What is a Coral

Introduction

There appears to be some confusion among hobbyists as to what organisms are corals and which ones aren't. While marine biologists don't always agree what should be included as a "coral" (see below) there is little disagreement on which cnidarians are not corals (e.g. anemones, zoanthids and corallimorphs).

Part of the problem is there is no one definition of a coral and there is no single taxonomic groups that constitutes all corals. What is important is that the term "coral" is merely a label to allow grouping of some animals for discussion. It is no different than say a grouping such as "dog" or "cat", both of which may be used for either a single species or a group of similar species.

Corals

The following are considered by most biologists to be corals:

• true stony corals - Order Scleractinia in Subclass Hexacorallia (or Zoantharia)
• octocorals - Orders Alcyonacea (soft corals and sea fans) and Helioporacea (blue corals) in Subclass Octocorallia
• hydrocorals - Orders Millepora (fire corals) and Stylasterina (rose corals) within Class Hydrozoa

The hydrocorals aren't even in the same class as stony corals or octocorals and share a class with hydroids and even some jellyfish. You could even add the black corals (Order Antipatharia in Subclass Ceriantipatharia) as corals.

With this grouping or definition of what is a coral, there is not always agreement. For example, in Corals of the World, Charlie Veron only includes the Scleractinia, the hydrocorals, the blue corals and one genus of soft coral (Tubipora) as these are the only "corals" to contribute to building a reef. Other biologists consider all the above to be corals.

Gallery

• 
  Acropora sp.
• 
  Sarcophyton sp.
  Leather Coral
• 
  Distichopora sp.
  Purple Lace Coral
• 
  Symphyllia agaricia
  Brain Coral

Non-Corals

There are a number of groups of organisms that, while closely related to some of the above groups, are not considered corals. Anemones, zoanthids and corallimorphs share the same subclass as the stony corals (Hexacorallia) but are not considered corals. Recent evidence even suggests that the corallimorphs actually evolved from stony corals, nevertheless, they are not considered corals.

It can certainly be argued that anemones, zoanthids and corallimorphs are anatomically and physiologically very close to corals. However, that is largely irrelevant because the aren't considered to be corals. For example, damselfish are closely related to anemonefish (they are in the same family) and yet, damselfish are not considered anemonefish, even though anatomically and physiologically they are almost identical.

That's just the way things go. A name is a name is a name. And the name "coral" is not used for anemones, zoanthids and corallimorphs.

Gallery

• 
  Entacmaea quadricolor
  Bubble Tipped Anemone
• 
  Zoanthus sp.
  Green Zoanthid
• 
  Discosoma sp.
  Corallimorphs

Resources

• Corals in the Reef tank. What They Are and How They Live by J. Charles Delbeek - Aquarium USA 1995