In the phylum of cnidarians, we find in the class Anthozoa, the order Alcyonacea, and the family Coralliidae the Mediterranean red coral or Corallium rubrum. It is a coral found throughout the Mediterranean Sea, and in certain parts of the Atlantic Ocean, from the Portuguese coast to the north coast of Africa. It usually lives on the walls of marine caves and in general in poorly lit environments up to 250 meters deep.
Corallium rubrum is a colonial cnidarian whose hard, bright red calcium carbonate skeleton adopts an arborescent form up to 50 cm high. The colony clings to the rocky substrate tightly at its base. The polyps that make up the colony, which have 8 retractile tentacles, are of a bright white color that contrasts with the color of the skeleton. It is a slow-growing coral, whose colonies grow at a rate of about 3 to 4 cm every 10 years. This growth does not take place in a single plane, but the ramifications arise in all possible planes, and hence its arborescent appearance, although those planes located transversely to the prevailing currents of the place where they live predominate.
Numerous animal species find shelter in the branches of Corallium rubrum, such as molluscs like Ocinebrina paddeui, gastropods like Pseudosimnia carnea, and even crustaceans like Balssia gasti.
Being a species that inhabits places with poor lighting, the feeding of Corallium rubrum is based on the capture by the polyps that make up the colony, of small invertebrates and in general zooplankton and small organic particles that capture thanks to the tentacles of the polyps.
The reproduction of Corallium rubrum can be asexual by fragmentation of the colony, although it is not a commonly observed form of reproduction in this species. The second form of reproduction is sexual, which is the most common. Within a colony we find polyps specialized in reproduction, which can be male or female. Male polyps release sperm into the water column. The sperm through currents reach the female polyps, and fertilize the eggs internally, developing inside the female polyps the larvaes for about a month. After this time, the larvaes are released into the current where they spend a few weeks before settle to the bottom and generate a new colony.
The skeleton of Corallium rubrum, with its intense red color, is highly valued in the jewelry industry, a fact that has caused the endangerment of the survival of this species due to the over-harvesting to which it has been subjected during the last centuries. There is some controversy in this regard, especially regarding the over-harvesting that has occurred in the coastal regions where it used to live. Recently, new habitats have been discovered, hitherto totally unknown, where Corallium rubrum specimens are extremely abundant and of sizes practically impossible to observe in more coastal areas.
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