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What is a soft coral? Well its a coral thats soft isn't it? Ah... . well no, not quite right.
In fact there are soft corals that are well armoured with Calcium Carbonate plates and others live within a Calcium Carbonate shell eg the difficult to keep Pipe Organ Coral (Tubipora) that is well known for its attractive red skeleton.
The true soft coral belongs to the subclass Octocorallia. As the beginning of the name suggests, the defining part of their structure is to do with the number eight. In fact they have eight (or a multiple of eight) tentacles on each polyp. However don't get excited if you count a different number of tentacles, as individual polyps can be deformed and may have an unusual number! Very close inspection of these eight tentacles will often reveal they have branches giving them a feathery appearance. A common example of this phenomenum is the Clove Polyp. These extensions off the tentacles are called pinnules. However many soft corals have evolved to lose these pinnules to a greater or lesser extent.
Soft corals contain small spicules of calcium carbonate called Sclerites within their tissues. These are increasingly used to distinguish different soft corals. Useful when you consider that the same coral will often look different depending upon the adaptions it makes to live with localised conditions, such as waterflow, light levels and amounts of available food in the water column.
This is particularly true in the reef aquarium where conditions can vary greatly. For example when I moved over from skimming to a mud style refugium system, the increased polyp size dramatically changed the appearance of many corals.