Posidonia oceanica is a very peculiar aquatic plant, endemic to the Mediterranean Sea. Its features are similar to those in the terrestrial plants, such as roots, rhizomatous stem and ribbon-shaped leaves up to a meter long. Also, it flowers underwater, and its fruits can float. But the most interesting fact is that it forms large underwater meadows hosting a great biodiversity and with a huge ecological importance. Therefore, this underwater plant is considered a good bioindicator for measuring the quality of coastal marine waters.