Transparent Immortal Jellyfish

This illustration shows the different life stages of Turritopsis jellyfish. Image via Wikimedia Commons
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.
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