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Coral reefs are warm, clear, shallow ocean habitats that are rich in life. The reef's massive structure is formed from coral polyps, tiny animals that live in colonies; when coral polyps die, they leave behind a hard, stony, branching structure made of limestone.
The coral provides shelter for many animals in this complex habitat, including sponges, nudibranchs, fish (like Blacktip Reef Sharks, groupers, clown fish, eels, parrotfish, snapper, and scorpion fish), jellyfish, anemones, sea stars (including the destructive Crown of Thorns), crustaceans (like crabs, shrimp, and lobsters), turtles, sea snakes, snails, and mollusks (like octopuses, nautilus, and clams). Birds also feast on coral reef animals.
Types of Corals: There are two types of coral, hard coral and soft coral. Hard corals (like brain coral and elkhorn coral) have hard, limestone skeletons which form the basis of coral reefs. Soft corals (like sea fingers and sea whips) do not build reefs.
 Where are Coral Reefs?: Coral reefs develop in shallow, warm water, usually near land, and mostly in the tropics; coral prefer temperatures between 70 and 85 ° F (21 - 30 °C). There are coral reefs off the eastern coast of Africa, off the southern coast of India, in the Red Sea, and off the coasts of northeast and northwest Australia and on to Polynesia. There are also coral reefs off the coast of Florida, USA, to the Caribbean, and down to Brazil.
The Great Barrier Reef (off the coast of NE Australia) is the largest coral reef in the world. It is over 1,257 miles (2000 km) long.
Types of Reefs: The different types of reefs include:
- Fringing reefs are reefs that form along a coastline. They grow on the continental shelf in shallow water.
- Barrier reefs grow parallel to shorelines, but farther out, usually separated from the land by a deep lagoon. They are called barrier reefs because they form a barrier between the lagoon and the seas, impeding navigation.
- Coral Atolls are rings of coral that grow on top of old, sunken volcanoes in the ocean. They begin as fringe reefs surrounding a volcanic island; then, as the volcano sinks, the reef continues to grow, and eventually only the reef remains.
Coral Reefs in Danger: Many coral reefs are dying. Major threats to coral reefs are water pollution (from sewage and agricultural runoff), dredging off the coast, careless collecting of coral specimens, and sedimentation (when silt or sand from construction or mining projects muddies the waters of a reef and kills coral, which needs light to live).
Acknowledgment and thanks go to http://www.enchantedlearning.com
Soft corals, typified by their internal fleshy skeletons.
Classification: Taxonomy, Relation With Other Groups
Here I'll present some of the higher taxonomy of the group and their kin, as I've found an understanding of the same to be a tremendously helpful tool in identification and keeping straight the husbandry of related forms.
The soft corals are members of the stinging-celled animals, Phylum Cnidaria ("Nigh-dare-ee-ya"), formerly Coelenterata; a group that includes the anemones, jellyfishes, hydroids, sea-pens, the true corals and other coral-named groups.
Cnidarians are tissue-grade life characterized by having just two germ layers (ecto- and endoderm), stinging-cells, and principal radial symmetry. Other salient characteristics; they have a single body cavity (the coelenteron) that is sac-like, with one opening that serves as both an mouth and an anus; lack a central nervous system (have simple nerve nets), no head or gas exchange, excretory or circulatory systems.
The phylum Cnidaria is separated into three Classes roughly by the principal form (bell-shaped free-living medusoid, or attached polypoid) they take as life stages.
Class Scyphozoa, the jellyfishes, are mainly medusoids.
Class Hydrozoa, the hydroids and hydromedusae, display alternation of generations with asexual benthic polyps alternating with sexual planktonic medusae.
Class Anthozoa, sea anemones, corals, sea pens; have no medusoid stage. They are further subdivided into two subclasses.
Subclass Hexacorallia (=Zoantharia), sea anemones and true corals (Order Scleractinia), have tentacles and mesenteries (internal body divisions) in multiples of six ("hex"), 0,1,2 siphonoglyphs (slot-like mouth/anus openings).
Subclass Octocorallia (=Alcyonaria), the octocorals called soft, blue, organ-pipe corals, sea fans, sea pansies. Colonial polypoids, with eight-numbered mesenteries and hollow tentacles (pinnately, i.e. side-branched like a bird's feather), one siphonoglyph. A few orders of note to aquarists:
Order Gorgonacea, sea fans, sea whips. Non-living central structure with living "rind" covering.
Order Pennatulacea, sea pens, sea pansies.
Order Heliporacea, blue "coral"
Order Stolonifera, red organ-pipe "coral" (Tubipora).
And finally, the Order Alcyonacea ("Al-see-oh-nay-see-ah"), the soft corals. Made up of either encrusting or erect colonies, mostly fleshy and flexible with a bizarre assortment of internal structural elements called sclerites, rendering shape and structure.
Briefly, you can see that it is not only the lack of external hard, stony, calcareous skeletons that the "true" corals (Order Scleractinia, Subclass Hexacorallia) from the soft fleshy or leathery corals, the Alcyonacea and their relatives; but major elements of body plan and symmetry (tentacular and body segmentation number, mouth-anus openings).
Natural Range
Soft corals are found worldwide, more in tropical than temperate reefs, mainly in mid-depths of 5-30 meters. Abundance on reefs in the Indo-Pacific and Red Sea is often conspicuous compared with stands of colonies in the Caribbean, Hawaii and elsewhere.
Behaviour:
Territoriality
One of the obvious strange attributes of soft corals is the lack of "fouling organisms" that bedevil them. Take a look; do you see algae, worms, barnacles, anything growing on or in them? Apparently not. Alcyonaceans have been investigated for chemical reasons for such cleanliness for bio-pharmacological and anti-fouling properties. Yes, they have allelopathic (toxic to other species) chemical effects; terpenoids and other molecules that poison would-be settlers and neighboring life forms.
Predator/Prey Relations
Few organisms are known to be really deleterious to soft corals in the wild; some mollusks and butterflyfishes will chew on them, but rarely to their destruction. You might want to check new arrivals for the presence (and eradication) of commensal crabs; these may prove destructive.
Reproduction, Sexual Differentiation:
Soft corals reproduce asexually by budding and fragmentation, and sexually via egg and sperm production. They can be artificially propagated by way of cutting and grafting. A typical simple pro forma involves slicing off a reasonable size piece, dipping it into sterilizing solution and attaching it with epoxy or "super" glue to a hard substrate.
Acknowedgment and thanks go to http://www.wetwebmedia.com
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