Coral reefs are built by and made up of thousands of tiny animals—coral “polyps”—that are related to anemones and jellyfish. Polyps can live individually (like many mushroom corals do) or in large colonies that comprise an entire reef structure.

Most coral polyps have clear bodies. Their skeletons are white, like human bones. Generally, their brilliant color comes from the zooxanthellae (tiny algae) living inside their tissues. Several million zooxanthellae live and produce pigments in just one square inch of coral. These pigments are visible through the clear body of the polyp and are what gives coral its beautiful color.

They live in symbiosis with polyps.
When the temperature of the water raise for even 2 degrees for longer time, they stop feeding polyps and they start starving.