TAILIEUCHUNG - Toefl ibt internet based test 2006 - 2007 part 78

Tham khảo tài liệu 'toefl ibt internet based test 2006 - 2007 part 78', ngoại ngữ, toefl - ielts - toeic phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 023 024 025 Q26 20 EXPLANATORY OR EXAMPLE ANSWERS AND AUDIO SCRIPTS FOR MODEL TESTS MODEL TEST 2 599 But how do these little polyps build such impressive reefs Well hard coral secrete a shell of calcium carbonate around their bodies. The potyp isn t hard you see but the shell is. And these shells are the material that forms a coral reef. So a coral reef is just a colony millions and millions of coral animals whose shells are connected. And reproduction is really the basis for the construction of a large reef. You see as each potyp matures it converts the calcium and other minerals in ocean water to a hard limestone exoskeleton called a corallite. And this is fascinating. Although the polyps themselves don t appreciably increase in size they continue to build new shells periodically um connecting them with . with partitions. Now coral can reproduce sexually through an activity called mass spawning. During one night in the spring when the moon is full coral polyps release egg bundles that contain both eggs and sperm. Most polyps have both male and female reproductive cells. The egg bundles are round about half the size of marbles I would say. They re brightly colored in orange or red or pink and they float up to the surface to form a thick layer of uh. well think of them as rather fragrant beads. So with the water so saturated with them predators will only be able to devour a small number compared with the huge number that will survive and break open. The sperm cells swim away to fertilize the eggs from another bundle. So. once fertilized the little egg begins to mature from a coral larva to a planulae which can swim for a few hours days or even a few weeks. Ultimately it locates a hard surface on which to attach itself and from which it will not move for the rest of its life except for the movement involved in the process of building a new. neighboring shell as . as it continues to mature. But actually sexual reproduction isn t the way that coral reefs are really .

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