Coral is often mistaken for a rock or a plant, but it is actually a complex living organism that is part animal, part mineral, and part plant-like algae. The large structures known as reefs are vast underwater cities created by millions of tiny animals living together in colonies. Understanding coral requires separating the structure into its three distinct components: the hard foundation it builds, the soft animal that secretes it, and the microscopic life that sustains the whole system. This unique biological partnership allows these organisms to thrive in nutrient-poor tropical waters and construct the most diverse ecosystems on the planet.
The Structural Foundation: Calcium Carbonate
The solid, rock-like material that forms the backbone of a coral reef is a mineral compound known as calcium carbonate. This substance is secreted by the coral animal in a crystalline form called aragonite, creating a hard external skeleton. The hard corals, also called stony or scleractinian corals, are the primary architects of reef structures, constantly laying down new layers of this mineral beneath themselves. Each individual animal sits within a cup-shaped depression in this skeleton, which is built up over centuries by successive generations of colonies.
The growth rate of this skeletal material varies widely; some branching species increase in length by up to ten centimeters per year, while massive, dome-shaped corals grow much slower. Soft corals, by contrast, do not build a single rigid structure. Instead, they rely on small, spiky internal pieces of calcium carbonate, called sclerites, for minimal support within their flexible tissue.
The Living Animal: The Coral Polyp
The living part of the coral structure is the polyp, a small, sac-like invertebrate that is related to sea anemones and jellyfish. Coral polyps are individual animals, though they live connected to one another within a colony by a thin sheet of tissue. Each polyp has a cylindrical body with a single mouth opening, which functions for both food intake and waste expulsion.
Surrounding the mouth is a ring of tiny tentacles used to capture food from the water column. These tentacles are armed with specialized stinging cells called nematocysts, which can paralyze small prey like zooplankton. This carnivorous feeding behavior is an important supplemental source of energy for the coral. The polyp’s soft body tissue is housed within the protective calcium carbonate cup, allowing it to retract for safety when threatened.
The Symbiotic Partner: Zooxanthellae
The immense productivity of coral reefs is made possible by a remarkable partnership between the coral animal and microscopic, single-celled algae called zooxanthellae. These golden-brown dinoflagellates live within the gastrodermal tissues of the coral polyp, a relationship that is mutually beneficial. The coral provides the algae with a protected environment and essential compounds like carbon dioxide and nitrogenous waste from its own metabolism.
In return, the zooxanthellae perform photosynthesis, converting sunlight into energy-rich compounds such as glucose, glycerol, and amino acids. This process is so efficient that the algae transfer up to 90% of the organic material they produce directly to their coral host, satisfying the vast majority of its energy needs. The presence of these algae is also what gives most corals their vibrant colors.
This delicate balance is disrupted when the coral is stressed by environmental changes, most notably elevated water temperatures. When this happens, the coral expels the zooxanthellae from its tissues, causing the coral to turn stark white in a phenomenon known as coral bleaching. While the coral animal can survive briefly by capturing plankton, prolonged bleaching can lead to starvation and death without the primary energy source provided by its symbiotic partners.