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.
The Cnidaria is a small jellyfish found in the deepest depths of the oceans. This easy-to-spot creature won't attack unless approached too closely. It delivers a sedative sting that can knock out enemies if striked repeatedly. While it cannot be tamed, survivors will hunt it to gather its sedative, which is used for advanced tranquilizers.