What is the largest species of jellyfish by bell diameter?

The lion's mane jellyfish is the largest known jellyfish species by bell diameter.

Its scientific name is Cyanea capillata, and exceptionally large individuals can have bells measuring roughly 2 meters—or about 7 feet—across. A famous specimen recorded off Massachusetts in the 19th century also had trailing tentacles extending roughly 36 meters, or 120 feet.

The lion's mane jellyfish is named for its dense mass of reddish or golden tentacles, which resemble a lion's mane. Its bell is divided into eight lobes, and the tentacles carry stinging cells used to capture plankton, fish, and other small animals. The species prefers cold northern waters, including parts of the Arctic, North Atlantic, and North Pacific.

A common mix-up is confusing largest bell with greatest body mass or longest overall span. Nomura's jellyfish can be extremely heavy, while the lion's mane is especially notable for its broad bell and exceptionally long tentacles. Portuguese man o' war is not technically a single jellyfish animal but a colonial siphonophore.