The water in the ocean is saline or salty. Coral polyps require saline water. They cannot live in parts of the ocean where there is water that has little or no salt at all, known as freshwater . Usually, freshwater flows into the ocean from the land via rivers and streams. Since corals polyps do not like freshwater, they do not grow near rivers or streams flowing into the ocean.
Coral polyps require sunlight.
As we learned in Lesson 10, during a process known as photosynthesis sunlight and excess coral polyp wastes combine to promote the growth of tiny plants called zooxanthellae-algae. This algae is an important food source for coral polyps. Without sunlight, algae could not grow depriving the coral polyps the needed food. In order for sunlight to reach corals underwater, the ocean water must also be clear and clean. If ocean water is muddy or murky, sunlight cannot reach the corals and algae food cannot be produced.