Which sea creature can reverse its own ageing?

Turritopsis dohrnii, a jellyfish a few millimetres across. A jellyfish normally lives as a stalked polyp attached to the sea floor and then buds off the free-swimming medusa everyone recognises, which breeds and dies. This one, when starved, damaged or simply stressed, can convert the medusa back into a polyp - reversing the life cycle - and start again.

The mechanism is transdifferentiation: cells that are already specialised turn directly into other kinds of cell, which is something animal cells are generally not supposed to do. Muscle can become nerve. In principle the loop has no limit, and it is why the species is described in the press as immortal.

The word needs qualifying. Nothing stops it being eaten, and in practice most are. It also does not evade death so much as refuse to age - and the same trick that fascinates gerontologists makes it an unusually effective invader, since it can survive being carried across the world in ballast water in conditions that would kill anything else. It is now found in oceans it certainly did not start in.