Coral is often mistaken for a plant due to its stationary nature and reliance on sunlight, but it is unequivocally classified as an animal. This misconception arises from the complex way coral obtains its nutrition, combining the traits of an animal predator with a partnership involving plant-like organisms. The biological evidence clearly places coral within the Animal Kingdom, supported by its cellular structure, feeding mechanism, and life cycle.
The Definitive Biological Classification of Coral
Coral belongs to the phylum Cnidaria, which includes creatures such as jellyfish and sea anemones. Within this phylum, corals are classified into the class Anthozoa, meaning “flower animals” due to their radial symmetry and tentacled appearance. The individual animal unit is called a polyp, a small, sac-like body anchored to the seafloor or to a communal skeleton.
Animals are defined by heterotrophic feeding, meaning they must consume organic carbon for energy. Coral polyps are equipped to do this using specialized stinging cells called nematocysts embedded in their tentacles. When small prey, such as zooplankton, drift past, the polyp extends its tentacles and fires these microscopic, venomous harpoons to immobilize and capture the food. The prey is then guided into the central mouth opening to be digested in the gastrovascular cavity.
The coral life cycle also confirms its animal status through its mobile larval stage. Coral colonies reproduce sexually by releasing gametes into the water, which fertilize and develop into a tiny, free-swimming larva known as a planula. This larva is covered in tiny, hair-like cilia that allow it to actively swim through the water column for days or even months. The planula eventually settles onto a suitable hard surface, where it undergoes metamorphosis into a polyp, establishing a new, sessile colony.
The Symbiotic Relationship That Causes Confusion
The confusion regarding coral’s classification stems from a remarkable partnership it maintains with single-celled algae called zooxanthellae. These algae, a type of dinoflagellate, live within the coral polyp’s tissues, giving the coral its characteristic color and contributing to its stationary, light-dependent nature. This relationship is a form of mutualism, where both the animal host and the algal symbiont benefit significantly.
The zooxanthellae perform photosynthesis, using sunlight, water, and the carbon dioxide produced by the coral to create energy-rich organic compounds. Up to 90% of the sugars, glycerol, and amino acids generated by the algae are transferred directly to the coral polyp, providing the bulk of the animal’s nutritional needs. This energy subsidy allows reef-building corals to thrive in the clear, nutrient-poor waters of the tropics, acting almost like a plant by converting solar energy into food.
In return, the coral polyp offers the algae a protected environment within its gastrodermal cells. The polyp also supplies the zooxanthellae with carbon dioxide and waste products from its own metabolism, which the algae require to fuel their photosynthetic process. When corals are stressed by factors like high water temperature, they expel these algae, causing the coral to turn stark white, a phenomenon known as coral bleaching.
Anatomy and How Coral Builds Reef Structures
Individual coral polyps function as part of a unified colonial organism. The polyps are genetically identical and remain physically connected by a thin sheet of tissue called the coenosarc. This living connection allows the entire colony to share nutrients and coordinate its activities.
Hard, or stony, corals build the rock-like formations that characterize coral reefs. These corals are defined by their ability to precipitate calcium carbonate, or limestone, from the seawater. Each polyp secretes this hard, skeletal material beneath its base, forming a protective cup called a corallite. Over many generations, as the polyps multiply through asexual budding, they continuously deposit new layers of calcium carbonate, building vertically and horizontally.
The continuous process of calcification creates a complex calcium carbonate structure that is the foundation of the entire reef ecosystem. In contrast, soft corals, which belong to the subclass Octocorallia, do not produce a rigid limestone skeleton. Instead, they form a flexible, internal structure composed of a protein called gorgonin, often reinforced by small, spiny pieces of calcium carbonate called sclerites. Soft corals do not contribute significantly to the reef-building architecture that stony corals create.