Another case of symbiosis is the one between the mollusc Tridacna, which lives on the reefs of the Asian Pacific region, and the zooxanthellae. The latter live on the edge of the coating of the mollusc, which contains “hyaline bodies”, i.e. transparent bodies, that make it easier for light to penetrate; the zooxanthellae crowd around these bodies to perform the photosynthesis. The zooxanthellae are symbionts, also of Coelenterata, and reproduce through eggs. In the Mediterranean Sea, they live only in some sea anemones, but in the Tropical seas the symbionts live in very many species of Coelenterata, especially in the Madreporaria of the reefs. The polyp of the madrepores contains these algae that give it its brown-reddish colour. The algae supply the polyp with energy in the form of sugars and amino acids, produce oxygen and remove carbon dioxide (which could form carbonic acid and damage the calcareous skeleton of the polyps). In return, the polyps offer protection to these small algae. Every square centimetre of a madrepore contains approximately one million algae sp. Zooxanthellae.

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