As amanmazleifg said muscle wire (Nitinol) seems suitable, though festo does not use it. I looked at that jellyfish and other stuff that festo made in the past, they are amazing.
Ummm, maybe you can use some kind of elastic film shaped like a jellyfish and change its shape with a fan blowing/not blowing or adapting the speed. That can create a smooth jellyfish movement.
There is a little introductory text I had written about Nitinol, maybe it can help http://www.ndsgn.com/ShapeMemoryAlloyTips.pdf