What Are Coral Polyps? A Complete Guide to These Marine Creatures

Coral polyps are a collective term for many kinds of organisms in the class Anthozoa. Their bodies are cylindrical in shape, with eight or more tentacles, and a mouth at the center of the tentacles. They mostly live in colonies, joining together into a group. They are marine cylindrical coelenterates: food enters through the mouth and food waste is expelled through the mouth. They live by preying on tiny plankton in the ocean, and during growth they can absorb calcium and carbon dioxide from seawater, then secrete limestone, which becomes their own shell for survival. Coral is the secretion of coral polyps and forms the supporting structure of the polyps.

Coral polyps are tiny, with many small tentacles around the mouth used to capture tiny organisms in the ocean. They can absorb minerals from seawater to build a shell that protects the body. Coral polyps exist only as polyp-type individuals, shaped like a hollow cylinder, with the lower end attached to the surface of an object and the top end bearing a mouth surrounded by one or more rings of tentacles. The tentacles are used to collect food and can extend to a certain degree; they bear specialized cells (nematocysts), and when stimulated, the nematocysts evert their threads to paralyze prey with the stinging threads.

Coral polyps generally live in colonies, and each individual is connected to the others by a structure called coenosarc. The coenosarc part can secrete a horny or calcareous exoskeleton. The tentacles of coral polyps are very small and all grow around the mouth. When seawater passes through the digestive cavity, the food and calcium in it are absorbed by the polyps. Inside the bodies of coral polyps live algae that live together with them; these algae live on the waste excreted by the polyps and at the same time provide oxygen to the polyps. The algae need sunlight and a warm environment to survive, and the higher the coral piles up, the more favorable it is for the algae to survive.

Coral polyps reproduce rapidly and grow quickly in tropical regions. The old ones keep dying and the new ones keep growing, and the skeleton also increases and expands accordingly. Sand accumulates to form a tower, from small to large, and over a long period of time they become massive coral reefs and coral islands. The Dongsha Islands and Xisha Islands in the South China Sea of China, the Maldives in the Indian Ocean, Fiji in the South Pacific, and the world-famous Great Barrier Reef were all built by tiny coral polyps.

The eggs and sperm of coral polyps are produced by the gonads on the mesenteries and are released through the mouth into seawater. Fertilization usually occurs in seawater, and sometimes also in the gastrovascular cavity. Usually fertilization occurs only between eggs and sperm from different individuals. The fertilized egg develops into a ciliated, free-swimming planula larva that can swim. After several days to several weeks, it settles on a fixed surface and develops into a polyp-type body. Coral polyps can also reproduce by budding; after the bud forms, it does not separate from the original polyp, and new buds keep forming and growing, thus forming a colony. When new polyps grow and develop, the old polyps below die, but their skeletons remain on the colony.