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Sea jellies have thrived in the oceans for 500 million years, and they’ve done it without blood, bones, or brains. What these delicate pockets of protoplasm—jellies are 95 percent water!—do have venomous stinging cells that let them take on sturdier predators and prey.
Jellies range in size from mere drops that could float in a contact lens to 100-foot behemoths. Some jellies are carried passively on currents for thousands of miles, others follow the sun’s movement from east to west, and some bounce like rubber balls. Still others pulse in place, like seafloor flowers. A few have internal algae that manufacture food for them, all capture planktonic prey, and many eat other jellies. Enter their mysterious world to learn more.