The Jellyfish That Reabsorbs Itself and Starts Over

In 1992 a team led by Italian biologist Giorgio Bavestrello reported something odd in the Mediterranean species Turritopsis dohrnii. The adult medusae they'd collected refused to die. When stressed by injury, starvation, or bad water, they did not decompose. They pulled in their tentacles, shrank, sank, and re-attached to a surface as a polyp — the colonial stage that's supposed to come before the medusa. From there they began producing free-swimming medusae again, genetically identical to the one that had just rolled itself up.

The biology underneath has a name: transdifferentiation. Mature, specialised cells abandon their identity and become a different cell type, a muscle cell becoming a nerve cell or a stinging cell becoming part of an intestinal lining. Most animals cannot do this in any meaningful way. T. dohrnii does it across multiple tissue types. Genome work has flagged enrichment in DNA-repair and telomere-maintenance genes during the regression.

It is not, despite the headlines, immortal in any everyday sense. The adults are about 4 millimetres across, and predators eat them constantly. What's missing is the senescence pathway. Given the choice between dying and rebooting, the cells reboot. Repeatedly.

The longest-running captive observation belongs to Shin Kubota, a marine biologist at Kyoto University's Seto Marine Biological Laboratory in Shirahama. He keeps a colony in seawater tanks, feeds them plankton he collects every morning, has watched the same lineage rejuvenate at least ten times, and in his off hours writes karaoke songs about the species. Outside his lab, the animals are effectively impossible to keep alive long enough to study.

What aging research has wanted from this jellyfish for thirty years is the switch. So far no one has cleanly identified it.