In the warm coastal waters of the Mediterranean, and now globally distributed via ship ballast water, lives a translucent jellyfish about the size of a pinky-nail clipping, with a bright red stomach visible through its bell. Turritopsis dohrnii looks unremarkable. It feeds on plankton, swims at the mercy of currents, and gets eaten in large numbers by fish, sea turtles, and other predators that consume jellyfish without much discrimination. What distinguishes T. dohrnii from the thousands of other jellyfish species is what happens when one of these medusae becomes injured, starved, sick, or simply old. Instead of dying, the animal can transform itself back into its juvenile polyp stage and start its life over.<\/p>\n
The phenomenon was first described in 1992 by Giorgio Bavestrello and colleagues at the University of Genoa, working with Christian Sommer at Ruhr University Bochum. Bavestrello and Sommer had collected specimens of what they assumed was a typical hydrozoan jellyfish, intending to rear them for unrelated research. Instead, they observed that under stress, the medusae did not die. They settled to the bottom, lost their swimming ability, contracted into a \u201ccyst\u201d of cells with no medusa-like features, and then over 24 to 72 hours regrew as polyps \u2014 the juvenile colonial stage that normally precedes the adult medusa in the jellyfish life cycle. According to a September 2025 feature in The Scientist on the species<\/a>, Stefano Piraino of the University of Salento, who had been involved in the investigation since the early 1990s and was lead author on the foundational 1996 paper characterising the cellular mechanism, recalled the initial reaction: \u201cThis was certainly a point of interest for the media, because they claimed that we had discovered the elixir of immortality.\u201d<\/p>\n How the reversal works<\/p>\n The cellular process underlying the reversal is called transdifferentiation. In ordinary animal development, cells start out as undifferentiated stem cells, gradually commit to specific lineages (muscle, nerve, epithelium, and so on), and then remain in their committed state for the rest of the cell\u2019s life. In T. dohrnii, this is not what happens. Under stress, fully differentiated cells in the medusa change their commitment, transforming into cells of a different lineage entirely. Muscle cells become epithelial cells. Tissues that had specialised for one function reorganise into tissues with another function. The overall result is a complete reorganisation of the animal\u2019s body plan, from adult medusa to juvenile polyp, accomplished by reprogramming the cells already present rather than by generating new ones from a reserve of stem cells.<\/p>\n Not every medusa can perform the reversal. According to tissue-excision experiments published in 1996 by Piraino, Boero, Aeschbach and Schmid in the Biological Bulletin<\/a>, the transformation of medusae into polyps occurs only if differentiated cells of the exumbrellar epidermis (the outer cell layer) and part of the gastrovascular system (the circulatory canal system) are present in the dying medusa. Severely damaged individuals with these tissues missing cannot complete the reversal and die. A 2021 transcriptomic study of the species, building on the 1996 cellular work, identified gene expression changes in the cyst stage that include upregulation of telomere maintenance, DNA repair pathways, and developmental transcription factors, alongside silencing of regulators that normally maintain cellular commitment. The transition appears to be controlled by stress signals that include senescence: as the medusa ages, the same pathways that trigger reversal under injury become active spontaneously.<\/p>\n What the genome shows<\/p>\n The most extensive genetic analysis of the species came in 2022 from Maria Pascual-Torner, Carlos L\u00f3pez-Ot\u00edn, and colleagues at the University of Oviedo, who published the first whole-genome assembly of T. dohrnii alongside a comparison genome of Turritopsis rubra, a sister species that the team described as incapable of postreproductive rejuvenation. According to the Pascual-Torner team\u2019s paper in PNAS<\/a>, T. dohrnii carries approximately double the number of genes associated with DNA repair and damage protection compared with T. rubra, as well as variants in genes affecting cell division regulation and telomere stability. During the life-cycle-reversal process, the team observed activation of pluripotency-associated transcription factors comparable to the Yamanaka factors used in mammalian cell reprogramming, and silencing of polycomb repressive complex 2 targets that normally maintain differentiated cell states.<\/p>\n