The immortal jellyfish is so-called because it can revert to a previous stage in its life cycle due to stress or injury. However, this isn't immortality as we typically think of it: the jelly turns back into a stationary polyp that spawns several genetically identical medusae.

Nomura's jellyfish is among the largest jellyfish species in the world — measuring up to 2 metres (6.6 ft) in diameter and weighing up to 200 kg (440 lb). Starting out as small as a grain of rice, it can grow this large in less than a year.