A heavy brown-purple jellyfish up to 60 cm (24 in) across, its bell raised into a knobbly crown that gives the family 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 is mild on most skin and unpleasant on the face and neck, and it comes from those trailing filaments rather than the bell. A swarm passing a dive site is a nuisance rather than a hazard, though a diver who surfaces through one usually remembers it.
Every large bell carries passengers. Juvenile jacks, drums and occasionally small crabs travel in the shelter of the arms, using a stinging animal as a mobile refuge in open water, and they will follow the bell as it drifts rather than leave it.
Like other true jellyfish it alternates generations: the medusa spawns, the larva settles as a tiny polyp on hard bottom, and that polyp clones itself and buds off young medusae later, so a bloom 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, with thread-like filaments trailing
- swims strongly for a jellyfish, pulsing steadily rather than drifting
- bell up to 60 cm across
