Given the task definition and input, reply with output. Given a scientific passage and an answer, generate a question for the given answer.

Passage: Corals form large colonies in shallow tropical water. They are confined to shallow water because they have a symbiotic relationship with algae that live inside of them. The algae need sunlight for photosynthesis, so they must stay relatively close to the surface of the water to get enough light. Corals exist only as polyps. They catch plankton with their tentacles. Answer: symbiotic
Coral and the algae living inside of them have what type of relationship, since the algae relies on the coral to stay close to the water's surface?