A transparent bell barely 2 cm (0.8 in) across with a deep groove running round it, splitting the top from a fringe of sixteen small lobes. The groove is what gives the crown jellyfishes their name, and it makes them separable from every other small medusa in the plankton.
The interesting half of the life cycle is not the jellyfish. The polyp stage lives inside sponges - a colony of tiny polyps in horny tubes embedded in sponge tissue, drawing water from the sponge's own current, and for a long time those tubes were described as part of the sponge rather than as another animal.
From there the polyps bud off young medusae, which is why a sponge can suddenly release a cloud of them. The medusa lives a matter of weeks, spawns and dies, while the polyp colony inside the sponge persists for years - so the animal a diver sees is the short-lived, disposable part of it.
The sting is negligible on human skin. Whether the sponge suffers from the arrangement is not settled, which is a fair summary of most partnerships this small.
- tiny transparent bell with a deep groove separating the top from the rim
- sixteen short marginal lobes and eight tentacles between them
- dark brown or reddish gonads visible through the bell
- bell up to 2 cm across
