There are two ways in which coral reefs grow.
First, they grow by secreting more limestone around the base of their cup. This creates the limestone skeleton that grows outward and upward.
Second, some coral polyps reproduce new polyps by creating clones using asexual reproduction, while others procreate sexually using eggs and sperm.
Once new polyps appear, they float to the bottom of the sea floor and attach to a solid surface, either starting their own colony or joining a pre-existing colony. Polyps connect to each other at their base and are connected laterally using a tissue called coenosarc. Polyps combined with this tissue make up the living portion of the coral reef that can be seen by the naked eye.
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