Their attacks shock you (which makes you unable to move), and they often stick in groups. If you get surrounded by these, you're basically dead. Don't use melee weapons, use a crossbow.

They drop bio toxins, which can be used for a new tranq dart that's more powerful AND shocks the target.

Once grabbed they won't let go till dead I had a Lvl 215 spino and was having a jolly swim then BANG WAM I get jumped by 8 jelly bastards and the spino died :(

Basilosaurus is the by far the best way to kill these things. Doesn't get shocked. High level ones harvest hundreds of biotoxin in minutes with no threat.

Dismount your Dino when ambushed by jellyfish, whistle follow then swim down deeper. After the initial shock wears off the Dino finds it easier to escape downward unmounted. Don't necessarily wait on it either, swim to a safe distance and it will catch up.