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A coral is a colony made of small animals, called polyps. The polyps live in symbiosis with algae called zooxanthellae, that make photosynthesis, that's why the corals live only in lighted places. The photosynthesis products of the algae are eaten by the polyps and their waste products are used by the algae to synthesize biochemical compounds.
As they require light, current and warm waters, between the coral species there is a strong competition for space. Even if corals live fixed to the sea bottom, between two colonies that live very close to one another sometimes fierce "battles" can be triggered for conquering a larger space, with more food supply. This way, it was found that the species named fire coral (Millepora alcicornis), an extremely aggressive one living in calm waters under 10 m (33 ft) depth, if it detects the presence of another coral in a range of 30 cm (1 ft) around, it starts the "hostilities".
Special ramifications which have an odd hand shape travel centimeter by centimeter, in a few months, the distance till the neighboring colony, sneak amongst its branches, fix on them and suffocate them. In its fight, the offensive coral does not touch algae, sponges or other bushes of Millepora. The fire coral attacks especially the splendid Gorgonia species, known for their varied and special color patterns. Interestingly, the aggressor attacks only if its "rival" is placed upstream to the current.
Specialists believe Gorgonia releases a chemical, which transported by the water current, is detected by this "shark" amongst corals. Fixing itself on the branches of its adversary allows the Millepora a much easier development as it conquers an already built coral skeleton, on which to found a new colony.
It should be noticed that the fire corals are the most dangerous for scuba divers as in contact with the human skin, they provoke an intense, even if short, pain but also reddening, hives and itches. These effects are due to the defensive cells, called nematocysts that go off, penetrating the skin with some microscopic harpoons which inject venom. If the contact is made with a smaller animal, this can suffer paralysis and even death.
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