Marine Science Chapters


The Coral Animal

Coral polyp extendedCoral polyp withdrawn
Coral polyp extended with its ring of tentacles surrounding the mouth (left). Coral polyp withdrawn, having pulled in all the tentacles around the mouth (right). (LJF images)

The polyp is the basic body form of a coral animal. It is essentially a round animal with a mouth in the middle and a ring of tentacles around the mouth. The tentacles possess stinging cells called nematocysts or cnidocytes that can be used to sting, paralyze, and catch prey. This trait is what places them in the phylum Cnidaria along with jellyfish and sea anemones. Once paralyzed by the coral polyp, the prey is wiped off into the mouth and digested internally in a one-way digestive tract (there is no anus). Undigested material must be regurgitated through the mouth. When the animal is disturbed, or not feeding, it may close up, withdrawing its tentacles but the circular outline of the polyp is still there with its mouth in the middle.

Solitary coral corallite side viewSolitary coral corallite top view
Solitary coral corallite, side view (left). Solitary coral corallite top view (right). Notice all the radiating septa on the top view making the corallite almost solid calcium. (GA images)

Corals secrete a hard skeleton, called a corallite, under their skin. Each polyp secretes a hard, circular corallite (made of calcium carbonate). This circular corallite is attached at the bottom and has thin walls, starting at the outer circle, radiating toward the middle (but leaving room for the central mouth cavity). The thin (but numerous) radiating walls are called 'radiating septa.' The corallite is usually permanently attached to the solid surface upon which it lives (except in species like mushroom coral). The numerous radiating septa cause the corallite to be extremely dense and strong. The coral animal can add calcium carbonate to its corallite and extend it upward, keeping its living tissues in the uppermost part of the corallite, leaving a hard, permanently attached, base beneath.

Colonial mounding, coral side view of corallitesColonial mounding, coral top view of corallitesColonial mounding, coral close up of corallites from a dividing polyp
Colonial mounding coral, side view of corallites (left). Colonial mounding coral, top view of corallites (center). Colonial mounding coral, close up of corallites from a dividing polyp (right). (GA images)

Colonial branching coral, side view of corallitesColonial branching coral, top view of corallitesColonial branching coral, close up of corallites from a dividing polyp
Colonial branching coral, side view of corallites (left). Colonial branching coral, top view of corallites (center). Colonial branching coral, close up of corallites from a dividing polyp (right). (GA images)

Coral polyps can be solitary or colonial. Solitary forms remain as one polyp and one corallite. Colonial forms can reproduce the polyp asexually (cloning) and the new polyp forms another corallite that is attached to the first corallite. These colonial forms may have a general growth pattern that is somewhat horizontal, across a surface, like in the mounding or plate-like forms. Other colonial forms may grow vertically, branching like a tree. Each species of coral has its own polyp size (some tiny - less than a quarter of an inch, and some large - over a foot) and, if colonial, its own growth pattern.

Corallites of tropical corals, solitary and colonial forms
Corallites of tropical corals, solitary and colonial forms. (GA image)

Reef building corals are colonial, creating large 'coral heads' that may exist for thousands of years, providing tons of calcium carbonate that remain as the base for the coral reefs growing up and out over time.

Coral reef with a diversity of corals
Coral reef with a diversity of corals. (GA image)

Reef building corals are also hermatypic - a condition where they are in a symbiotic relationship with another species called a zooxanthellae. Coral can be found all over Earth where it is just the coral animal itself - these species are called ahermatypic. Ahermatypic corals must feed for themselves as other animals do. Now, the hermatypic corals do not have to catch all of their own food because their symbiotic zooxanthellae make extra food and give it to the coral. This allows corals to thrive in the tropical waters with low productivity (where there are few life forms in the water to be used as food by the corals).

Blue-green coralOrange coral
Blue-green coral (left) and orange popcorn coral (right) are just a few examples of the variety of colors found in tropical corals. This color may be primarily from their symbiotic zooxanthellae (as in the blue-green coral) -or- produced by the coral animal itself (as in the orange popcorn coral). (SA images)

Zooxanthellae are producers and photosynthesize to produce their own food. They are tiny, mostly microscopic, and can exist living within the tissues of other organisms. Many people call them 'plants' because they can photosynthesize. Most zooxanthellae can not live well outside of the coral animal's body because there are not enough nutrients in the tropical ocean to allow proper photosynthesis. It is often the zooxanthellae that give each coral species its characteristic color. If the zooxanthellae leave, the coral becomes 'bleached' (without color) and can die if it does not recover its zooxanthellae.

Cavernous star coral head
Cavernous star coral heads may be over five feet wide. (SA image)

Cavernous star coral closerCavernous star coral closest
Cavernous star coral closer (left), and closest (right) so you can see where the individual polyps are located on this colonial coral head. (SA images)

Coral animals provide their zooxanthellae with nutrients in the form of their excrement. The unique relationship between the coral and the zooxanthellae creates an environment where tropical zooxanthellae can live without the nutrient limitations of most marine tropical waters. The coral animals provide the nutrient fertilizers for their symbiotic zooxanthellae.

Coral polyps
Coral polyps have retracted into their corallites in the yellow-green coral. You can easily see the radiating septa (walls of calcium from the outside of the circular corallite pointing to the middle) of each corallite. (SA image)

Zooxanthellae produce food for their coral host in the form of simple sugar molecules that are formed in photosynthesis. They produce enough for themselves and enough to provide over eighty percent of the food requirements for many tropical reef building corals. Without the zooxanthellae, the coral would starve. Without the coral, the zooxanthellae would be severely nutrient limited.

Coral head with seastar
Coral head with seastars. (SA image)

A mutualistic symbiosis between the coral and its zooxanthellae allows both species to exist in the tropics. This relationship is also reef building, facilitating the ability of corals to lay down calcium carbonate to create acres and acres of coral reefs. Due to the creation of calcium carbonate and the high rate of photosynthesis from zooxanthellae, coral reefs are often referred to as carbon sinks. These ecosystems hold or sequester carbon for decades through the creation of the reef. In addition, these reefs provide spaces and an environment for many other species, creating the beginnings of a habitat with great diversity - the tropical coral reefs.

Coral reef diversityCoral reef diversity
Coral reefs have a diversity of species. (SA images)

Often compared to the diversity of the tropical rain forest, the coral reef community is both colorful and diverse. There is much more to a coral reef than just the hermatypic corals. There are ahermatypic corals, sponges, many invertebrates, fish, mammals and seaweeds. Many coral reefs themselves may have a large amount of calcium material that has come from sources other than coral, such as the shells of animals (snails, sea urchin spines) and coralline algae. These are all cemented into the reef structure.

Coral colony getting ready to spawn
This colonial coral is getting ready to spawn, you can see the pink spawn right below the surface of the mature polyps. (SA image)

Sexual reproduction of coral reefs begins with a spawning event. Coral animals reproduce sexually by releasing their gametes (eggs and/or sperm) into the ocean through their mouth (broadcast spawning).

A spawning coral head
A spawning coral head. (SA image)

Coral spawn is usually tiny pink balls that are released through the mouth of the spawning polyp. Once spawning starts, the ocean is full of these pink bundles.

Floating coral spawn
Coral spawn floating on the surface of the ocean. (SA image)

Floating on the surface of the ocean much of the excess spawn may be seen as a floating mat after a spawn. The gametes unite in the water to form the fertilized egg (called a zygote).

Coral Zygotes
Close up showing zygotes right after fertilization. (SA image)

The zygotes are one cell right after fertilization.

Dividing Coral Zygotes
Close up showing several zygotes that have begun cell division. It also shows a micrometer (in millimeters) so you can see the actual size of the spawn. (SA image)

Development of the zygote begins with cell division – first to two cells and then four cells and so on. The zygote develops into an oval planula larval form that hatches and swims in the water. The planula larval form is microscopic and can sink and swim with all the cilia on its oval surface. After a time planula larvae become 'competent' and can settle on a solid surface, metamorphose into a polyp, and secrete their corallite. Then, if it is a colonial form it can asexually reproduce (clone) and form a large coral head (of many polyps/corallites, all joined together).

Synchronous spawning is common on coral reefs no matter how many species are on the reef. This is when all species on a reef spawn on the same night each year. This synchronous spawning is very interesting to scientists and many are gathering information to try to predict this event on each reef. Exactly what causes synchronous spawning is not yet clear. Often the clear tropical water is cloudy for a few days after these spawning events. You may even see clumps of the pink spawn.

Coral reef.
Coral reef. (SA image)

Reef building coral need a hard substrate for attachment along with clear, sunlit water (for their zooxanthellae). They do not do well with a lot of plankton or sediment in the water (reducing the light) or reduced salinities (due to fresh water). It is the microscopic planula larvae that begin new coral reefs on uninhabited new oceanic islands to form the fringing reefs. They can also begin a new coral head in an area of a reef that has been damaged due to physical or biological factors.

Competition between brain coral and cavernous coral.Two coral species competing for space.
Coral competition between brain coral and cavernous star coral (left). A close view of two coral species fighting for space (right). The pink species is winning at this time - notice the new white corallites overgrowing the brownish species. (SA images)

Reef building coral species may compete with each other for space. These competitions, when two coral heads meet, may go on for hundreds of years with the advantage changing as conditions change. Over time, the reef itself may go through many stages as a fringing or barrier reef and then an atoll. It may continue as an atoll or sink below the ocean as a seamount or guyot after thousands of years. Each reef is different and has a different history.

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(Revised 17 Oct 2018 RMartin)
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