Everyone knows about coral reefs and almost everyone knows that the rock-like features of a reef are made small animals known as corals.  What a lot of people do not know is what kind of animal a coral is – it’s not what most people think.

It may surprise you to know that corals are actually a type of jellyfish.  We learned in a previous article on Portuguese Man-of-Wars, that jellyfish come in two forms – polyps and medusa.  The medusa are the ones most people think of when they think of jellyfish – a bell-shaped head with trailing tentacles.  The polyps, you might recall, are on the bottom of the sea, attached by their heads and extending their tentacles upwards into the water to capture food drifting by.  The example we gave was the sea anemone.  Corals are in the same class as sea anemones.

Polyp jellyfish fall into one of two classes – the hydrozoans and the anthozoans.  Hydrozoans possess sting cells (called nematocysts) in their tentacles.  Anthozoans possess nematocysts in their tentacles AND along the lining of their stomach.  The Portuguese Man-of-War is a hydrozoan.  Corals are anthozoans.

Anthozoans are the largest class of jellyfish with over 6000 known species.  Corals are subdivided into two groups – hard and soft corals.  Though some of the coral polyps can be large (10 inches in diameter), most we are familiar with are very small (0.05 inches in diameter).  Working with a symbiotic partner called zooxanthalle, they produce a calcium carbonate shell (same as the snails and oysters).  This shell is cup-like in shape and the corals live within.  At night they extend their tentacles to feed on plankton.  During the day they retract them and all we see is the stony rock of their calcium carbonate shells.  When the coral dies their offspring build their cups on top of their ancestors and the coral rock “grows” larger – with only the outer layer actually alive.  A large coral boulder could be centuries old.

Those who have snorkeled or dove the coral reefs of the Florida Keys are very familiar with the rock formations of the stony hard corals, but they have also observed the softer structures of the soft corals.  These would include the sea fans and sea whips we see attached to the stony corals wafting in the currents on the “calm side” of the reef (away from destructive waves).

We do not think of corals as jellyfish because they do not sting the way your classic medusa jellyfish do.  But they are jellyfish and they possess stinging cells, it is just the toxicity of their venom is weak, and we do not feel it.  There are exceptions to this rule.  Any diver who has encountered fire coral knows what I mean.  They do not call them fire coral for no reason.

So… corals are jellyfish… many did not think that.

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