Jellyfish are ancient creatures that have no brain, are almost all water, and many of them are transparent -often beautifully so. You'd think such an animal would be harmless, but no. They sting, and even worse, they multiply like crazy. Out-of-control jellyfish blooms are becoming more and more of a problem. In nature, that's usually because something else is out of balance. In this case, it's because the jelly's natural predator is suffering from declining populations. That's the sea turtle. There's a reason that it's illegal to disturb sea turtle nests or hatchlings. They suffer enough from trash in the ocean. Mariela Pajuelo and Javier Antonio Quinones put together this TED-Ed lesson to warn us about the importance of the leatherback sea turtles who are not afraid to swim with the jellies. They are keeping the oceans from turning into a morass of jellyfish goo.