Jellyfish can sting with their tentacles if they brush against you when you're swimming in the ocean. You also can get stung if you step on a jellyfish, even a dead one. Usually, jellyfish stings will hurt, but are not emergencies.
Depends on where you step on the jellyfish, how long it's been dead, and the species. If you step on the top of most jellyfish, you will not be stung. If the jellyfish has been dead for a while, you will not get stung. Those are true in most cases, but when you have jellyfish such as any box jelly you need to be careful.
Hey now, people are stupid and if we tell them some jellyfish will be safe to step on on the tops then might just go and step on them.... Actually, Darwinism.
HEY EVERYONE, THE TOPS OF JELLY FISH ARE SAFE TO STEP ON, GO DO IT!
For reference though, if you can avoid it then don't step on a jellyfish. I am not certain how long nematocysts survive or stay active but I believe that as long as they are not dry that they can still put their toxic barbs into you as the process of doing so isn't voluntary.
It all pretty much varies from species to species as some nematocysts can last longer than others after being detached from their "host."
I think everyone should experience a nice cannonball jelly sting at least once in their life as it's a mild irritant on skin and could cause some swelling. I'm not just saying this because I've been stung 7 separate times by them either. A nice sting will make people think at least twice about going around them while in water and quite possibly while on land too.
Cannonball jellies are okay to touch on land as they lose their tentacles when dead and essentially turn into something to throw at your friends.
Note: All of the times I've been stung have been on my legs when a jellyfish was washed into me by waves. I did not actively seek them out.
That is interesting to know, Hey @berniesanders, this man here ^^
Anyways I have never been stung by a jelly in any of my times swimming in the ocean, though while on the West Coast Trail we (the group members that finished eating) sat at the crab shack one day (like the only day you can) while part of our group was still eating and watched a little jelly handing around. It was pretty cool, no idea what type. Closest I came to having indepth knowledge about them was from a bio20 project that I did on jellyfish
Where I'm studying marine science I get firsthand knowledge from my professors who conduct research over almost any marine organism you can think of as well as other parts of the marine environments.
What was your project over? Jellyfish in general? You can discord me so we don't have a comment chain going on here if you want.