Longest-lived jellyfish

The longest-lived jellyfish is the aptly named immortal jellyfish Turritopsis dohrnii, a species of hydrozoan jellyfish native to the Mediterranean Sea and the waters off Japan, which can become quite literally immortal once it reaches adulthood. For if a sexually adult free-swimming medusa specimen is threatened, injured or faces inhospitable environmental conditions, it will simply transform back into a larval attached polyp and then reproduce asexually, budding off a series of genetically identical medusae that can each then mature into a sexually adult medusa. Each of these can then, in turn, transform back into larval polyps if threatened, and so on, with this cycle continuing indefinitely if need be.

Although this species was initially discovered as long ago as 1883 (at which time it was thought to belong to what is now known to be a separate but closely related species, T. nutricola), its extraordinary immortality cycle was not uncovered until 1996. It is achieved by a process called cellular transdifferentiation, in which the cells of the medusa transform themselves into different, larval cell types – e.g. adult nerve cells can transform into larval muscle cells.