DevBio / Wikipedia:
Hydrozoan cnidarians usually have a complex life cycle, wherein a colonial stage leads to the sexually mature, solitary, adult stage. Eggs and sperm from solitary, sexual, adult medusa (jellyfish) develop into an embryo and planula larva, and they then form the colonial polyp stage. Medusae are formed asexually from polyps. These medusae have a limited lifespan and die shortly after releasing their gametes.
The hydrozoan Turritopsis nutricula has evolved a remarkable variation on this theme, and in so doing appears to have achieved immortality. Its medusa (jellyfish) form can revert to the polyp stage after becoming sexually mature. It is the only known case of a metazoan capable of reverting completely to a sexually immature, colonial stage after having reached sexual maturity as a solitary stage. It does this through the cell development process of transdifferentiation. Theoretically, this cycle can repeat indefinitely, rendering it biologically immortal although in nature, most Turritopsis, like other medusae, are likely to succumb to predation in the plankton stage, or disease, without ever reverting to the polyp form.
The relatively large stomach is bright red and has a cruciform shape in cross section. Young specimens 1 mm in diameter have only eight tentacles evenly spaced out along the edge, while adult specimens have 80-90 tentacles. The medusa is free-living in the plankton.
Turritopsis nutricula also has a bottom-living polyp form, or hydroid, which consists of stolons that run along the substrate, and upright branches with feeding polyps that can produce medusa buds. These buds develop over a few days into tiny 1 mm medusae, which are liberated and swim free from the parent hydroid colony.
Turritopsis are found in temperate to tropical regions in all of the world's oceans. Since the species is immortal, the number of individuals could be spiking. "We are looking at a worldwide silent invasion," said Smithsonian Tropical Marine Institute scientist Dr. Maria Pia Miglietta.