Turritopsis dohrnii, the immortal jellyfish (family Oceaniidae), is a small hydrozoan medusa renowned for its unique ability to revert from the sexually mature medusa stage back to the juvenile polyp stage after stress or senescence—a process of transdifferentiation enabling indefinite repetition of its life cycle and making it the only known animal capable of biological immortality at the individual level. Distributed widely in temperate and tropical oceans globally through shipping ballast water, it has become a cosmopolitan species. It is an important model for aging and regeneration biology. Genomic data have been generated for this species and are available in public databases, enabling comparative studies of cnidarian evolution and ecology.