Electrical Aptitude Practice Test 7

Building a Reef, One Polyp at a Time

¹A coral reef may look like a pile of colorful rock, but it is built by animals. Reef-building corals are colonies of tiny, soft-bodied animals called polyps. Each polyp produces a hard skeleton of calcium carbonate, the same mineral found in limestone. As old polyps die, new ones grow on top of their skeletons, and over many generations the colony slowly builds a massive stony structure.

²Most reef-building corals depend on a partner. Living inside the coral's tissues are microscopic algae called zooxanthellae. The coral gives the algae a protected home and supplies the carbon dioxide and water they need. In return, the algae use sunlight to make sugars and proteins, passing as much as 90 percent of the organic material they produce to the coral. The coral uses these products to grow and to build its skeleton. Because the algae need light, reef-building corals live in clear water with little suspended material. When corals are stressed, the polyps expel their algae and the colony turns stark white. This is called coral bleaching, and a coral that stays bleached too long can starve and die.

³Reefs grow slowly. Massive, boulder-shaped corals add only 0.3 to 2 centimeters per year, while branching corals may grow up to 10 centimeters per year. A reef can take up to 10,000 years to form, and large barrier reefs and atolls can take from 100,000 to 30 million years.

⁴Scientists group reefs into three main types. Fringing reefs, the most common type, grow seaward directly from the shore. Barrier reefs also follow a shoreline but lie farther out, separated from the land by a lagoon of open, often deep water. Atolls are rings of reef, usually circular or oval, around a central lagoon. In 1842, Charles Darwin proposed that the three types could be stages of one process. A fringing reef forms around a volcanic island; as the island slowly sinks, the coral keeps growing upward, becoming a barrier reef with a widening lagoon. When the island finally sinks below sea level, only the ring of coral, an atoll, remains.

⁵Darwin's idea was tested more than a century later. In 1952, scientists drilled deep holes at Enewetak Atoll in the Pacific Ocean. The drills passed through reef limestone and reached volcanic rock more than 4,000 feet below the surface. Because reef corals grow only in shallow, sunlit water, such a thick layer of reef rock meant the island had sunk while the reef kept building, just as Darwin had predicted.