Hard corals, classified scientifically under the order Scleractinia, are the primary organisms responsible for constructing the massive, intricate structures known as coral reefs worldwide. These marine invertebrates are distinct from soft corals, which possess a fleshy, pliable body structure supported by microscopic skeletal elements called sclerites. Hard corals, conversely, form a rigid, stony framework. The individual animals, or polyps, typically exhibit hexamerous symmetry, meaning their tentacles occur in multiples of six, separating them from the eight-tentacled soft corals.
The Defining Skeletal Structure
The defining characteristic of a hard coral is the production of a dense, external skeleton that provides support and protection to the soft-bodied polyp. This structure is composed almost entirely of calcium carbonate, specifically the crystalline form known as aragonite. The individual coral polyp sits within a small, cup-like depression in the skeleton called a corallite.
The process of forming this skeleton is called calcification, where the coral polyp extracts calcium and carbonate ions from the surrounding seawater. This material is secreted by the basal layer of tissue, the calicoblastic epithelium, which lays down the aragonite structure beneath the living animal. The corallite features vertical partitions called septa that radiate inward from the wall, providing internal bracing for the soft polyp.
As the polyp grows upward, it continuously secretes new skeletal material, progressively isolating the lower, older portion of the structure from the living tissue. The entire colony’s skeleton is a composite material, roughly 97.5% aragonite, with the remainder consisting of organic matrix materials like proteins and sugars that help regulate the crystal growth. This mineral framework is what remains long after the polyp dies, serving as the literal building block for the reef structure. The skeleton’s density and architecture are also influenced by environmental factors, helping the colony withstand strong currents and wave action.
The Symbiotic Relationship with Zooxanthellae
Hard corals achieve high rates of calcification and growth primarily through a mutualistic association with microscopic algae called zooxanthellae. These golden-brown dinoflagellates reside within the cells of the coral polyp’s gastrodermis. The coral provides the zooxanthellae with a stable, protected environment and a steady supply of compounds like carbon dioxide and metabolic waste products, such as nitrogen and phosphorus.
In return, the zooxanthellae perform photosynthesis, converting sunlight and the waste products into energy-rich organic compounds. Up to 90% of the organic material produced by the algae, including sugars, glycerol, and amino acids, is transferred directly to the host coral tissue. This energy subsidy satisfies the majority of the coral’s nutritional requirements, allowing the animal to devote energy to growth and reproduction.
This reliance on photosynthesis means that reef-building hard corals are restricted to shallow, clear, and sunlit tropical waters, typically less than 50 meters deep. The presence of these algal symbionts is also responsible for the vibrant colors of healthy corals. When corals experience prolonged environmental stress, such as elevated water temperatures, they expel the zooxanthellae, leading to a white state known as coral bleaching.
Colonial Existence and Reef Building
The characteristic size and shape of hard corals result from their colonial lifestyle, where a single founding polyp reproduces asexually to create thousands of genetically identical individuals. Colony expansion occurs through budding, where new polyps are generated either within the tentacle ring of the parent (intratentacular) or between existing polyps (extratentacular). These polyps remain physically connected by a thin sheet of living tissue called the coenosarc, which overlays the shared skeleton.
Hard corals that actively build reefs are known as hermatypic corals, functioning as ecosystem engineers by creating structural habitat for countless other marine species. Their varied growth forms directly contribute to the complexity of the reef architecture. Massive corals, such as brain corals, grow slowly but are dense and resistant to physical damage from storms.
Branching corals, like Acropora species, grow quickly, extending their structures outward like tree limbs. This creates high-relief habitats but makes them more vulnerable to breakage. Other forms include plating or foliaceous corals, which optimize surface area to capture sunlight in deeper or shaded areas. While most hard corals are hermatypic, ahermatypic corals do not form massive reef structures, often lacking zooxanthellae and existing as solitary or small colonial forms in deeper, colder waters.