Crown Jellyfish

The jellyfish is the disposable half of this animal. The half that lasts lives inside a sponge.

The polyp stage of a crown jellyfish is a colony of tiny polyps in horny tubes embedded in sponge tissue, drawing water from the sponge's own current and taking food out of it. For years those tubes were described as part of the sponge itself rather than as a second animal living in it. From there the colony buds off young medusae, which is why a sponge can suddenly release a cloud of them.

The medusa lives a few weeks, spawns and dies. The colony inside the sponge persists for years. So the drifting bell a diver notices is the short-lived, expendable part, sent out to breed, and the animal proper is somewhere behind, inside something else.

What you see is a transparent bell barely 2 cm (0.8 in) across with a deep groove running around it, splitting the top from a fringe of sixteen small lobes with eight tentacles between them. That groove is what gives the crown jellyfishes their name and separates them from every other small medusa in the plankton. Dark brown or reddish gonads show through the tissue.

The sting is negligible on human skin. Whether the sponge gains anything from the arrangement, or merely tolerates it, has never been settled - which is a fair summary of most partnerships this small.