Corals are marine invertebrates that form large underwater structures. Often mistaken for plants or rocks, corals are sessile marine invertebrates, meaning they attach themselves to the ocean floor. However, corals are animals belonging to the phylum Cnidaria, similar to jellyfish, and unlike plants, do not produce their own food through photosynthesis. What is commonly called “a coral” is actually a colony of hundreds or thousands of individual animals called polyps. These polyps secrete a hard, cup-shaped skeleton composed of calcium carbonate, which forms the foundation of coral reefs.
The Coral Polyp: Building Blocks of Formation
The individual coral polyp is the fundamental unit of coral formation. Each polyp has a sac-like body with a central mouth surrounded by tentacles, which it uses to capture food such as zooplankton from the water. The polyp secretes a hard outer skeleton of calcium carbonate around its base. This secreted skeleton provides protection for the soft body of the polyp, allowing it to retract inside when threatened.
The process of forming this hard skeleton is called calcification, where polyps extract calcium and carbonate ions directly from seawater. Most corals harbor microscopic algae called zooxanthellae within their tissues in a symbiotic relationship. These zooxanthellae are plant-like organisms that perform photosynthesis, converting sunlight into energy. In return for a protected habitat and metabolic waste products from the coral, the zooxanthellae provide the coral with nutrients and energy, which enhances the coral’s ability to secrete calcium carbonate and build its skeleton. As individual polyps grow and reproduce asexually through budding, they continuously add new layers of calcium carbonate, slowly building the larger coral structure over time.
Environmental Requirements for Coral Growth
Coral polyps require specific environmental conditions to thrive. Warm water is a primary requirement, with optimal temperatures ranging between 23°C and 29°C (73°F and 84°F). Temperatures outside this range can stress corals and hinder their growth. Sunlight is also necessary because of the symbiotic zooxanthellae living within coral tissues. These algae rely on light for photosynthesis, which provides the coral’s energy.
Clear, low-nutrient water is important for healthy coral growth. Turbidity, caused by suspended sediments, can reduce light penetration, thus impacting zooxanthellae. High nutrient levels, often from runoff, can promote the growth of algae that can outcompete and smother corals. Corals prefer a salinity range of 32 to 42 parts per thousand. Water movement, such as currents and waves, benefits corals by bringing in fresh food supplies, distributing larvae, and removing waste products from the colony.
Types of Coral Reef Structures
The accumulation of calcium carbonate skeletons by coral polyps forms large coral reef structures.
Fringing reefs are the most common type and form close to shorelines or around islands. They grow outwards directly from the landmass, separated by a shallow or non-existent lagoon. The reef crest, the highest point, is often exposed at low tide, and the reef slope descends into deeper water.
Barrier reefs are distinguished by a lagoon that separates them from the coastline or island. These reefs often begin as fringing reefs, but as the land subsides or sea levels rise, the corals continue to grow upwards, maintaining their position near the surface, while the former shoreline recedes or sinks, creating the lagoon. The Great Barrier Reef off the coast of Australia is a prominent example, stretching over 2,300 kilometers.
Atolls are ring-shaped coral reefs that enclose a central lagoon, without any landmass in the center. Their formation begins with a fringing reef growing around a volcanic island. Over time, the volcanic island subsides and sinks beneath the ocean surface. The coral continues to grow upwards, forming a circular reef that marks the original outline of the island, with the lagoon occupying the space where the island once stood.
Ecological Significance of Coral Reefs
The intricate structures created by coral formation hold significant importance for marine ecosystems and human societies. Coral reefs are recognized as biodiversity hotspots, providing habitat, shelter, and breeding grounds for an estimated 25% of all marine species, despite covering less than 1% of the ocean floor. This includes many fish, invertebrates, and other organisms, contributing to ocean life.
Beyond their biological richness, coral reefs serve as natural barriers, offering significant coastal protection. They dissipate wave energy from storms and tsunamis, reducing erosion and safeguarding shorelines, coastal communities, and infrastructure. These underwater structures also provide substantial economic value. They support productive fisheries, offering food and livelihoods for millions globally. Reefs also drive tourism through activities like snorkeling and diving, contributing billions to local economies, and hold potential for new medical discoveries from the diverse organisms they host.