Coral have a symbiotic relationship with what in order to eat?

Most have a symbiotic relationship with photosynthetic algae, called zooxanthellae.

The corals have a relationship with algae (called zooxanthellae) that live in the tissues of the corals. The coral provides protection for the algae and the algae provides the coral with the compounds they need for photosynthesis

essentially, one bacterial cell splits into two daugurer cells. bacterial offspring don't need or get care from their parents because they are in a sense their own parents. that said, some bacteria can and do depend on other bacteria especially in harsh environments such as bacterial microfilms in hot water tanks.