Coral
Coral refers to a diverse group of marine invertebrates belonging to the class Anthozoa within the phylum Coelenterata. These organisms are primarily found in warm, tropical waters across various geographical locations, including the Mediterranean Sea, the Caribbean, and the South Pacific. Corals are known for forming intricate and colorful reef structures, which are essential to marine ecosystems. The individual units of coral are called polyps, which possess tentacles and are anchored by calcium carbonate exoskeletons.
Corals can be classified into two main subclasses: Hexacorallia, which includes stony corals that build reefs, and Octocorallia, which encompasses soft corals. Coral reproduction primarily occurs through asexual budding, allowing colonies to grow over time. However, corals also rely on symbiotic relationships with algae for nourishment, which can lead to coral bleaching when environmental stressors, such as temperature changes, occur. This bleaching phenomenon poses a significant threat to coral reefs, highlighting the impact of climate change on these vital ecosystems. Corals also produce precious varieties, such as red and black corals, used in jewelry, although their trade faces environmental scrutiny.
Coral
Coral Facts
- Classification:
- Kingdom: Animalia
- Phylum: Coelenterata
- Class: Anthozoa
- Subclass: Hexacorallia (six-tentacled or six-tentacle multiples), Octocorallia (eight-tentacled)
- Orders: Include Alcyonacea (soft corals), Antipatharia (black corals), Scleractinia (stony corals), Zoantharia
- Geographical location: Mediterranean Sea, Sea of Japan, east coast of the Americas from Brazil through the West Indies and along the Florida coast, Bermuda and the Bahamas, South Pacific Islands, East Indies, Indian Ocean, Australia
- Habitat: Marine, warm to tropical waters
- Gestational period: None; reproduces via asexual budding of indeterminate duration
- Life span: Can live indefinitely as living polyps grow on the skeletons of dead ones
- Special anatomy: Tentacles, exoskeletons
The word coral refers to a large group of marine invertebrates, class Anthozoa, in the phylum Coelenterata. Anthozoans are well known for forming reefs, intricate and brilliantly colored coral bushes, and precious coral (which humans use for jewelry). The individual organisms involved are coral polyps. They have protective and supportive calcium carbonate (limestone) exoskeletons or other structural elements.
Stony corals of the order Scleractinia, also called true corals, form coral reefs. The reefs are limestone formations in oceans made by billions of polyps. These polyps have saclike shapes, with a mouth at the upper end, surrounded by tentacles. Coral polyps form reefs from calcium taken from ocean water and deposited around their bodies.
Corals of the order Alcyonacea, also called soft corals, have skeletons of flexible, horny material. Soft corals make coral bushes and lacy-looking coral fans. These formations are covered with live polyps, which are yellow, pink, purple, brown, and even black. Wherever they occur, such as in West Indian waters, the forms produced look like parts of underwater gardens.
Precious coral is used to make jewelry. It is produced by polyps of the order Alcyonacea, typically within the genus Corallium. This hard coral polishes into beautiful red, rose, or pink material that can be made into jewelry beads. Most precious red coral grows as small “bushes” in the Sea of Japan and the Mediterranean. Other forms of precious coral include black coral, of the order Antipatharia, and Hawaiian gold coral (Kulamanamana haumeaae), of the order Zoantharia. The use of precious coral for jewelry has been criticized by many environmentalists, and trade in some species has been banned or restricted. However, coral poaching and smuggling remains problematic.
Physical Characteristics of Coral Polyps
There are two colonial anthozoan subclasses differing in radial symmetry. The first, Octocorallia, is composed of eight-tentacled animals. Included are soft corals (alcyonaceans) and red precious coral. Members of the second subclass, Hexacorallia, have either six or multiples of six tentacles. Hexacorallia includes stony corals or scleractinians.
Most scleractinians are colonial. Their polyps have a diameter of about one tenth of an inch and are interconnected. The shapes of coral colonies, such as branched forms, depend on the coral species. Colonial corals can grow in deep waters, but reef-building corals occur only in warm, shallow ocean regions. Stony corals secrete calcium carbonate from the bottom of each polyp, forming skeleton cups where each polyp is anchored and into which it withdraws for safety. In an oral apparatus atop the stalk is an opening with tentacles and cilia. It serves as both mouth and anus. At night, the tentacles catch plankton and carry them to the mouth. Stinging cells on the tentacles paralyze prey.
Life Cycles of Coral Polyps
Each coral colony grows by asexual polyp budding. Live polyps build on limestone or horn deposits of past generations. In this way, they make new reefs or other coral masses. The polyps remain associated in colonies. The combined limestone- or horn-forming activity of a colony yields huge masses of coral. They may be dome-shaped or towering and branched.
Live polyps cover the outside of each mass and create its gorgeous colors, including various shades of tan, brown, orange, yellow, pink, purple, and green. When reef coral is removed from the ocean, the polyps die and wash off, leaving behind white skeletons. These corals build foundations for tropical atolls and barrier reefs.
Reef corals must live at depths where light penetrates because symbiont algae (zooxanthellae) that live in their tissues require sunlight for photosynthesis, and polyps cannot exist without the algae. The algae produce most of the carbon compounds that polyps use to make energy and protoplasm, while the plankton caught by polyps provides the algae with nitrogen, phosphorus, and other nutrients otherwise unavailable. The coral’s dependence on the algae varies with coral species and habitat.
When a reef can no longer provide the nutrients necessary to sustain its zooxanthellae, it expels them. This phenomenon is called coral bleaching, as it results in the reef taking on a bleached appearance. Coral bleaching can be caused by any one of a number of factors that put stress on the reef, including decreased salinity in the surrounding water, increased exposure to sunlight, and the introduction of toxins or otherwise foreign material. One major cause is temperature change, and an extreme increase in temperature can lead to mass bleaching events. While bleached coral can potentially recover in time, if the stressor is severe or prolonged, the coral will eventually die. The increase in cases of coral bleaching in the late twentieth and early twenty-first centuries has been linked to the phenomenon of global climate change.
Principal Terms
Budding: the development of asexually produced protuberances that become complete organisms
Exoskeleton: an external skeleton
Horn: hard, smooth, keratinous material forming an external covering
Limestone: an insoluble substance composed of various crystals of calcium carbonate
Plankton: microscopic plants and animals that float in huge numbers in oceans
Radial Symmetry: symmetry around a central axis
Bibliography
"Coral Basics." Flower Garden Banks National Marine Sanctuary, NOAA, flowergarden.noaa.gov/education/coralbasics.html. Accessed 9 Oct. 2024.
"Coral Reef Ecosystems." US National Oceanic and Atmospheric Administration, www.noaa.gov/education/resource-collections/marine-life/coral-reef-ecosystems. Accessed 9 Oct. 2024.
Dubinsky, Zvy, and Noga Stambler, eds. Coral Reefs: An Ecosystem in Transition. Springer, 2011.
Ethan, Eric, and Marie Bearanger. Coral Reef Builders. Stevens, 1997.
Faulkner, Douglas, and Richard Chesher. Living Corals. Potter, 1979.
Köhler, Annemarie, and Danja Köhler. The Underwater Explorer: Secrets of a Blue Universe. Lyons, 1997.
Ronai, Lili, and Arabelle Wheatly. Corals. Crowell, 1976.
Sheppard, Charles R. C., Simon K. Davy, and Graham M. Piling. The Biology of Coral Reefs. Oxford UP, 2009.
Veron, J. E. N. A Reef in Time: The Great Barrier Reef from Beginning to End. Belknap, 2008.