Figure 30.6: Reef-Building Corals Live Symbiotically with Algal Cells Corals, unlike most animals, are photosynthetic and require sunlight. (A) Coral reefs consist of a diverse array of corals. Each coral is a large colony of hydra-like polyps, which build the coral’s skeleton. (B) Algal cells (often called zooxanthellae) live inside the cells of a polyp’s gastrodermal cell layer. The algae carry out photosynthesis, synthesizing glucose and other nutritionally important molecules for the polyp. Polyps also feed heterotrophically by using stinging cells on their tentacles to capture tiny animals that swim by.