A coral skeleton serves as the hard, supportive framework for individual coral polyps, which are tiny, soft-bodied animals. This external structure provides protection and the necessary foundation for the polyp to grow. Each individual coral polyp builds its own skeleton, forming a cup-shaped depression called a corallite where the polyp resides.
What Coral Skeletons Are Made Of
The primary material composing coral skeletons is calcium carbonate (CaCO3). This substance is found in a crystalline form known as aragonite, which is also present in eggshells and limestone. Corals extract calcium ions (Ca²⁺) and carbonate ions (CO₃²⁻) directly from the surrounding seawater to form this mineral.
How Corals Build Their Skeletons
The process of skeleton formation, known as calcification, involves the coral polyp secreting calcium carbonate. This occurs in a specialized space between the coral’s cells and the surface of its existing skeleton, called the sub-calicoblastic space. Coral polyps actively pump hydrogen ions (H⁺) out of this calcifying space, which increases the concentration of carbonate ions, allowing them to bind with calcium ions to form calcium carbonate.
Symbiotic algae, called zooxanthellae, live within the coral’s tissues and play a significant role in this process. These algae perform photosynthesis, converting inorganic carbon dioxide into organic compounds. A large portion of these energy-rich compounds, known as photosynthates, are then transferred to the coral host. This energy fuels the calcification process, allowing corals to build their skeletons more efficiently, especially in nutrient-poor ocean environments.
The Skeleton’s Foundation for Reefs
The accumulation of countless coral skeletons over vast periods forms the intricate, three-dimensional structures known as coral reefs. Each coral polyp continuously secretes layers of calcium carbonate beneath its body, causing the skeleton to grow larger. As new polyps form through asexual reproduction, they build upon the remnants of previous generations, gradually constructing massive reef frameworks.
These solidified skeletons provide the structural backbone for entire marine ecosystems. They create diverse habitats, offering shelter, protection from predators, and feeding grounds for a wide array of marine life. The complex architecture of coral reefs supports over 25% of all marine species, making them biodiversity hotspots.
Threats to Coral Skeletons
Coral skeletons face threats from changes in ocean chemistry and temperature. Ocean acidification occurs as the ocean absorbs increasing amounts of carbon dioxide (CO2) from the atmosphere. This absorption leads to a decrease in ocean pH, making the water more acidic and reducing the availability of carbonate ions, which are the building blocks for coral skeletons. This makes it harder for corals to form and maintain their calcium carbonate skeletons, making them more vulnerable to damage.
Rising ocean temperatures also pose a threat, leading to coral bleaching. When water temperatures become too warm, corals expel their symbiotic zooxanthellae algae. This expulsion causes the coral to turn white and lose its primary food source, weakening the coral and impairing its ability to calcify. While corals can sometimes recover from bleaching, prolonged or frequent events can lead to their death, ultimately impacting the integrity and growth of their skeletons and the reefs they form.