Stony corals, with their intricate and often colorful formations, are living animals that form the foundation of coral reefs. Their presence supports diverse marine life and performs functions that extend beyond the ocean, impacting human societies globally.
Defining Stony Corals
Stony corals, also known as hard corals or Scleractinians, are marine invertebrates belonging to the phylum Cnidaria, a group that also includes jellyfish and sea anemones. Their defining characteristic is the ability to secrete a hard, external skeleton composed of calcium carbonate, commonly known as limestone. This skeletal material forms a protective cup, or calyx, in which the individual coral animal, called a polyp, resides. While some stony corals are solitary, most are colonial, meaning they live in large groups of genetically identical polyps that share a connected skeleton, forming expansive structures over time.
The Living Structure: Polyps and Symbiosis
Each individual coral polyp has a soft, cylindrical body topped with an oral disc and a ring of tentacles surrounding a central mouth. These tentacles contain stinging cells, or nematocysts, used to capture microscopic plankton and small organisms for food, typically at night. Beyond external feeding, polyps build their calcium carbonate skeletons by extracting calcium and carbonate ions from the surrounding seawater, a process called calcification that allows the coral to grow and maintain its hard structure.
Stony coral biology includes a symbiotic relationship with microscopic algae called zooxanthellae, which are a type of dinoflagellate. These algae live within the coral polyps’ tissues, where they perform photosynthesis, converting sunlight into energy. Zooxanthellae provide the coral with up to 90% of its energy, while the coral offers the algae a protected environment and compounds necessary for photosynthesis. This partnership also gives most shallow-water corals their vibrant colors, as the algae contain various pigments.
Architects of the Ocean: How Reefs Are Built
Stony corals construct coral reefs through the continuous secretion of calcium carbonate. Each polyp builds upon the skeletal remains of previous generations, gradually expanding the colony. This slow growth, occurring at rates between 0.3 to 2 centimeters per year for massive corals and up to 10 centimeters for branching forms, results in complex, three-dimensional reef frameworks that can take thousands to millions of years to form.
The diverse shapes of stony coral colonies contribute to the intricate architecture of reefs. Growth forms include branching (with numerous arms), massive (large, dome-shaped structures), plate-like or foliose (spreading horizontally), encrusting (low-spreading layers), and columnar (pillar-like structures). This variety in form creates diverse habitats that enhance reef complexity.
The Vital Role of Stony Corals
The reefs built by stony corals are among Earth’s most biodiverse ecosystems, often called “rainforests of the sea.” They provide habitat, shelter, and feeding grounds for an estimated 25% of all marine species, including fish, invertebrates, and plants. This biodiversity supports commercial and subsistence fisheries, offering food security globally.
Beyond supporting marine life, coral reefs provide services to coastal communities. They act as natural barriers, protecting shorelines from erosion and dissipating wave energy from storms, preventing billions in coastal damage annually. Reefs also hold economic value through tourism and recreation, generating billions each year from activities like diving, snorkeling, and fishing. Additionally, these ecosystems are a source of compounds with potential pharmaceutical applications.
Threats to Stony Corals
Stony corals and their reefs face numerous threats, with climate change being a primary concern. Rising ocean temperatures cause coral bleaching, a phenomenon where corals expel symbiotic zooxanthellae algae due to stress. This expulsion causes the coral to lose its color and primary food source, leaving it vulnerable to starvation, disease, and often widespread mortality if conditions do not improve.
Another climate-related threat is ocean acidification, resulting from the ocean absorbing increased carbon dioxide from the atmosphere. This leads to a decrease in seawater pH, making it more acidic and reducing carbonate ions corals need to build and maintain their skeletons. This impairs coral growth, weakens existing reef structures, and makes them more susceptible to physical damage.
Localized threats further jeopardize coral health. Pollution from land-based sources, such as sedimentation from coastal development, agricultural runoff containing excess nutrients, and toxic substances from industrial and urban areas, can smother corals, block sunlight, and foster algal overgrowth. Overfishing can disrupt the delicate balance of reef ecosystems by removing fish that graze on algae, allowing algae to smother corals. Physical damage from destructive fishing practices, boat anchors, and recreational misuse also directly harms coral colonies.