Purple Jellyfish

This animal has no mouth. It lost the one it was born with.

Rhizostome jellyfish start with a single central mouth like any other medusa and close it over as they grow, replacing it with thousands of tiny openings scattered along the arms. Each one draws in plankton caught in mucus and passes it into a branching canal system that runs back into the bell. The result is an animal that feeds along a meter of trailing tissue at once instead of at a single point, and that has no throat, no gut opening and no way to take anything large.

What you see is a smooth pink-purple dome of 20 cm (8 in) trailing eight ribbon-like arms several times its own length, twisted and frilled, tapering to fine points. There are no tentacles around the rim, which is the quickest way to separate this group from the jellyfish that sting badly.

The sting lives in the arms, not the bell: brushing the dome does nothing, and what usually catches a swimmer is a fine thread shed from an arm. The marks fade within the hour.

The drifting animal is only half the species. Larvae settle on hard bottom as tiny polyps that clone themselves for years and release young medusae when conditions turn, which is why these arrive in numbers rather than one at a time, and why they turn up in the same bays season after season.