The immortal jellyfish is an amazing creature that has the ability, even
once completely mature, to revert back to the polyp stage and start over
again. It is able to do this though a cell development process called transdifferentiation -- basically altering the state of an existing cell
into a new cell.
This metamorphosis begins with the umbrella of the medusa as it reverts itself into a new polyp colony and then reabsorbs the tentacles. Each of the tiny polyps grows into a new jellyfish. When fully grown the jellyfish are less than a quarter of an inch long. This repeated process renders them biologically immortal. Despite this "immortality" most of the jellyfish are likely to be consumed by predators or die of disease in the plankton stage before ever reverting back into the polyp state. The jellyfish also reproduce sexually
The immortal jellyfish is believed to have originated in the Caribbean, but now exist worldwide. The theory is that the little medusas have spread through ships offloading ballast water while in port.