Coral reefs

Coral reefs are underwater ecosystems built by colonies of tiny animals called corals, which form symbiotic relationships with zooxanthellae (photosynthetic algae). These structures rank among Earth's most biodiverse environments, supporting roughly a quarter of all marine fish species despite covering less than 1% of the ocean floor.

Corals are actually colonial animals that secrete calcium carbonate skeletons, creating the rigid framework of the reef. The algae living in their tissues provide energy through photosynthesis, while corals provide the algae with nutrients and protection. This symbiotic arrangement enables reefs to thrive in nutrient-poor tropical waters, typically between 23–25°C.

Reefs function as nurseries, feeding grounds, and shelters for countless organisms—from microscopic ions in the water column to large predators. They also protect coastlines from storms and provide sustenance to over half a billion people globally.

However, coral reefs face existential threats. Ocean acidification, warming waters (causing coral bleaching), overfishing, and pollution have degraded roughly half of the world's reefs. The Great Barrier Reef exemplifies both their magnificence and vulnerability.

Related

Marine Ecology, Symbiosis, Biodiversity, Ocean currents, Reefs, Conservation