Every large bell carries passengers. Juvenile jacks and drums, sometimes small crabs, travel inside the curtain of arms, using a stinging animal as a moving shelter in open water where there is nothing else to hide behind. They stay with the bell as it drifts rather than leaving it, and looking under a jellyfish is one of the more reliable ways to see a fish you would otherwise never find.
The host is a heavy brown-purple animal up to 60 cm (24 in) across, its bell raised into the knobbly crown that gives the group its name, with a dense bunch of frilly arms underneath trailing fine filaments. It pulses steadily and makes real headway, which most jellyfish do not.
The sting comes from those filaments rather than from the bell, and it is mild on most skin and unpleasant on the face and neck. A swarm crossing a dive site is a nuisance rather than a hazard, although a diver who surfaces through one tends to remember it. It is mild enough that the species is fished for food in 🇯🇵 Japan and 🇨🇳 China, and turtles eat it whole.
Like other true jellyfish it alternates generations. The medusa spawns, the larva settles on hard bottom as a tiny polyp, and that polyp clones itself and buds off young medusae later - so a swarm reflects conditions on the seabed months earlier rather than a sudden explosion of adults.
- brown-purple bell with a raised knobbly crown on top, like a cauliflower
- dense bunch of frilly mouth arms below, trailing thread-like filaments
- pulses steadily and makes real headway instead of drifting
- usually carries juvenile fish riding under the arms
- bell up to 60 cm across
