Luckily enough, during the middle of the last century, Leuckart identified them as animals. In spite of their elegant and harmless semblance, they are carnivorous and sometimes active predators. The actinia are present in almost all the seas. However, the most beautiful and colourful species can be found in the tropical seas. Usually they are benthonic animals, and they live at the bottom of the sea, where they are attached to rocks, stones and seashells through a foot provided with an adhesive disc. Some species have the lower part shaped differently, so they can anchor themselves into the sand. The sea anemones that live a pelagic life are rare.
In the Mediterranean Sea, the Actinia range from a few millimeters up to about 20 centimeters in size (Anemonia sulcata). Certainly, they are very small compared with an Australian sea anemone of the Stoichactis type which can have a diameter of more than one meter.
Their structure is astonishingly simple. The cylindrical main body is sustained by some sort of jelly (the mesoglea), and is rich in water, which is stored between the two sides of the "cylinder". The upper part has a mouthlike disk, surrounded by a crown of tentacles which can be used for both attack and defense because of their urticant properties. In the middle of the mouthlike disk, there is the mouth, which leads to a single cavity (celenteron), which has a septum through which the food is digested and the waste is eliminated. When a prey (which usually is a small worm or fish) gets near an actinia, the tentacles envelop it. The tentacles are provided with special cells (cnidoblasts) which contain poisonous matter that could be injected into the victims body through particular structures called cnidocysts. This poison can paralyze and eventually kill the unlucky prey.
Certainly, many of us have experienced the urticant effects produced after touching these animals. The relation between the actinia and the other living organisms, both animals and vegetables, is very peculiar. Some sea anemones live in mutual symbiosis (which is advantageous for both of them) with unicellular algae of the gender zooxantella. In other cases, fish of the gender amphiprion, which are immune from the urticant substances, find refuge between the tentacles of some actinia to escape from the natural enemies. However, the most known mutual symbiosis is that between the crustaceans Eupagurus Bernhardus, the Pagurus Arrosor,and the Paguristes Oculatus, with the actinia Calliactis Parasitica. The actinia protects them from predators with its urticant tentacles, and at the same time can find more food because of the movement of the crustaceus.
Indeed, it is surprising that animals which are so primitive and which are mainly made by water (their body contains up to 95% of water), can have such a complex relation and can adapt to the surrounding environment in such an efficient way.
PHYLUM - Celenterati CLASSE - Antozoi ORDINE - Attiniari