Under stress the adult medusa can revert to a polyp and start its life over, which is a real finding wrapped in a bad name.
A hydrozoan medusa a few millimetres across, transparent, with a bright orange-red stomach visible through the bell and a fringe of fine tentacles around the margin. Small, and easy to pass over on a blackwater dive unless you are looking for the colour of the stomach.
The name comes from a genuine observation. In Turritopsis dohrnii, an adult medusa that is starved, injured or otherwise stressed can settle onto a surface, reabsorb its tentacles and bell, and transform back into a polyp — the juvenile stage — which then buds new medusae. The cells change type to do it. The process is called transdifferentiation and it reverses the normal one-way direction of the life cycle.
Two corrections are worth making, because the animal is written about badly. It is not immortal: it is eaten, and it dies of disease and of being a few millimetres long in an ocean. And the reversal is demonstrated for T. dohrnii, a species described from the Mediterranean. Several Turritopsis species occur in the Indo-Pacific, they are not separable in a photograph, and how widely the ability is shared across the genus is not settled. This entry claims the genus and nothing more.
Ratings are per location: 1 means found on most dives that look for it, 5 means a handful of records. Where enough dives have been logged the figure is calculated from them; otherwise it is an editorial estimate, and says so. We publish habitat and destination, not the dive site.
