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Pic#1 is a close-up of a single polyp under UV light Pic#2 is a close-up of several polyps under UV light Pic#3 is the whole colony under UV light Pic#4 is the whole colony under regular LED light A coral "group" is a colony of myriad genetically identical polyps. Each polyp is a sac-like animal typically only a few millimeters in diameter and a few centimeters in length. A set of tentacles surround a central mouth opening. An exoskeleton is excreted near the base. Over many generations, the colony thus creates a large skeleton that is characteristic of the species. Individual heads grow by asexual reproduction of polyps. Corals also breed sexually by spawning: polyps of the same species release gametes simultaneously over a period of one to several nights around a full moon.
An UV light was used to stimulate photosensitive pigments in the hard corals, which then emit fluorescent colours. Scientists suspect that it's a defence against being bleached by the sun.