Coral

Coral animals belong to the cnidarian group, cousins of jellyfish and sea anemones. Each coral polyp is a tiny tube with tentacles that capture plankton. Polyps live together in colonies and secrete calcium carbonate skeletons that stack upward through time, forming reefs that can stretch for hundreds of kilometers.

Most reef-building corals host microscopic algae called zooxanthellae inside their tissues. The algae share sugars made through photosynthesis, while the coral provides shelter and nutrients. Because the algae need sunlight, reefs thrive in clear, shallow tropical water between 18 and 30 °C.

Reefs create complex habitats. Branching corals shelter fish, massive boulders protect turtles, and sandy lagoons support seagrass beds. Coastal communities rely on reefs for food, tourism, and natural breakwaters that soften storm waves. Scientists call reefs the "rainforests of the sea" because they host such high biodiversity.

Stressful conditions cause corals to expel their algae, turning the colony white in a process called bleaching. Heat waves, pollution, sediment runoff, and ocean acidification can trigger bleaching or slow skeleton growth. Crown-of-thorns starfish outbreaks and destructive fishing practices also damage reefs.

Protecting coral requires reducing greenhouse-gas emissions, managing coastal development, and establishing marine protected areas. Citizens can help by using reef-safe sunscreen, avoiding anchoring on corals, and supporting reef restoration projects that plant nursery-grown coral fragments back onto damaged reefs.