Until I saw this I could not imagine a jellyfish could grow to this size. The Lions Mane jellyfish, on the picture, is the largest known spices of jellyfish. They usually are 2.5 meters in bell diameter and 35 meters in length (longer than blue whales).
The tentacles are extremely sticky and are grouped into eight clusters, each cluster containing over 100 tentacles. They are poisonous and help the creature capture and eat passing fish. The Lions Mane jellyfish live 20 meters deep in arctic waters and exist since before dinosaurs.