This is the questions I get asked all the time? No the jellyfish are not real! Jellyfish are all glass, the glass is heated up to 2500 degrees then hand blown into paperweights to look like a jellyfish. If you were to try and put a jellyfish in glass, a real jellyfish is 95% water, what do you think would happen? Exactly it would disintegrate. Don't believe all you see at craft shows and retail stores.
Jellyfish are naturally phosphorescent, one of the reason we infuse phosphorus into the glass blowing process. Once the glass is exposed to light, it will glow in the dark at night time for an hour or more.