A coral reef is a giant, colorful underwater structure built in the ocean. It is home to more animals than almost anywhere else on Earth, acting like an enormous apartment building made of stone. Reefs create shelter and safety for millions of sea creatures, making them some of the busiest neighborhoods deep below the waves.
Coral is an Animal, Not a Rock
Many people think coral is a type of rock or a sea plant, but it is actually an animal. Coral is made up of thousands of small, squishy creatures called polyps. Polyps are related to jellyfish and sea anemones, looking a bit like tiny, upside-down jellyfish with a ring of stinging tentacles around their mouth.
Each polyp anchors itself to the ocean floor and uses its tentacles to catch tiny bits of food floating in the water. To keep safe from predators, the polyp creates a hard, protective skeleton around its soft body. This skeleton is made of calcium carbonate, which is the same material found in chalk or seashells.
Building the Coral Neighborhood
The entire coral reef structure grows because polyps live in large groups called colonies. When a polyp dies, its hard calcium carbonate skeleton remains attached to the reef, acting as a foundation for a new polyp to settle upon. Over hundreds and even thousands of years, these tiny stacked-up houses form the massive, rock-like structures we recognize as coral reefs.
The polyps have a special partnership with a type of microscopic algae called zooxanthellae. These tiny algae live inside the coral polyps’ tissues. Using sunlight, the zooxanthellae perform photosynthesis, which makes food that the coral polyps use for up to ninety percent of their energy needs. This team effort allows the coral to grow fast enough to build huge reefs, and the algae also give the coral its vibrant colors.
The Busy Underwater City
A coral reef is a lively ecosystem that supports around twenty-five percent of all ocean life. The reef’s many holes, branches, and crevices offer a maze of hiding spots and safe nurseries for countless animals. This dense habitat serves as a cafeteria for some creatures and a safe refuge for others seeking protection from predators.
Specific animals rely completely on this habitat, such as the clownfish, which hides safely within the tentacles of sea anemones that live on the reef. Blue tangs and parrotfish graze on the algae growing on the coral, which helps keep the coral clean and healthy. Larger animals also use the reef as a resting or hunting ground, including sea turtles and small reef sharks that patrol the edges looking for food.
Taking Care of the Reef
Coral reefs are important because they protect coastlines from strong waves and storms, acting as a natural barrier. They also provide habitats for many of the fish that people around the world rely on for food. However, these delicate neighborhoods are vulnerable to changes in the ocean environment.
When the ocean water gets too hot, the stressed polyps will push out their colorful zooxanthellae partners, causing the coral to turn completely white. This event is called coral bleaching, and if the water stays too hot for too long, the coral can get sick and die. Dirty water from land, which carries things like soil and chemicals, can also block the sunlight the zooxanthellae need to make food.
Even small actions can help protect the reef. Reducing the amount of plastic you use prevents trash from harming ocean wildlife. Choosing to walk or ride a bike instead of driving helps keep the air cleaner, which helps keep the ocean from getting too warm.