This Blue Button Jellyfish, or porpita porpita, at Enoshima Aquarium is not actually a jellyfish. It’s a hydroid made up of individual zooids that float together at the water’s surface and each function differently: some eat, some reproduce, some defend, etc. From Coastal and Coral Culture:
Porpita porpita are hermaphrodites and have two main body structures. The first part is the float which is a round disc like shape and is a golden-brown color. It is typically 1.5 inches wide or less, and has a single mouth underneath the float which is used for both the intake of nutrients and the dispersal of wastes. The second part is the hydroid colony (jellyfish like tentacles) that are bright blue, turquoise or yellow. Each strand is covered in branchlets and end in knobs of stinging calls called nematocysts.
Porpita porpita stings usually do not hurt but can cause skin irritation. They have gaseous bodies which allow them to float on the surface and are propelled by wind and ocean currents.