This is Atolla wyvillei, a deep-sea scyphozoan jellyfish. They reach about 15cm in diameter.
They are bioluminescent only when disturbed. Researchers have described this as their 'burglar alarm' - when attacked, they glow in the hope of attracting attention from something larger, which might distract their predator! In the ensuing confusion, the jellyfish can escape.

http://1.usa.gov/UAZdzk