A mosaic jellyfish floats serenely in the waters of the Coral Sea, about 100 nautical miles from Cairns, Australia. Jellyfish are ubiquitous in the Earth’s oceans. They can thrive in warm water and cold, along coastlines or out in the deep. Their bodies are about 95 percent water. And though they have no brains, jellyfish have somehow been smart enough to survive for over 500 million years.