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Corals

Corals are invertebrates classified in the phylum cnidaria (stinging nettle) and the word coral means ěhardî in Greek. Until 1831 corals were considered to be plants, but they are now considered to be animals. The main structure of a coral is a polyp, which means "many footed". A coral polyp is like an upside down human wisdom tooth. The tentacles extend up into the water to collect food, while the mouth and the stomach are in the base that attaches to a corralite skeleton. Corals eat various forms of plankton and are aided by Xooxanthellae algae. In an aquarium environment with a protein skimmer, it will be advisable to blend seaweed, shrimp, mussels, clams, oysters, fish roe and brine shrimp into a mixture that will provide adequate nutrients to corals.  

Many corals form colonies of polyps. A reef consists of living corals covering an extensive base of dead coral, which creates a gigantic limestone structure. Corals occur worldwide but always near the equator. Some corals are toxic to fish in order to protect themselves from predators. Most corals are sessile invertebrates, which means that they cannot move about.

Classification of the Phylum Cnidaria

  • Hydrazoa - fire and lace corals

  • Scyphozoa - jellyfish

  • Anthozoa - most corals and anemones, as detailed below

Classification of the Class Anthozoa

  • Octocorallia - blue coral, soft coral, mat polyps, gorgonians and sea pens

  • Hexacorallia (Zoantharia) - sea anemones, button polyps (colonial anemones), mushroom corals and stony corals

  • Ceriatipatharia - black corals and tube anemones

In the aquarium trade, stony corals (hard calcium exterior skeleton) are further grouped into:

  • SPS (small polyped stonies)

  • LPS (large polyped stonies)

Anemones

Although anemones look like plants, they are actually animals that lack a spine.  Anemone's require special lighting and excellent water quality.  Anemones have been popularized by the movie Finding Nemo, which features Nemo living in an anemone.  There are hundreds of species of anemone.  The most popular types are:

  • Heteractis Anemone - large anemone that attach to rocks and corals.  These are very difficult to keep.

  • Carpet Anemone - anemone that spreads out to produce a smooth carpet-like appearance.

  • Tube Anemone - has a cylindrical base with the tentacles at the top.

Anemones should be fed about twice a week.  They will eat fresh pieces of shrimp, scallop and mussel.

Water Quality

Invertebrates are very sensitive to water quality and should not form part of a beginner's aquarium.  Copper sulphate is the most common cure for saltwater fish disease, but even trace amounts will kill invertebrates.  Don't use common copper in your reef tank!