The animal that is 98% water is the Jellyfish.
Jellyfish have a minimalist structure as they have no brain, heart, bones or blood as they are made up of so much water.
Jellyfish also rely on diffusion to breathe and a simple nerve net, instead of a central nervous system to survive and when the jellyfish are washed ashore, the jellyfish also almost entirely disappears as their body water evaporates.
And despite the jellyfish's simple structure they are also ancient creatures, which have also existed for over 500 million years.
The immortal jellyfish also known as the Turritopsis dohrnii never technically dies of old age.
The Turritopsis dohrnii or immortal jellyfish is a tiny, 4.5 mm (0.18 inch) jellyfish that is found in the Mediterranean and worldwide.
The Turritopsis dohrnii jellyfish can technically achieve what is known as biological immortality through a process that is called transdifferentiation.
When the Turritopsis dohrnii jellyfish faces stress, physical damage or even starvation, the Turritopsis dohrnii jellyfish can revert it's mature cells back into young cells and technically be immortal and not die.
The Turritopsis dohrnii jellyfish has the ability to do a life cycle reset, where it transforms from it's adult medusa stage, back into a juvenile polyp stage, and essentially starts it's life over again.
And although the Turritopsis dohrnii jellyfish can repeat this process indefinitely, immortal jellyfish can still die from disease or even being eaten by it's predators.
Turritopsis dohrnii (the "immortal jellyfish") does actually die frequently, despite being biologically immortal.
While they can reverse their life cycle to avoid death from old age, they are still easily killed by predators, disease, or environmental dangers.
They only cheat death by aging, not by physical destruction.
Most T. dohrnii die from being eaten by predators like fish and turtles or by contracting diseases or facing harsh conditions in the ocean.
And while the immortal jellyfish can start over, they are not completely immune to dying or death, which means that their population is controlled naturally.
If the immortal jellyfish does not get diseased or eaten they can technically live forever.
But like mentioned above, many of the immortal jellyfish do get eaten by predators or die eventually of some diseases.