Stony corals are the primary architects of tropical reefs, constructing the immense, complex structures that form the foundation of these underwater cities. The reef is not the living coral tissue itself, but rather the hard material secreted by countless generations of coral polyps. This dense, rock-like material serves as the structural framework for one of the most biodiverse ecosystems on the planet. Understanding this structure is paramount to appreciating the scale of the coral reef ecosystem. This article will define the structure, explain the biological processes that enable its creation, and discuss the threats currently jeopardizing its integrity.
The Calcareous Skeleton
The hard, external structure secreted by the coral colony is generically called the skeleton, but its component parts have specific scientific names. The entire skeletal mass of a coral colony is technically known as the corallum. This material is a crystalline form of calcium carbonate (CaCO3) known as aragonite, which is continuously deposited beneath the living coral tissue.
The skeleton of a single coral polyp is termed a corallite, a cup-shaped enclosure where the soft-bodied polyp rests and into which it can retract. Inside the corallite, vertical plates called septa radiate inward, providing the base structure for the polyp’s body plan. The porous skeletal material that connects the corallites across the entire colony is called the coenosteum, binding the thousands of skeletal cups into a single, cohesive structure.
The Biological Process of Calcification
The creation of the corallum occurs through calcification, or biomineralization, performed by the living coral polyp. The tissue layer responsible for this construction is the calicodermis, a specialized layer of cells that rests directly on top of the existing skeleton. This calicodermis creates a tiny, confined space, known as the calcifying space, where it regulates the chemical environment.
In the calcifying space, the coral actively pumps calcium ions (Ca2+) and carbonate ions (CO32-), derived from seawater, to precipitate the aragonite. The process is tightly controlled and requires the coral to manage the acidity within this localized area. Specifically, the coral must expel hydrogen ions (H+) to increase the pH in the calcifying space, which favors the formation of carbonate ions needed for the CaCO3 crystal growth.
A significant facilitator of this process is the symbiotic relationship between the coral and the microscopic algae living within its tissues, known as zooxanthellae. During photosynthesis, the zooxanthellae consume carbon dioxide (CO2) produced by the coral’s metabolism. By removing CO2 and increasing the pH within the coral tissue, this photosynthetic activity dramatically enhances the coral’s ability to efficiently deposit the calcium carbonate skeleton. This light-enhanced calcification explains why reef-building corals are restricted to shallow, clear, sunlit waters.
Ecosystem Function of the Reef Framework
The collective skeletal mass of the reef framework performs functions that extend far beyond supporting the coral polyps. Over millennia, the accumulation of countless coralla creates a massive, three-dimensional structure that serves as the primary physical architect of the entire reef ecosystem. This structure is often referred to as the “rainforests of the sea” due to the immense biodiversity it supports.
The complex network of caves, crevices, and branches within the reef framework provides shelter, nursery grounds, and foraging habitat for approximately 25% of all marine species. This structural complexity is directly responsible for concentrating biological activity, which drives the health of local and regional fisheries.
The reef framework also serves a function in coastal protection by acting as a natural breakwater. The structure can dissipate a high percentage of incoming wave energy, with some studies estimating up to 97% reduction in wave force. This wave attenuation shields shorelines from erosion and flooding, making the physical presence of the reef a defense against storm surges and hurricanes for coastal communities. Furthermore, the reef stabilizes sediments along the shoreline, helping to maintain beach integrity and the health of other adjacent ecosystems like mangroves and seagrass beds.
Threats to Skeletal Integrity
The structural stability of the aragonite skeleton is jeopardized by changes in ocean chemistry, primarily driven by the absorption of atmospheric carbon dioxide (CO2). This uptake leads to ocean acidification, where the CO2 reacts with seawater to form carbonic acid, lowering the ocean’s pH. The increased acidity reduces the availability of carbonate ions, the building blocks corals require to construct their skeletons.
Corals must expend significantly more energy to pump hydrogen ions out of their calcifying space and gather the necessary carbonate ions from the increasingly acidic water. This stress leads to a reduced calcification rate, resulting in skeletons that are less dense and more susceptible to physical damage or bioerosion. If ocean acidification becomes severe, the surrounding seawater can become corrosive, causing the existing CaCO3 skeleton to dissolve.
The integrity of the skeleton is also indirectly compromised by thermal stress, which triggers coral bleaching. When ocean temperatures rise above a certain threshold, the coral expels its symbiotic zooxanthellae, losing its primary source of energy and the photosynthetic boost to calcification. While a bleached coral is not immediately dead, the lack of energy significantly slows or stops skeletal growth and repair. This leaves the structure vulnerable to breakage and erosion until the coral can recover the symbiotic algae. The combination of reduced growth and increased dissolution threatens to shift coral reefs from net calcium carbonate accumulation to net loss.