Acropora
Stony thorny coral Acropora settled in the middle of the soft coral Xeniidae . The reefs grow from hard corals, not from soft ones.
Stony or hard corals are animals that live on the seabed. Each separate animal is a polyp of a few millimeters size. They form colonies and grow hard calcium sceleton of CaCO 3 . The polyps reproduce by budding and remain near each other. So a hard coral grows from the seabed like a crystal. Each coral species is of specific form. When polyp dies its home-sceleton remains. New polyps attach to this building. Many structures of hard corals are collected in natural museums. Most beautiful are even in shops.