A moon jellyfish.

Image ID: reef2547, The Coral Kingdom Collection
Photographer: Florida Keys National Marine Sanctuary Staff
Credit: Florida Keys National Marine Sanctuary

Photo Source:
The NOAA Photo Library - http://www.photolib.noaa.gov
The National Oceanic & Atmospheric Adminstration (NOAA) - http://www.noaa.gov/
Copyright Information - http://www.photolib.noaa.gov/aboutimages.html

The common jellyfish, moon jellyfish, moon jelly, or saucer jelly (Aurelia aurita) is a widely studied species of the genus Aurelia. The moon jellyfish is translucent and can be recognized by its four horseshoe-shaped gonads, easily seen through the top of the bell. Aurelia aurita is found along the eastern Atlantic coast of Northern Europe and the western Atlantic coast of North America.