The tentacles contain stinging cells called nematocysts. When an animal brushes against the nematocyst a spear-like projection shoots out and captures the animal. Corals need the sunlight and clear, shallow water so that the zooxanthellae in their tissues can photosynthesize. In waters that are either too deep or too murky, the zooxanthellae does not get enough light to photosynthesize. Corals come in many shapes and sizes.
Corals get their common name from how they look.
The shape of a coral is determined both by how they build their skeleton and the environmental factors with which they must cope. In shallow areas with high surf you find corals with low, sturdy forms like lobe, rice, blue, and cauliflower coral. Branching, delicate corals such as shelf and finger coral are found in calmer or deeper waters. Coral reefs face a number of threats, both natural and human made. Natural factors affecting reefs include hurricanes and storms that can quickly destroy large areas of the reef.