Depending on what species of jelly you are, turtles, mola mola, sea birds, and other jellies prey on jellyfishes.
Storms wash them up, climate changes can break or make cycles, marine debris can negatively contribute, but considering all that may, or may not be against them, jellyfish around the planet continue to thrive and in many cases are blooming faster than ever. There are many places that a major balance has taken hold, such as off Florida and the Gulf and others as well.
Yes
First of all, the man-of-war is not a true jellyfish. Its body is filled with air. To be a jellyfish the "bag" of their body has to be filled with water. Also a box jellyfish is more dangerous. The box jellyfish is the most dangerous jellyfish in the world.
The box jellyfish is lethally dangerous if you have been stung over more than a few square centimeters of skin.
Clear
Their venom.
Most jellyfish are not dangerous. The Man o' War has the ability to do some painful damage to your skin, but it is not a true jellyfish . . .
They can sting you and it hurts! Trust me, I know...
Yes, both dangerous and friendly :-)
Some jellyfish stings are dangerous to humans.
A jellyfish has tentacles which can sting by injecting a toxin. Jellyfish can be extremely dangerous, despite being a very primitive type of organism with no skeletal system.
IDK.Box jellyfish, Portuguese Man o war, Sea nettle, Earacounge (Ear-a-con-g). IDK the rest
Yes, it is. It has been proven numerous times in several experiments that swimming with jellyfish is extremely dangerous and can for a fact cause serious injuries and/or death. Trying this is highly discouraged for your own safety.