The 'immortal jellyfish' can turn itself back into a juvenile and start again.
When stressed, sick or injured, the tiny jellyfish Turritopsis dohrnii can do something almost unheard of in the animal kingdom: revert from a sexually mature adult to its juvenile polyp stage and begin life again. In theory it could repeat the trick indefinitely. In practice, most still get eaten.
Turritopsis dohrnii is a hydrozoan whose adult, or medusa, is a bell no more than about 4.5 millimetres across. Like many of its relatives, it lives a two-part life. Larvae settle on the seabed and grow into colonies of polyps, which bud off tiny free-swimming jellyfish. These become sexually mature within a few weeks, depending on water temperature. For most species that is a one-way street: the medusa breeds and eventually dies.
T. dohrnii's medusae can go back. In experiments, jellyfish at every stage, from newly released to fully mature, reverted when starved, exposed to sudden temperature changes or lower salinity, or even cut with scissors. The bell and tentacles break down, the animal settles, and within days it grows root-like stolons and new polyps that form a fresh colony. The key is transdifferentiation, in which specialised cells turn into entirely different cell types. In one experiment the reversal happened without any stress at all. Shin Kubota of Kyoto University, one of the very few people to keep the species long-term, reported his colony rejuvenating itself 11 times in two years.
That is why it is called biologically immortal: it appears to have no fixed maximum lifespan. But immortal is not the same as indestructible. The reversal has never been observed in the wild, partly because it happens quickly, and most medusae probably fall to predators or disease long before they get the chance. They are eaten by other jellyfish, sea anemones, tuna, sharks, turtles and penguins.
Researchers want to know how it resets its cellular clock. Studies point to mechanisms that maintain telomeres, the protective caps on chromosomes, and a 2022 comparison with a closely related jellyfish that cannot rejuvenate highlighted genes involved in DNA repair and stem-cell renewal. Meanwhile the species has quietly spread through temperate and tropical seas worldwide, probably carried in ships' ballast water, in what one Smithsonian scientist called a worldwide silent invasion.
Source: Wikipedia — Turritopsis dohrnii · Text summarised from Wikipedia (CC BY-SA 4.0)
