A smooth pink-purple dome trailing eight ribbon-like arms several times its own length - up to a meter of them behind a bell of 20 cm (8 in). There are no tentacles around the rim, which is the quickest way to tell this group from the jellyfish that sting badly.
It has no mouth either, in the ordinary sense. Rhizostome jellyfish lose the single central mouth as they grow and replace it with thousands of tiny openings scattered along the arms, each drawing in plankton trapped in mucus and passing it into a branching canal system. The animal feeds along its whole trailing length rather than at one point.
The sting is mild on human skin, and the arms are where it lives - brushing the bell does nothing. Fine threads shed from the arms are what usually catch a swimmer, and the marks fade within an hour.
As with other true jellyfish, the drifting animal is only half the life cycle: larvae settle on hard bottom as tiny polyps that clone themselves for years and bud off young medusae when conditions turn, which is why these appear in numbers rather than one at a time and why they arrive in the same bays season after season.
- smooth domed bell in pink to purple, with no tentacles around the rim at all
- eight long ribbon-like arms trailing far behind, several times the bell diameter
- arms twisted and frilled along their length, tapering to fine points
- bell up to 20 cm, arms to about 1 m
