BIOLOGICAL NOTES
In the clear and shallow waters of the tropical seas, where the light is plentiful and the temperature never dips below 16 degrees, vast rocky constructions can be found. The walls of this reef have been built from calcium carbonate by successive generations of living creatures, and though they look like rock, they are in fact formed by millions of corals, tiny organisms that, despite appearances, are very much part of the animal kingdom. Each coral polyp, a kind of minuscule jellyfish, surrounds itself with a calcium carbonate exoskeleton that provides its home. All that can be seen of this animal are its tiny tentacles, which stretch out from the stone branches, and are deployed or folded, depending on the time of day. All the polyps in the colony play their part in building the coral, and each species has its own shapes and sizes. There are branching bushes, rounded blocks, massive forms, and fragile ramifications. Over thousands and thousands of years, they gradually construct gigantic reefs. The most spectacular example is the Australian Great Barrier Reef, which stretches for some 3,000 km. By analogy, coral looks rather like a tree, with the polyps its leaves, and the massed exoskeletons forming the branches.
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