Coral live in a mutualistic relationship with zooxanthellae (“zooks”)—photosynthetic algae-like protozoa that live inside the epidermal cells of the coral. The two species share nutrients. Zooks also raise the pH of the coral cells slightly, which helps the coral lay down its coral skeleton. Though they come in a tremendous variety of shapes and colors due to species differences and even growing conditions, all coral have the same basic body plan: the polyp—a tube-like structure attached at the base to a substrate. At the top of the tube are stinging tentacles that trap food floating by, which is taken in by the mouth and sent to the digestive sac.