Corals belong to the phylum Cnidaria and class Anthozoa. Corals exist as individual polyps or in colonies. Eight hundred species of reef-building corals and hundreds of non reef-building corals are known. Coral polyps live in a relatively narrow band of the Earth's oceans and thrive only in oceans within 30 degrees north and 30 degrees south of the equator. In these areas, temperatures must average above 70 degrees Fahrenheit and the water must be of a low nutrient quality.
Coral polyps are multicellular organisms with limited organ development. The average polyp grows from 1 to 3mm in diameter and can exist as a solitary individual or as a group of interconnected polyps called a colony. Colonial polyps are connected by the coenosarc, allowing polyps to communicate and share nutrients. Similar to cnidarians, polyps contain three body tissues: epidermis, mesoglea, and gastrodermis. Corals are categorized as scleractinian, also called hard corals, or as Alcyonacea, soft corals, depending on whether or not they produce a calcium carbonate skeleton.
Corals that create a rigid calcium carbonate skeleton are the hard corals. These are the primary corals involved in reef building and are termed hermatypic. Typically, scleractinian coral polyps are characterized as having sets of six tentacles, septa, and mesenteries. The tentacles encircle the mouth and are used in defense and the capturing of prey. They contain stinging cells called nematocysts. These poison-injecting cells are used to incapacitate small prey, as well as deter predators. The mouth is the only opening to the gastrovacular cavity and is used to ingest food and expel waste. The gastovascular cavity contains mesenteries, with internal folds that increase the surface area of the stomach. Mesenteries are known to contain digestive filaments that aid in food capture. The mesenteries are supported by septa.
Hard coral polyps attach to the substrate via the calyx and basal plate. Calcium carbonate is secreted at the base of the animal. As polyps die, they leave the secreted calcium carbonate skeleton behind. In living scleractinian coral colonies new polyps will grow in place of the dead polyp. The buildup of calcium carbonate over time leads to the formation of coral reefs.
Soft coral polyps are similar in structure to those of hard corals. However, ahermatypic corals do not have calyx, septae, or basal plates and do not secrete calcium carbonate. The polyps are also characterized as having tentacles and mesenteries grouped into sets of eight. These corals are often specialized for filter feeding.
Symbiotic relationship with zooxanthellae
Symbiosis can be defined as any of several living arrangements between members of two different species. Symbiotic relationships can be categorized into three different types including, mutualism, commensalism, and parasitism. The species involved in these relationships are called symbionts.
In a mutualistic symbiotic relationship, both species involved benefit. An example of this type of relationship is the one between oxpecker birds and rhinos. An oxpecker bird will land on the back of a rhino and eat the ticks, fleas, and other parasites that live on them. Both symbionts benefit. As the oxpecker receives nutrients from eating the parasites, the rhinos are getting rid of the parasites feeding on them. In many cases the mutualistic symbiotic relationship is obligative meaning that both symbionts rely on the benefits gained so much that without them neither symbiont would survive.
In commensalism one symbiont, called the commensal, benefits from the relationship while the other symbiont, called the host, remains unaffected. An example of this type of relationship could be when smaller fish follow bigger fish in order to eat leftover food. The smaller fish benefit by gaining nutrients from the leftover food while the bigger fish are unaffected as they are already done with the food.
Parasitism is a non-mutual symbiotic relationship where the parasite, benefits at the expense of the host. A classic example of this type of relationship is when a tapeworm attaches itself to the insides of the intestines of other animals and feed on the food that the animal ingests. The parasite benefits by gaining nutrients, but does so at the expense of the host because it now needs to eat more in order to survive.
Zooxanthellae are photosynthetic algae that live inside coral polyps. Coral polyps and zooxanthellae have a mutalistic symbiotic relationship. The calcium carbonate skeleton produced by the coral polyps provides the zooxanthellae with a protected environment suitable for photosynthesis. In return, the photosynthetic algae produce oxygen and aid the coral in removing waste. Additionally, zooxanthellae provide the coral with nutrients such as glucose, glyserol, and amino acids, which are products of photosynthesis. These compounds are utilized by the coral as building blocks in the manufacture of fats, as well as the synthesis of their calcium carbonate skeleton.
Some polyps are able to capture enough nutrients to survive without zooxanthellae, but these are rare. It has been found that zooxanthellae donate up to ninety percent of the organic material produced during photosynthesis to the host coral tissue. When coral polyps become stressed due to a change in the environment, often caused by human activity, they can lose the zooxanthellae. When this occurs, coral polyps previously accustomed to receiving a large amount of nutrients from the zooxanthellae begin to die. This process is often indicated by coral bleaching, where corals lose their zooxanthellae, which give corals their color. Therefore, the relationship between polyps and zooxanthellae is obligative in most cases.
Zooxanthellae may be responsible for up to ninety percent of coral polyp energy needs. Even so, it is common for polyps to capture prey. Prey include small fish, zooplankton, bacterioplankton, and other small particles. Prey capture involves use of nematocysts in the tentacles to stun or kill prey. The tentacles will then move the prey to the mouth of the polyp. When the food particles are too large to fit into the mouth of the coral, the coral can digest it externally using filaments that travel through the mouth or temporary openings in the polyp body. 
Coral polyps are capable of reproducing sexually or asexually, and may use both methods of reproduction within their lifetimes.
In sexual reproduction, many coral polyps are referred to as broadcast spawners. Depending on the species of coral, the polyps will release either male sperm, female eggs, or both in large quantities out into to the water. For example, brain and star coral produce both sperm and eggs at the same time while all polyps within one colony of elkhorn and boulder coral produce only sperm and require eggs from another colony to fertilize. Because coral polyps cannot move and come in physical contact with one another to reproduce, spawning often occurs as a massive synchronized event where all coral species in the same area release their eggs and sperm at approximately the same time. By precisely timing this event coral polyps increase the probability that fertilization will occur. The coral larvae, called planulae, swim toward the ocean surface exhibiting positive phototaxis. After free floating for a period typically ranging anywhere from a few days to a few weeks, the planulae will eventually swim back down to the bottom. If conditions are suitable the planulae will settle, metamorphose into polyps, and attach themselves to the surface they have settled on..
In asexual reproduction, coral polyps reproduce through a process called budding. In the process of budding a parent polyp will grow to a certain size and then divide itself. In some cases polyps remain connected with the colony during the process of budding and are able to grow to great sizes. In other cases environmental factors will break off large portions of the colony and deposit it elsewhere. Once settled this colony may begin to thrive again. This process is called fragmentation. .
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