What are corals? (taxonomy question)

[ludnix]

When I show my aquarium to visitors I'm often asked what things are, I'll try and explain what corals are and the common names for some of them. Often though there is a lot of confusion when it comes to anemones, my guest will look at my LTA after I explain that it is an anemone, then look at frogspawn and ask if that is an anemone.

I usually say that they are both cnidarians but anemones are usually considered as "invertebrates" and not as corals.

What is this difference really though? Stores online seem to include any sessile cnidarian as a coral while any that moves is usually an invertebrate. Some text-book definitions of coral say that it's all the animals that fall under the class Anthozoa, but anemones fall under this class as well.

So are anemones really "corals" or does that word have a more specific definition that somehow excludes the Actiniaria order?

 

[xJake]

Corals and anemones (as well as a host of other organisms) are classified as cnidarians. The defining feature of a cnidarian is the presence of nematocysts (stinging cells), so in that sense, yes corals and anemones are very closely related.

Most biologists, including myself, generally think of the general term "coral" as any cnidarian that uses dissolved calcium carbonate to build significant amounts of skeletal tissue. This is generally seen as the main difference between "corals" and other cnidarians. Obviously there are some exceptions to this taxonomic feature, such as "corallimorpharians," some species of "hexacorallians," and some species of "octocorallians," yet these animals still generally require dissolved calcium carbonate for growth and produce what are commonly known as "spicules" (tiny crystalline slivers) of calcium carbonate.

IIRC, anemones don't use any significant amounts of calcium carbonate for growth and contain little-to-no skeletal tissue. The same is true of most non-sessile cnidarians such as "jellies."

 

[ludnix]

Thanks, xJake, I hadn't realized the "soft" corals take in any appreciable amount of calcium carbonate let alone any more than an anemone might.

 

[LockeOak]

Phylum Cnidaria (= Coelenterata)

Class Scyphozoa - True Jellyfish

Class Cubozoa - Sea Wasps

Class Hydrozoa - Hydroids, fire "corals", hydrocorals, Man-of-War, Velella, a few others. No true corals.

Class Anthozoa
Subclass Alcyonaria (= Octocorallia). Gorgonians, soft corals, sea pens.
Subclass Zoantharia (= Hexacorallia).
Order Actiniaria - Sea anemones.
Order Corallimorpharia - Corallimorphs (note, see below)
Order Scleractinia - True (stony, hard) corals.
Order Zoanthidea - Zoanthids, inc. Palythoa
Order Ceriantharia - Ptychocyst-bearers
Order Ptychodactiaria - Anemone-like.
Order Antipatharia - Black or thorny "corals".


Within the Scleractinia the phylogenies are up in the air. A recent study by Nancy Knowlton's lab using nuclear and mitochondrial markers essentially found that almost no families within the Scleractinia are monophyletic--that is, that the current phylogenies are incorrect, though better phylogenies are still in the works. Scleractinia was found to be a monophyletic order. Corallimorphs were shown to be the closest relatives to the true corals (Scleractinia).

Fukami et al. 2008. Mitochondrial and Nuclear Genes Suggest that Stony Corals Are Monophyletic but Most Families of Stony Corals Are Not (Order Scleractinia, Class Anthozoa, Phylum Cnidaria). PLoS One.