Jellies are 95% water. The creatures don't have brains, stomachs, intestines, or lungs.
They move by rapidly contracting their mushroom-shaped bell to expel water, which propels them forward.
Jellies are opportunistic feeders, meaning they'll ingest just about anything: microscopic plankton, crustaceans, and fish larvae are all fair game.
The absence of complex body parts allows jellies to adapt easily to changing ocean conditions.
In the last 100 years, average ocean surface temperatures have risen by about 1.6 degrees Fahrenheit (0.9 degrees Celsius), according to the National Oceanic and Atmospheric Administration. Last year was the hottest on record for the seas.
Surprisingly, maritime shipping and undersea drilling industries also benefit jellyfish, since one of the creatures' reproductive stages, the polyps stage, requires them to settle on a hard surface.
When inland rivers carry fertilizer run-off from agriculture to coastal waters, that can create competition-free buffets for jellies.
Overfishing is also fueling the global jellyfish proliferation.
A 2012 study from the University of British Columbia concluded that "jellyﬁsh populations appear to be increasing in the majority of the world's coastal ecosystems and seas."
Groups of jellies are called "blooms" or "outbreaks."
For one, they prevent swimmers and beachgoers from entering the water. Some 150 million jellyfish stings occur annually worldwide.
Some types, like the Chironex fleckeri species of box jellyfish, can kill a human in 3 minutes.
In January, nearly 4,000 people were stung in one weekend by blue bottle jellies that drifted ashore in Queensland, Australia.
In large numbers, jellies can clog power-plant pipes and force them to shut down.
In 2011, jellyfish overwhelmed the cooling system at a coal power plant near Hadera, Israel.
Jellyfish swarms can also be deadly for other marine creatures.