When people first looked upon coral animals they thought them plants. Upon further investigation and study they found two distinct types of animals - reef-building corals and non reef-building corals.
The coral animal itself is simple in design with only sponges being simpler. Corals have two main layers of skin/tissue with the outer layer called the ectodermis and the inner layers the endodermis or gastrodermis. Between these two is a less prominent middle layer called the mesoglea. One or more mouths, sometimes numbered in the thousands, open to allow food to enter where it finds its way to a connected stomach-like area called the gastrovascular cavity.
Food is digested in this cavity and waste products are either expelled through the same opening or secreted through body tissue to the surrounding water since corals have no excretory system or kidneys. A ring of stinging tentacles often surrounds the mouth to aid in capturing various types of food/prey.
Most corals have tentacles for one basic reason - to capture food. The larger the tentacle the larger the food they can catch. Those that do not have tentacles use their body mucus to catch bacteria and plankton. Small filaments called cilia transport the captured foodstuffs to the mouth where it is then transported to the stomach and digested.
Besides tentacles armed with nematocysts, a harpoon-like projectile that stuns and captures prey, some corals can extend mesenteric filaments outward from the stomach where they partly digest prey too large to be swallowed immediately. Besides containing a vast amount of different chemical compounds, used for either defense or offense, some corals exhibit long sweeper-like tentacles during evening hours that can clear out the competition for growing space downstream.
Corals do not have a brain or nervous system, nor a respiratory system, i.e., gills or lungs. Nor do they contain a circulatory system with blood or even the vessels to transport it. Dissolved oxygen is simply absorbed through most of their outer tissue layer, as is organic and inorganic compounds.
Some corals grow into large colonies while other live a solitary existence. Many build an internal calcium carbonate skeleton to support themselves. They take many forms and often consist of numerous polyps/mouths joined together by a tissue covering where they share the internal flow of nutrients.
Ahermatypic corals, called non reef-building corals, do not build reefs. They also differ from hermatypic corals, called reef-building corals in the way they attain most of their nutrients. Non reef-building corals are found in many warm and cold environments in both shallow and deep areas. They can be considered carnivores as they consume whatever they can catch. Most are slow growers and do not precipitate calcium carbonate as fast as reef-building corals. However, this is not to say they do not contain any calcium carbonate structures, as some form small skeletal pieces called sclerites, sometimes referred to as spicules. These small structures help provide some support and have become a means to identify some animals. They and zooxanthellae reside in the middle mesoglea layer.
Hermatypic corals are also carnivorous, yet receive a vast amount of their nutritional needs from a single-celled dinoflagellate algae belonging to the genus Symbiodinium called zooxanthellae. They live in more warm and shallow environments where sunlight penetrates and photosynthesis occurs, as zooxanthellae only survive in mid 60 to a high 80º Fahrenheit temperature range (16 - 27ºC).
In the mutualistic symbiosis formed the zooxanthellae take in waste carbon dioxide from the coral animal and return oxygen. Since there is no transport system within the animal, oxygen is now both available from outside and within the structure of the animal. Such a method allows its tissue to take many different forms instead of a very flattened shape that would it be dependent upon oxygen from outward sources.
It is thought that up to 90% of zooxanthellae waste products are utilized by the hosting animal as a food supply. Besides oxygen, glucose, glycerol and other nutritious organic substances are produced by the zooxanthellae. Along with providing valuable foodstuffs, zooxanthellae contribute to the precipitation of the calcium carbonate skeleton material in reef building corals by maintaining a higher internal pH. It could be asked what do zooxanthellae get besides a safe and comfortable home. It is thought that since the water surrounding the reefs is so nutrient poor, zooxanthellae get the compounds, e.g., phosphate and nitrogen, they need for their existence directly from coral animal tissue.
When it comes to the corals kept in aquariums, ahermatypic non-reef-building corals are referred to as soft corals. Their polyps have eight tentacles, hence the name "Octocoral." Sometimes the tentacles have side branchlets called "pinnules" that make the tentacles look feather-like.
There are, however, soft corals that do not fit the soft coral term, such as Heliopora coerulea, called the Blue coral, and Tubipora called Organ Pipe coral. Blue coral, which has an internal sky-blue skeleton because of a concentration of iron salts, forms a hard crust of calcium carbonate that is brown due to zooxanthellae living in its outer covering. Organ Pipe forms upright red-colored calcium carbonate tubes joined in flat floor-like plates. Yet they both have eight tentacle polyps. There are other exceptions, such as some Sinularia that fuse together sclerites at their base and help form reef calcium deposits.
Hermatypic reef-building corals are referred to as stony corals, with those having small polyps as small polyp stony (sps) corals. One or several rows of tentacles surround each opening/mouth in multiples of six.
As noted in the other sections/groupings of this Animal Library, there's room for additions, and no doubt corrections. I've laid the groundwork, so please let's not criticize, simply pitch in and help! Together we can make this 'Animal Library' for hobbyists and divers the best in the world. I should also add that since the supply of invertebrate photos from individuals appears to be somewhat scarce in certain categories, I've searched the web to find sites where none-copyrighted examples appeared to be available. I've posted these here with the website where located, such as the wonderful site at National Oceanic & Atmospheric Administration (NOAA). By visiting these sites you might be able to gather more information about the species of interest. And if you have better photos, and/or more species information, please send it to me, as sharing is what this non-profit site is all about! Send to saltcorner@cs.com. Also, where photos were not available, I've noted three books where you can find a photo of the discussed species. They are: "Indo-Pacific Coral Reef Field Guide" by Dr. Gerald R. Allen & Roger Steens (ISBN 981-00-5687-7); "Baensch Marine Atlas" Vol. 2 (ISBN 1-890087-11-4); and "Marine Invertebrates And Plants of the Living Reef" by Dr. Patrick L. Colin (ISBN 0-86622-875-6. And I should add some of the information related to here comes from 'Aquarium Corals, Selection, Husbandry, and Natural History' by Eric Borneman, and also Julian Sprung's 'Corals: A Quick Reference Guide.' These, and the three book set of "Corals of the World" by JEN Veron (ISBN 0-642-32236-8, 0-642-32237-6, and 0-642-32238-4) deserve to be in your aquarium library. - Bob
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