What is Coral?
Coral refers to a group of animals related to sea anemones and jellyfish. Most corals are found in tropical and subtropical waters around the world.
By Matt Lybolt¹
¹Florida Fish and Wildlife Conservation Commission-Fish and Wildlife Research Institute
Jump to coral habitats
In brief, coral refers to a group of animals related to sea anemones and jellyfish. All animals in this large group called Cnidarians (ni-Dairy-enz) have tentacles with stinging cells called nematocysts (ne-Mat-oh-sists). All corals have the same basic body structure; tentacles containing stinging cells surround a central body cavity. There are two general categories of coral: hexacorals and octocorals.
Hexacorals are animals that have 6 tentacles or multiples of 6 (i.e. 6, 12, 18, 24…). Their skeleton is described as "hard" and is usually made up of calcium carbonate.
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Credit: Florida Fish and Wildlife Conservation Commission
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Octocorals always have 8 tentacles. Skeleton material is described as "soft" and is made of flexible proteins.
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Credit: Florida Fish and Wildlife Conservation Commission
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For both groups, the body cavity, or coelom, partially encases the animal. Coral tentacles capture food (plankton) and sweep sediment from the surface of the animal.
Where do corals live?
Credit: World Resource Institute
Corals can be found in oceans all over the world, on the deep sea floor, and even under the ice cap! Reef building corals have specific habitat requirements that explain why they are found only in shallow tropical and subtropical waters around the world. Shallow water reef building corals require; warm seawater (19° C–29° C), high salinity (34 S), clear nutrient-poor water, high wave energy, moderate to intense sunlight.
In the Florida Keys, there are four general types of coral habitat: hardbottom, patch reef, shallow offshore, and deep offshore reef.
Credit: Florida Fish and Wildlife Conservation Commission
| Hardbottom habitats are generally dominated by octocorals with low stony coral cover and low stony coral species diversity. |
| Patch reef habitats are usually characterized by high diversity of both octocorals and stony corals. A mature patch reef is roughly circular and the perimeter is buttressed by large boulder corals. |
Credit: Florida Fish and Wildlife Conservation Commission
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Credit: Florida Fish and Wildlife Conservation Commission
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Shallow offshore habitats are high-energy environments with large branching corals and expanses of bladed fire coral and yellow sea-mat. Sea-mat is a different type of cnidarian called a zooanthid (a colonial anemone). |
| Deep offshore reef habitats are inhabited by a diverse array of benthic (bottom dwelling) organisms including large sponges, octocorals, and stony corals.
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Credit: Florida Fish and Wildlife Conservation Commission
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Vocabulary Quiz: What is the difference between "hard grounds" and "hardbottom?" Answer: "Hard grounds" is a geological descriptor. The term illustrates the geological character of the substrate, but does not describe the overlying biological benthos in any way. "Hardbottom" is a biological descriptor, which characterizes the living benthos. It happens that hardbottom habitat, with a thin veneer of sand, usually -but not always- overlies hard grounds. "Hardbottom" and "live bottom" are functionally interchangeable terms; however, "live bottom" is a more general descriptor than "hardbottom." If you perform a search, bear in mind that "hardbottom" is the preferred term; "hard bottom" spelled as two words is not favored in the literature.
How long does stony coral live?
Coral has a life span like all organisms. The question of coral life span is really two questions: 1) How long does the whole coral colony live? 2) How long does an individual coral animal (polyp) live?
- How long does the whole coral colony live? The answer depends on the species. One of the small stony corals named Favia fragum, which never grows larger than a golf ball, has a life span of about five to ten years. The star coral Montastraea annularis may live for several centuries and grow to the size of an automobile. Its growth rate ranges from about 5 mm to 15 mm (1/4 to 3/4 of an inch) of yearly, outward expansion. Among shallow water coral, boulder-shaped colonies grow more slowly, and live longer than branching coral colonies.
- How long does an individual coral animal (polyp) live? Once again, the answer depends on what the species. In general, polyps on a bumpy colony have a short life span, and polyps on a smooth colony have a long life span. Here’s why: A polyp grows outward in a basically straight line. You can see this on corals that are broken in half. The polyp skeleton (corallite) looks like a straw, and the polyp was living on the very top of the straw. The whole colony is like a bundle of straws. On bumpy colonies some of these polyps are “aimed toward” other polyps. They get closer and closer as the polyps grow, and eventually they meet. One will live the other will die. This is what determines the life span of individual polyps on a bumpy colony. Life span of an individual polyp on Porites (a common bumpy coral) is about 2–3 years. A good source for polyp life span is; W.M. Darke and D.J. Barnes, Growth trajectories of corallites and ages of polyps in massive colonies of reef-building corals of the genus Porites, Mar. Biol. 117:321–326, 1993.
OK, what about smooth colonies? Well the corallites are essentially the same. On smooth colonies none of the polyps are “aimed toward” each other, they are all “aimed” outwards. Polyps on a smooth colony do not have to die as the colony grows. The best example in the Caribbean is a brain coral named Colpophyllia natans. Presumably, the very first polyp on a colony of this brain coral is living somewhere on a 400-year-old colony.
How are coral communities measured?
Ecological measurements of living organisms consist of two broad classes of data. The first is a measure of selected ecological components (mass, volume, biomass or square area). The second is a measure of the number of species that make up each of the selected ecological components. Most coral monitoring projects use percent cover as a measure of area and species richness or species diversity as the measure of number of species. For more information about Florida's coral reefs read Coral Sea Stats. See the Coral Reef Monitoring Project for more information about coral monitoring programs at the Fish and Wildlife Conservation Commission.
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