Corals are marine cnidarians belonging to the subphylum Anthozoa that typically form compact colonies of genetically identical polyps. Some, known as hard corals, secrete calcium carbonate to build rigid skeletons, while others, called soft corals, lack a stony skeleton and remain flexible, swaying with ocean currents. Many hard corals contribute to reef formation (hermatypic), whereas others are non-reef-building (ahermatypic).
Most corals live symbiotically with microscopic algae called zooxanthellae, which provide energy through photosynthesis and contribute to their growth and coloration. However, many deep-sea species lack these symbionts (azooxanthellate) and acquire their nutrition from captured prey.
The symbiotic relationships make corals the cornerstone of entire marine ecosystems. Although reefs cover only 1% of the world’s oceans, they provide habitat for at least 25% of the world’s marine life, including over 4000 species of fish, as well as mollusks, crustaceans, sponges, and other aquatic animals.
Being a broad group, corals show considerable variation in size, ranging from less than an inch to several inches in diameter. For example, mushroom corals (family Fungiidae) can reach up to 10 in (25 cm) across, while smaller species typically measure only 0.04 to 0.12 in (1 to 3 mm) in diameter.[1]
Coral colonies are typically a few feet high, although some species, like the great star coral (Montastraea cavernosa), reach an average colony height of 10 to 13 ft. (3 to 4m).
In 2024, scientists discovered the largest known single coral colony in the southwest Pacific Ocean. Measuring approximately 111 ft (34 m) wide, 104 ft (32 m) long, and 18 ft (5.5 m) tall, this specimen represents the largest coral structure ever recorded.[2]
Reef-building corals can extend for several kilometers. For instance, the largest reef, the Great Barrier Reef in Australia, covers an area of about 344,400 sq. km (133,000 sq. mi).[3]
For most of their lives, corals exist as colonies of sac-like polyps, with each polyp connected to its neighbor by a living tissue called the coenosarc. The polyps are embedded within skeletal cups called the corallites.
The body wall of each polyp consists of two distinct cellular layers, the outer ectoderm and the inner endoderm, separated by a middle layer of mesogloea.
The upper end of the polyp bears the mouth surrounded by tentacles. As cnidarians, corals possess specialized stinging cells, or cnidocytes, on the tentacles, which they use for defense and capturing prey. They have a single internal cavity, the gastrovascular cavity, that serves both as the stomach and the circulatory system.
Depending on the skeletal type and structure, corals are of two types: soft and hard corals.
Members of the class Octocorallia lack a solid exoskeleton. Instead, they are equipped with internal supportive structures, such as spiky calcium-carbonate sclerites. Some branching forms, like red corals (Corallium), possess a central axial skeleton running through each branch. This skeleton is typically composed of a tough, flexible fibrous protein called gorgonin, which allows it to drift with water currents.
These corals typically have an octamerous symmetry, with each polyp having its body parts arranged in multiples of eight. Each of the eight tentacles is pinnate, characterized by extensive side branches.
Members of the class Hexacorallia possess a hard, bicomposite exoskeleton made of both minerals and organic compounds. The mineral component comprises calcium carbonate, deposited primarily as calcite or aragonite, while the organic part consists of proteins, lipids, and sulphated sugars.
Mineralization begins at specialized centers of calcification, from where elongated crystalline structures radiate outward. This process starts in a space beneath the ectoderm called the calcifying space. The ectoderm produces a protein-rich secretion, the Skeletal Organic Matrix (SOM), which plays a crucial role in this process. Several proteins, such as CARPs (Coral Acid-Rich Proteins) and SAPs (Skeletal Aspartic Acid-Rich Proteins), facilitate the movement of calcium ions (Ca²⁺) into the calcifying space. These proteins also guide crystal nucleation and determine the final architecture of the skeleton. The conversion of carbon dioxide to bicarbonate is catalyzed by the enzyme α-carbonic anhydrase (CA).
Stony corals, as opposed to the octamerous soft corals, have a hexamerous symmetry, with the body parts of each polyp arranged in multiples of six. Moreover, unlike soft corals, their tentacles are smooth and unbranched.
According to the World Register of Marine Species, all coral species are classified into 2 classes and 8 confirmed, extant orders.
The oldest reef-building organisms similar to modern corals first appeared during the Cambrian Period (around 535 million years ago). About 100 years later, during the Ordovician Period, the first true corals of the groups Tabulata and Rugosa appeared, forming early reef systems. However, a major decline in their diversity occurred during the End-Ordovician Extinction Event.
Corals reappeared during the Silurian Period, with tabulate and rugose corals thriving together, forming cushion-like or branching colonies. But, during the Permian–Triassic Extinction (around 250 million years ago), these corals were wiped out, along with most other then-existing marine animals.
In the Middle Triassic Period (around 247 million years ago), modern stony corals (order Scleractinia) emerged to fill the vacant reef-building niche. Unlike older forms with calcite skeletons, these corals evolved aragonite skeletons. They diversified in the Jurassic Period.
Corals are found in all oceans of the world, ranging from the tropics to the polar regions. Most reef-building corals are distributed throughout the tropical and subtropical Western Atlantic and Indo-Pacific Oceans, particularly between 30°N and 30°S latitudes. While the Western Atlantic reefs include Bermuda, the Bahamas, the Caribbean Islands, Belize, Florida, and the Gulf of Mexico, the Indo-Pacific reefs extend from the Red Sea and the Persian Gulf through the Indian and Pacific oceans to the western coast of Panama. They also grow on rocky outcrops in some areas of the Gulf of California.[4]
The high concentration of reefs in the tropics is attributed to the fact that they require warm ocean temperatures (68° to 82° F) for growth. However, some cold-water species, such as Lophelia pertusa, are found at temperatures as low as 39.2°F to 46.4°F in the North Atlantic Ocean.[5]
Most reef-building corals are found at depths of less than 150 ft (46 m), where sunlight penetrates easily. However, many deep-sea corals are found at depths as low as around 20,000 ft (6,096 m).
Depending on whether the coral is reef-building, its diet varies. Reef-builders, living in shallow, sun-lit waters, rely heavily on photosynthesis from the symbiotic algae or zooxanthellae living inside their tissues. To supplement their diet, they may capture zooplankton and organic detritus, especially at night.
Since non-reef-building corals occupy deeper waters where sunlight is minimal, they feed entirely on zooplankton, small fish, and detritus.
Most corals typically feed at night. They extend their tentacles and immobilize their prey using the stinging cells. Using their tentacles, they draw the prey into their mouths and swallow it whole. As the food enters the internal cavity, it gets broken down into simpler molecules by digestive enzymes.
Many members of the order Scleractinia are reef-builders. However, reef-building is not a solitary process. Corals function as holobionts, consisting not only of the coral polyp itself but also of a complex symbiotic community that includes zooxanthellae (photosynthetic dinoflagellates, typically of the genus Symbiodinium), along with bacteria, archaea, viruses, and fungi.
They house the zooxanthellae typically within the cytoplasm of the gastrodermal cells, as well as their mucus, and, in the case of hard corals, even in their hard exoskeleton.[6] These zooxanthellae undergo photosynthesis, producing glucose, glycerol, and amino acids, which fulfill most of the coral’s energy needs. During photosynthesis, these algae also absorb carbon dioxide from the coral’s tissues, raising the pH and making the surrounding fluid more alkaline, creating favorable conditions for calcium carbonate to form (calcification). They also aid in waste recycling by consuming the coral’s phosphate and nitrogenous waste.
In exchange for all the favors, the zooxanthellae receive a stable habitat within the coral’s tissues, as well as a steady supply of nutrients.
These reefs grow remarkably slowly. For example, some corals may increase in diameter by only 0.4 to 0.8 in (1 to 2 cm) per year.[7]
Coral reefs exist in various forms, including shoreline fringes (fringing reefs), those running parallel to the coastline (barrier reefs), and ring-shaped reefs encircling a central lagoon (atolls).
About two-thirds of coral species (around 67%) are simultaneous hermaphrodites, with each colony producing both eggs and sperm. The rest, including species like Favia fragum, are gonochoristic, meaning each colony is either male or female, producing either sperm or eggs.
About 75% of all reef-building corals release their gametes into the water, typically at night, allowing fertilization to occur externally (broadcast spawning). Most noon-reef-building species, particularly in areas of high current, undergo brooding, releasing only sperm in the water. The sperm then drift down, are captured by female coral polyps, and fertilization occurs internally.
The fertilized eggs develop into embryos, which hatch into pink, elliptical planula larvae. These larvae drift near the water surface for about 2 to 3 days, then settle and attach to a suitable substrate on the seafloor. They then undergo metamorphosis, transforming into sessile polyps with a developing mouth and tentacles. Each polyp eventually develops into a juvenile and then into an adult polyp. The adult then reproduces asexually, forming colonies, either by budding or fragmentation.
The reefs face several threats, including coral mining, pollution, destructive fishing practices, and coastal development. Globally, climate change poses the greatest threat through rising sea temperatures, rise in sea levels, and ocean acidification. In fact, rising sea temperatures of just 1 to 2°C above the normal summer maximum can trigger widespread coral bleaching, causing the stressed corals to expel their symbiotic zooxanthellae, losing color and their major source of nutrition.[8]
Currently, about 10% of coral reefs are dead, and nearly 60% are at risk, with Southeast Asia experiencing the greatest losses (80% reefs are already endangered).[9] Therefore, it is essential to protect the world’s existing reefs through active conservation efforts and public awareness. Many governments have already banned the removal of corals from their natural habitats, while the establishment of biosphere reserves, marine parks, and world heritage sites helps prevent further overexploitation.
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