The substrate

Fine-grained soils are to be avoided, a size of 2-5 mm being perfect. The bed must be calcareous and comprise sand made from corals and marine algae, which can be complemented by crushed oyster shells to guarantee a moderate supply of calcium carbonate.

The substrate in marine tanks consists of coral elements in fairly large grains.

ZOOXANTHELLAE

These are green micro-algae which live in the skin of Anthozoa and other organisms, such as sponges or giant clams, a type of bivalve mollusk. They exchange substances with the cells of their hosts. The carbon dioxide (CO2) resulting from the metabolism of the cells - i.e. the oxidation of the foodstuffs - is collected by the Zooxanthellae. These, in their turn, absorb nitrogenous and phosphorous substances, carry out photosynthesis, and produce organic substances that enhance both their own growth and that of their host. This type of mutually beneficial association taking place inside an animal is known as endosymbiosis. The Zooxanthellae contribute, to some extent, to the feeding process of Anthozoa, which therefore need only a very scanty external supply of nutrients: some organisms, such as the anemones, need to be fed only around once a week in an aquarium.

• The green coloring of this Anthozoa indicates the presence of a significant number of Zooxanthellae: this animal must therefore be placed under fairly intense lighting.

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