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What Are Corals?


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Corals, Family of Anthozoans
By Jim Wolf, Marine Biologist

There about 6000 species of anthozoans, and they all exist exclusively as a polyp. There are two general groups of anthozoa that will interest the hobbyist. The first is the anemones and true corals. These animals usually can be readily identified as a Cniderian due to their large conspicuous polyp like structure. The other group are the gorgonian like animals. These animals are a bit more difficult to characterize. Soft corals, sea pens and gorgonians all are included in this group. Usually the polyp is quite small, and will bear eight tentacles. A fairly crude yet accurate way to tell the difference is that many octocorals do not resemble anemones at first glance. The specimen may appear tree like, feather like, and or encrusting and the tentacles will bear smaller tentcles (pinnate).

One of the most fundamental points regarding Anthozoan biology is their sessile lifestyle. Unlike fish and other mobile critters, most Anthozoans can not move from a particular space if they do not like it. These organisms rely on the water to bring food and oxygen to them, as well as take waste products away. 

The three primary ways in which Anthozoans feed are: predation, suspension feeding, and "farming". The predation method is perhaps the most familiar. Many anemones and large corals will sting a small shrimp fish or other food item to death. It will then transport the prey to its mouth for ingestion. Almost all Anthozoans will eat some food in this manner. Suspension feeding is a common method in the Octocorals (gorgonians, sea pens, soft corals, etc.) and all Anthozoans feed to some extent in this way. This method involves the capture and collection of microscopic food particles on a sticky surface of the animal. After a large amount of these "microscopic morsels" are stuck, they are transferred to the mouth for digestion. 

Many species of corals and anemones actually "farm" algae living in their tissue. This algae is called Zooxanthelle, and strikes a symbiotic (mutually beneficial) relationship with the coral. The algae provides food and oxygen to the coral, and in exchange receives nutrients and a protected place to live. The coral can derive up to 90% of its food needs from the algae! A powerful light source is required to insure that the algae can produce enough food, other than light, many Anthozoans do not require much additional feeding. 

Anthozoans usually consist of greater than 90 percent water. To rid itself of waste many Anthozoans will simply "wring themselves out". A football sized anemone can easily contract to a fist sized lump. This is a crucial point, since anemones placed too close together may expand and come in contact, resulting in injury. This aggression is a common cause of injury in many reef building hard corals so they need adequate space around themselves to accommodate expansion. Since corals can not move, the most common way to get new space for growth is to simply sting your neighbor to death and overgrow them!! 

HERMATYPIC CORALS AND ANEMONES. If your coral or anemone contains Zooxanthelle algae, it will usually appear bright green or brown, and will fluoresce under ultra-violet (U.V. or actinic) light. If this is the case, then your Anthozoan belongs to a group of hermatypic, or light loving species. Place them in an area of good lighting and water flow, and insure that they do not come in contact with each other or any equipment (even when expanded!!). If provided with lots of light ( See reef lighting ), most hermatypic Anthozoans need only an occasional supplemental feeding (apx. once a week), of trace elements (Specifically iodine, molybdenum, strontium, calcium) and an organic coral food. 

AHERMATYPIC CORALS. These anthozoans do not require light to survive, are usually brightly colored and will not fluoresce under U.V. light. These Anthozoans, like hermatypic corals enjoy good water flow and proper placement, but want reduced lighting, and increased feeding. Since they can not rely on algae for food, they must be fed more regularly. Directionalized feedings of small plankton, and supplemental liquid invertebrate foods are a good start. 

ZOOANTHID AND ENCRUSTING OCTOCORALS. These organisms look like very small anemones. They usually have eight or multiples of eight tentacles, and you could easily place 5 on a quarter! Most enjoy light, but check for fluorescence to be sure. These organisms require FAST water flow, and an ample supply of liquid invertebrate food and supplements. It is not uncommon for the colony to shrink a bit at first, but under good conditions, they will grow and colonize new space! 

SOFT CORALS. These Anthozoans usually resemble gelatinous blobs with small polyps. A general rule for these organisms is the more dull the color, the more need for good quality lighting. Again, water flow and liquid supplements are the rule. Occasionally part of the colony may become necrotic (start to decay). To ward off further decay, QUICKLY wash off the affected area with a strong stream of cool FRESH water, and QUICKLY return the specimen to the aquarium (this will work for almost any colonial Anthozoan experiencing decay. Many sort corals can expand dramatically, and some even like to be hung upside-down! So inquire about the habits of your specific soft coral. 

GORGONIANS These are some of the most dramatic and demanding of the Anthozoans. They require a powerful continuous water flow, preferably oscillatory (to and fro). Gorgonians need to be anchored firmly onto the rocks, in an area of diminished lighting, and enjoy frequent small feedings. Many gorgonians can not be kept successfully in aquariums that lack of suitable food. A diet of living plankton (fed at night, when the colony is feeding) substantially improves their chances for success. Also avoid putting them into tanks with lots of mechanical filtration, since this will filter out their food before they have eaten their fill. 

TUBE ANEMONES AND SEA PENS. These two groups are distantly related, but share a common need for sandy substrate. They should be placed in a part of the tank with at least three inches of sand. This will provide them with a space to anchor their "foot". Tube anemones are voracious predators that can expand dramatically. They could possibly catch and consume an unwary tank mate, so be for- warned!. Feed them about twice a week when they are fully expanded. Sea pens are totally harmless, and require strong unidirectional flow, diminished light, and frequent liquid feedings. 


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