A coral reef is an animal structure built by tiny organisms called coral polyps. This confusion often arises because, unlike most animals, reef-building corals are stationary and possess components that function like plants. Each individual polyp is a miniature animal that forms colonies, constructing the collective limestone skeleton of the reef over centuries. The plant-like appearance is due to photosynthetic algae living within the animal’s tissue.
The Coral Polyp: An Animal Classification
The living part of the coral, the polyp, is classified within the Animal Kingdom under the Phylum Cnidaria, making it a relative of jellyfish and sea anemones. A coral polyp is a small, soft-bodied invertebrate with a cylindrical shape and a mouth surrounded by tentacles. These tentacles are armed with specialized stinging cells called nematocysts, which the polyp uses to capture small prey.
This method of feeding, known as heterotrophic nutrition, proves the coral is an animal because it consumes other organisms for energy. The polyp captures zooplankton and suspended organic matter, pulling it into its central gastrovascular cavity for digestion. While this feeding provides supplemental energy, the polyp’s existence is heavily dependent on a second source of nutrition.
A single polyp is typically minute, ranging from one to three millimeters in diameter, but it is connected to a colony of thousands of genetically identical individuals. These colonial animals anchor themselves to the non-living limestone structure they secrete, remaining fixed in one place. Their sessile nature and reliance on light-driven energy often confuse observers into thinking they are aquatic flora.
The Symbiotic Engine That Creates Confusion
The plant-like characteristics of coral stem from a partnership with a single-celled algae called Zooxanthellae. These microscopic organisms, a type of dinoflagellate, live within the coral polyp’s gastrodermal tissues. The algae perform photosynthesis, converting sunlight, water, and carbon dioxide into energy-rich compounds.
The coral animal benefits from this mutualistic relationship, receiving up to 90% of its required organic nutrients from the internal algae. In return, the polyp provides the Zooxanthellae with shelter and a steady supply of compounds needed for photosynthesis, including carbon dioxide and nitrogenous waste. This highly efficient recycling system allows coral reefs to flourish in the clear, nutrient-poor waters of the tropics.
The vibrant colors of reef-building corals are due to the pigments of the Zooxanthellae living within their transparent tissues. When the coral animal is stressed by high water temperatures or other environmental changes, it expels the algae, causing the colony to turn stark white in an event known as coral bleaching. This expulsion severely limits the coral’s food supply, demonstrating the profound dependence of the animal on its plant-like partner.
Understanding the Reef: Structure Versus Organism
It is important to distinguish between the living coral organism and the structure it creates. The term “coral reef” refers to the entire geological formation, which is built by generations of polyps.
Reef-building corals, known as scleractinians, continuously secrete a hard skeleton made of calcium carbonate (aragonite). This limestone skeleton forms a cup-shaped structure beneath each polyp, providing protection and a base for the colony to grow.
As the polyps grow, they lift their living tissue away from the older skeleton, leaving the hardened calcium carbonate below. Over thousands of years, the accumulation of these skeletons from countless individual polyps forms the enormous, complex structure of the coral reef.
The reef is essentially the collective, non-living architectural foundation that the living animal colonies inhabit. The continuous process of calcification means that the reef is constantly being built upward and outward by the living coral polyps on the surface. While the reef is an ecosystem that hosts many plants, the reef-building material itself is a mineral structure produced by an animal.