Stony corals, also known as scleractinian corals, are marine invertebrates important to ocean ecosystems. Often mistaken for plants or rocks, these animals construct a durable, hard skeleton. Composed primarily of calcium carbonate, this framework forms the underlying structure of some of Earth’s most diverse underwater habitats.
The Polyp and Its Skeleton
Each stony coral begins as a tiny individual animal called a polyp, a few millimeters in diameter. A polyp has a cylindrical body with a mouth surrounded by tentacles that capture small food particles. Its base secretes a cup-shaped calcium carbonate structure, known as a corallite, which provides protection and support.
Microscopic algae called zooxanthellae live within the coral polyp’s tissues. This mutually beneficial relationship involves zooxanthellae performing photosynthesis, converting sunlight into energy-rich compounds. Up to 90% of these nutrients are transferred to the coral host, fueling its growth and energy needs. In return, the coral provides the algae with a protected environment and access to carbon dioxide and nutrients, byproducts of the coral’s metabolism needed for photosynthesis. This symbiotic relationship also contributes to the vibrant colors of many stony corals.
Skeleton building, called calcification, involves the polyp extracting calcium and carbonate ions from seawater. Corals expend energy to create a localized environment for rapid deposition of calcium carbonate, forming the hard skeleton. The layering of this skeletal material beneath the polyp allows for vertical and radial growth, incrementally building the coral structure.
From Colony to Reef
While some stony corals are solitary, most are colonial organisms, forming complex structures from thousands of genetically identical polyps. These colonies expand primarily through asexual reproduction, known as budding. In budding, a new polyp grows from an existing one or an original polyp divides into two, remaining attached to the shared skeleton. This process creates a multi-polyp colony from a single founding polyp, sharing a common calcium carbonate framework.
Stony coral colonies exhibit various growth forms, influenced by species traits and environmental conditions like water movement and depth. Common shapes include branching forms, such as staghorn corals, which create intricate, three-dimensional thickets. Massive or boulder corals, like brain corals, grow as large, compact structures, offering resilience against strong currents. Other forms include plating, encrusting, and foliaceous structures, all contributing to the architectural diversity of coral reefs.
These diverse growth forms collectively build the complex, three-dimensional framework of a coral reef. As coral polyps secrete their skeletons and new polyps bud, the remains of dead polyps contribute to the reef’s foundation, providing a platform for subsequent coral growth. This accumulation of skeletal material over long periods forms the underwater structures that provide shelter, foraging grounds, and nurseries for a wide diversity of marine organisms.
Reproduction and Expansion
Stony corals use two primary methods to reproduce and expand their populations. Sexual reproduction, particularly broadcast spawning, is a widespread strategy for long-distance dispersal and genetic diversity. During these events, which often occur synchronously across many species, corals release eggs and sperm into the water. These gametes float to the surface, where fertilization forms free-floating larvae called planulae.
Planula larvae drift with ocean currents, sometimes for days or weeks, allowing for wide dispersal. Once a larva encounters a suitable hard surface, it settles, attaches, and metamorphoses into a single polyp, initiating a new colony. This method ensures genetic mixing among different parent colonies, enhancing the population’s resilience and adaptability.
Asexual reproduction through fragmentation provides another means for local expansion and survival. Fragmentation occurs when a piece of a coral colony breaks off, often due to physical disturbances like storms or impacts. If this detached fragment lands on a suitable substrate, it can reattach and grow into a new, genetically identical colony. This process is common in branching and plating coral species, whose delicate structures are prone to breaking yet capable of regenerating.
Vulnerability to Environmental Change
Despite their robust skeletal structures, stony corals are sensitive to changes in their marine environment. A significant threat is coral bleaching, which occurs when corals experience stress, primarily from elevated sea temperatures. When water temperatures rise above a coral’s tolerance, the symbiotic zooxanthellae within its tissues are expelled. This expulsion reveals the coral’s white calcium carbonate skeleton, leading to the term “bleaching”.
Without their zooxanthellae, corals lose their primary source of nutrition, as up to 90% of their energy comes from the algae’s photosynthesis. While corals can survive short-term bleaching by relying on stored energy or capturing food, prolonged bleaching often leads to starvation and widespread mortality. Large-scale bleaching events, intensified by phenomena like El Niño, have caused degradation to reefs globally.
Another major threat is ocean acidification, a direct consequence of increased carbon dioxide (CO2) absorption by the oceans from the atmosphere. When CO2 dissolves in seawater, it forms carbonic acid, which lowers the ocean’s pH. This reduction in pH decreases the availability of carbonate ions, the building blocks corals need for their skeletons. Corals must expend more energy to acquire these scarcer ions, making it harder for them to grow and maintain their skeletal structures. This impedes the growth of existing colonies and can hinder the settlement and growth of new corals, thereby compromising the long-term integrity of reef ecosystems.