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Up to the nineteenth century, cnidarians, among other organisms such as echinoderms and sponges, were classified as zoophytes, or animal – plant. This term, initially used by Wotton in 1552 and later by Linné and Cuvier (Daudin 1926), was only referring at this time to the external shape of the organisms, fixed branched one. This term was abandoned in the twentieth century; it is however curious to note that biological and physiological reasons might support this term. Indeed, Brandt at the end of the nineteenth century showed the presence of photosynthetic algae inside the tissues of these animals. He suggested that these algae were symbiotic and called them zooxanthellae (Brandt 1881; see Perru 2003, for a review). Zooxanthellae belong to the dinoflagellate phylum and were initially considered as a single species, called Symbiodinium microadriaticum (Freudenthal 1962). It was presently shown to be highly diverse and subdivided in Symbiodinium clades (Pochon et al. 2006). Associations of these different clades with their host did not evolve randomly and members of the same cnidarian species generally harbor the same Symbiodinium clade(s) (see review by Coffroth and Santos 2005, Stambler 2010 this book). [ABSTRACT FROM AUTHOR]