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They may seem like an underwater paradise, but coral reefs are in fact perpetual battlegrounds – for space. In these crowded, frenetic communities, every individual, even the corals, have to fight for it.
Coral larvae drift through the open-sea floating in a soup of minute marine animals. When one of these larvae settles in a suitable spot and survives, a new reef is founded. It takes just a few days for a coral larva to change form and become a polyp. Identical copies then bud off and gradually a colony develops. Each separate polyp surrounds itself with a hard skeleton that can grow at a rate of up to fifteen centimetres a year.
Coral feed at night and each polyp uses its stinging cells to draw plankton into its mouth. None of the corals however could lay down their stony skeletons at the rate they do without help, as they simply could not acquire enough food to grow that fast. The help comes from millions of single celled algae, called zooxanthellae, which live within the coral tissues.
Corals frequently overgrow each other - and that means trouble. When they get too close corals can detect each other chemically. The aggressor polyps then extrude their guts and simply digest their rivals alive.
Corals provide the basis on which the entire reef community survives. Many creatures live within the coral and some will even protect their coral hosts from attack, such as the shrimps in the game. For others however the coral provides food. Parrotfish for example have jaws so powerful that they can bite through rock and coral alike in their quest for algae and, in doing so, play a large part in the erosion of a reef. The consumed rock and coral will emerge later from the parrotfish as a fine sand which contributes to the tropical beaches we find so alluring.