In this beautiful photograph by Alexander Semenov (underwater photographer extraordinaire), we see Aurelia aurita, commonly known as the moon jelly. It is translucent and typically ranges between 5-40 cm in diameter.
They are carnivorous and primarily feed on small plankton organisms such as mollusks, crustaceans, tunicate larvae, copepods, rotifers, nematods, young polychaetes, protozoans, diatoms, and eggs. Aurelia are capable of only limited motion and drift with the current even when swimming. [Sources: Animal Diversity Web, Wikipedia]
If you’re into underwater photography you should definitely check out more of Semenov’s work. You can find him active on Flickr, as well as his personal webiste Clione.ru. For prints, check out his page on Artflakes.