By looking at their tentacles fluctuating with the current, it is hard
to believe that these singular organisms belong to the animal kingdom.
They resemble beautiful flowers so closely, that they were called sea
anemones. Furthermore, until recently, their primitive structure has
deceived so many scientists, that the sea anemones were classified as
vegetables. Later on, Linneo classified them as zoophyte
(animals-vegetables).
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