached coral is alive, but it has lost an important energy source and may starve if stressful conditions continue. If conditions improve, a bleached coral may regain its algae and recover.
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Questions
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Question 1 of 4
The passage is mainly about
The answer is C: how shallow reef-building corals depend on algae while deep-sea corals live differently
The passage explains the shallow partnership, contrasts deep-sea corals, and returns to bleaching.
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Question 2 of 4
What do algae provide to many shallow reef-building corals?
The answer is D: Products of food made using sunlight
The algae use sunlight to make food and share some of those products with the coral.
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Question 3 of 4
Why can deep-sea corals live without sunlight?
The answer is A: They capture food carried by currents instead of relying on photosynthetic algae
The third paragraph contrasts their current-borne food with the shallow algae partnership.
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Question 4 of 4
In this passage, “expel” means
The answer is B: push out
During bleaching a shallow coral expels, or pushes out, its algae.
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