Turritopsis nutricula may be the world's only "immortal" creature.

Jellyfish usually die after propagating but Turritopsis reverts to a sexually immature stage after reaching adulthood and is capable of rejuvenating itself.

The 4-5mm diameter creature, technically known as a hydrozoan, is the only known animal that is capable of reverting to its juvenile polyp state.

Theoretically, this cycle can repeat indefinitely, rendering it potentially immortal.

Found in warm tropical waters Turritopsis is believed to be spreading across the world as ships' ballast water is discharged in ports.

Though solitary, they are predatory creatures and mature asexually from a polyp stage.

The jellyfish and its reversal of the ageing process is now the focus of research by marine biologists and geneticists. It is thought to achieve the feat through the cell development process of transdifferentiation, in which cells transform from one type to another.

The switching of cell roles is usually seen only when parts of an organ regenerate. However, it appears to occur normally in the Turritopsis life cycle.