Turritopsis dohrnii: Immortal jellyfish

Summary

What most of us would recognise as a jellyfish - the otherworldly, gelatinous aquatic animals renowned for their sting-filled tentacles - is actually just the final stage of these animals' life cycle. At least, it usually is. Not all jellyfish play by the same rules, and one species may have discovered the secret to immortality. How does the immortal jellyfish live forever? The life cycle of most jellyfish species is similar. Museum curator Miranda Lowe explains, 'They have eggs and sperm and these get released to be fertilised, and then from that you get a free-swimming larval form. 'The larva will move about in the current until it finds a hard surface to establish itself. It will then start to mature and grow. Larvae mature into polyps, which will then bud off and mature into young jellyfish.' An adult jellyfish is known as a medusa. Jellyfish belong to a group called Cnidaria, which also includes sea anemones and corals. As animals, they are subject to the cycle of life and death - though one species is known to bend the rules. The hydrozoan Turritopsis dohrnii, an animal about 4.5 millimetres wide and tall (likely making it smaller than the nail on your little finger), can actually reverse its life cycle. It has been dubbed the immortal jellyfish. When the medusa of this species is physically damaged or experiences stresses such as starvation, instead of dying it shrinks in on itself, reabsorbing its tentacles and losing the ability to swim. It then settles on the seafloor as a blob-like cyst. Over the next 24-36 hours, this blob develops into a new polyp - the jellyfish's previous life stage - and after maturing, medusae bud off. This phenomenon has been likened to that of a butterfly which, instead of dying, would be able to transform back into a caterpillar and then metamorphose into an adult butterfly once again. The process behind the jellyfish's remarkable transformation is called transdifferentiation and is extremely rare. Medusa cells and polyp cells ar...