What is the difference between corals and sea anemones?

As you may know, the Anthozoa belong to the Cnidaria or cnidarians, such as the Hydra and the jellyfish, an extremely fascinating and variable group of animals! They consist of two layers of cells: the gastrodermis which lines the inner cavity and an epidermis which forms the outer side. The internal cavity communicates with the outside world through a single opening, the mouth; the mouth is surrounded by tentacles to catch the prey. In the epidermis (but also in the gastrodermis) are stinging cells or cnides that give the name to the group. The cnides of the jellyfish can be very aggressive, those of anemones and corals are (for us!) much less so.

The internal cavity (gastrocoel) of the Anthozoa is divided into compartments; either a multiple of six in the Hexacorallia or eight in the Octocorallia. All anemones – so the Anthozoa without hard parts – belong to the Hexacorallia. The skeletonized Hexacorallia have a calcareous skeleton that is produced by the epidermis UNDER the animal. Most species form large, often massive colonies and can therefore build strongly on the coral reefs. If the colony dies for one reason or another, the calcareous skeleton remains behind, on which other organisms can attach themselves.

The Octororallia not only have a gastrocoel with eight compartments, but also only eight tentacles, which are also feathered. They have a skeleton BETWEEN the epidermis and the gastrodermis (in the so-called coenenchyma). That skeleton can be soft (as with the dead man’s thumb in the North Sea) or hard, but always made of protein, sometimes with a little lime. Those Octocorallia make up the beautiful plume-shaped Gorgons and also the red coral from the Mediterranean Sea from which jewels are made (now fortunately protected species).

So, Sophie, I think your question has been answered and I hope your interest in these fascinating animals has been piqued. If you want to know more, send me an email. I can also send you some drawings.

Kind regards

Ernest.Schokaert@uhasselt.be