Academic Reading — All-Types Test 16

Reading passage

How Coral Reefs Build Themselves A A coral reef looks, at first glance, like a single vast structure of stone, yet almost all of it is the accumulated work of tiny animals no larger than a pinhead. These animals are coral polyps, soft-bodied creatures related to sea anemones and jellyfish. Each polyp is little more than a hollow column of tissue crowned with a ring of stinging tentacles, and each one draws dissolved minerals from the surrounding seawater to build a protective cup of limestone beneath its body. Thousands of polyps may live side by side in a single colony, all descended from one founder that divided again and again, and it is the slow piling-up of their millions of limestone cups, layer upon layer over thousands of years, that produces the ridges, walls and lagoons we recognise as a reef. B The chemistry behind this construction is deceptively simple. Seawater carries calcium and carbonate ions, and the polyp combines them to make calcium carbonate, the same hard compound found in chalk and seashells. A single polyp secretes this material at its base, gradually cementing itself to the skeletons left behind by earlier generations. When a polyp eventually dies, its stony cup remains and becomes the foundation on which new polyps settle. In this way the living tissue of a reef forms only a thin skin over a huge mass of accumulated skeleton; the reef is, in effect, a graveyard that continues to grow. The particular shape a colony takes, whether a delicate branching thicket or a rounded boulder, depends both on the species and on the strength of the currents and waves it must withstand. C None of this would be possible without an unlikely partnership. Living inside the tissues of most reef-building corals are enormous numbers of microscopic algae called zooxanthellae. These algae capture sunlight and, through photosynthesis, convert it into sugars, a large share of which passes to the coral host and supplies much of the energy the polyp needs. In return, the coral offers the algae shelter and a steady supply of the waste compounds they require. This mutual exchange explains why reef-building corals thrive only in clear, shallow, sunlit water: below a certain depth there is simply not enough light for the algae to sustain their partner, and where the water is clouded by sediment the same problem arises. The relationship also lends corals their vivid colours, for the pigments belong largely to the algae rather than to the animal itself. D The dependence on algae carries a serious risk. When the surrounding water grows too warm, the coral expels the algae living within it, losing both its colour and its principal source of food. The pale, weakened coral that results is described as bleached. A brief episode of warmth may be survived if conditions soon return to normal, but a prolonged one can starve the colony until it dies. Because the algae are so central to the coral's survival, even a rise of only one or two degrees above the usual summer maximum, sustained for several weeks, is enough to trigger widespread bleaching across a reef. Reefs that bleach repeatedly, with too little time to recover between events, are the ones most likely to be lost altogether. E Growth, in any case, is a patient business. Under favourable conditions the fastest branching corals may extend by more than ten centimetres in a year, but the massive, boulder-like species that form the sturdiest parts of a reef often add less than a centimetre in the same period. A reef structure hundreds of metres thick therefore represents an archive of accumulation stretching back many thousands of years. Set against this slow rhythm, the pressures now bearing on reefs seem abrupt. Warming seas, storms of increasing force, and the gradual acidification of the ocean, which makes the carbonate ions harder for polyps to obtain, all act far faster than a reef can rebuild. Whether these ancient structures can keep pace with such rapid change is a question that marine scientists are only beginning to answer.