sea blubber

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A scyphozoan jellyfish of the genus Cyanea, especially the largest jellyfish, C. arctica, which usually grows to more than 3.6 meters (12 feet) wide and has tentacles over 30 meters (100 feet) long.

Wiktionary

1. n. A large jellyfish, Cyanea capillata.