The sea Walnut (Mnemiopsis leidyi) is a ctenophore belonging to the Bolinopsidae family. The Mnemiopsis is easily mistaken for a jellyfish because of its transparency, but it is far from being it.
The transparent and oval body has six lobes, two of which are evident at first sight (each large lobe alternates with two small ones), on which rows of ciliated combs are arranged; two rows for each large lobe and only one for each small lobe. These eyelashes shine with a blue-green light when they are stimulated by light or contact. He uses his numerous tentacles to feed himself, but, unlike cnidarians, they are not stinging to humans: they are adhesive colloblasts that do not emit any toxins that are arranged along two long and fine tentacles that float in the water. The entire body is 97% water and small: it measures about 7-12 centimeters long by 2.5 cm wide.
It is a species native to the Atlantic introduced with ballast water from tankers in the Black Sea, the Baltic and the Caspian. With time it has also colonized the whole Mediterranean. Mnemiopsis leidyi is a destructive and very intrusive species when not controlled by its predators.
(extract from Wikipedia)