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Where better to look for an answer to this question than BBC’s Blue Planet II series? As David Attenborough explains, coral reefs are “undersea cities, crammed with life… they are complex, infinitely-varied structures providing all kinds of homes for their many residents.” These cities, though populated by thousands of different animals and plants, rise into being on the back of one animal in particular: reef-building coral. Related to jellyfish and sea anemones, corals have been an oceanic staple for the last 450 million years; existing long before dinosaurs who arrived on the scene roughly 247 million years ago. >Reefs are formed when coral polyps (explored below) build calcium-carbonate skeletons that protect from predators and create structures upon which other coral polyps can attach themselves. By constructing these limestone matrices, the coral is also creating an environment in which many other animals and plants can flourish. Incredibly, although coral reefs only account for less than 1% of the ocean floor, an estimated 25% of marine species call these ecosystems home. Unsurprisingly, coral reefs are some of the most diverse ecosystems on earth. Scientists estimate that these reefs could house millions of undiscovered organisms and species, in addition to numerous natural compounds that could be used in treatments for illness and disease. For the reefs’ inhabitants, however, this is nothing new. As captured in an episode of Blue Planet II, bottlenose dolphins have been taking advantage of the Gorgonian corals’ antiseptic qualities for some time now; rubbing their bodies against the corals’ fronds to prevent infection and teaching their young to do the same. As mentioned earlier, coral reefs are made up of calcium carbonate structures that are built by hundreds or thousands of coral polyps. As reef-building coral continue this construction, coral polyps inhabit the small “cups” on its surface. Together these coral colonies exist on an ever-expanding structure that itself is built upon the compacted skeleton of old coral from thousands of years ago.