IELTS Reading: Flow-chart Completion

Coral reefs, often described as the rainforests of the sea, are among the most biologically rich ecosystems on Earth. Understanding how they form requires an appreciation of the relationship between corals, the tiny animals that build them, and the photosynthetic algae known as zooxanthellae that live within their tissues. The process begins with free-swimming coral larvae, called planulae, which are released into the ocean in enormous numbers during mass spawning events. These planulae drift in ocean currents for days or weeks before they encounter a suitable hard substrate — typically bare rock, an existing reef, or a submerged volcanic formation — in shallow, warm, clear water with adequate sunlight. Once a planula identifies a suitable surface, it settles and attaches itself, metamorphosing into a sessile polyp. The polyp begins to secrete a hard calcium carbonate skeleton around its soft body. This process, known as skeletogenesis, is greatly accelerated by symbiotic zooxanthellae that take up residence within the polyp's cells. The zooxanthellae use photosynthesis to produce sugars that provide up to 90% of the coral's energy needs, much of which is channelled into skeleton construction. Over time, the original polyp reproduces asexually by budding, producing genetically identical copies of itself. These daughter polyps add their skeletons to the original, building a colony. Thousands of colonies grow alongside one another, and as they die, their calcium carbonate skeletons remain, accumulating over centuries and millennia. New polyps grow on top of this accumulated skeletal mass, progressively extending the reef upward and outward. Fully developed reefs provide habitat and shelter for thousands of species and act as nurseries for fish species of great commercial importance. They also protect coastlines from erosion by absorbing wave energy.