Jellyfish

A jellyfish is a soft, drifting sea animal made of about 95% water. It has no brain, no heart, no blood and no bones, and has been doing perfectly well like this for more than 500 million years, which is longer than dinosaurs, trees or any form of planning.^[1]

Jellyfish are not fish. This is a source of some embarrassment to the name, and none to the jellyfish.

The immortal jellyfish

One small species, Turritopsis dohrnii, can turn itself back into an earlier stage of its life when it is injured or stressed, reverting from an adult into a polyp and then growing up again. Because it can, in principle, do this over and over, it is often called the “immortal jellyfish”.^[2] It is the only animal known to respond to a bad day by starting again from the beginning.

In practice most of them are eaten or caught by disease, so the immortality is mostly theoretical.

Without a brain

A jellyfish senses its surroundings with a simple net of nerves spread through its body. It can swim, catch food and avoid light without a brain. Scientists studying this have found it humbling, and have been asked by their departments to stop saying so in grant applications.

See also

• Water – most of a jellyfish
• Ocean – where it drifts
• Bubble – another see-through thing
• Shark – another sea animal with no bones, nearly

References

1. Drift, D. (1997). No Brain, No Problem. Current Press.
2. Polyp, P. (2011). “Starting Again: A Life History”. Marine Biology Digest 1: 1 (and again).
3. Gelatin, G. (2003). Not Fish. Wobble Books.