They are fairly aggressive but slow and for some reason they have no cool down on their stun so they will infinitely stun a creature with no resistens doing like 40 damage until the creature dies or if you can get it away (the stained creature wont be able to move so kill the jellyfish or pick up the creature) if you have good o2 and the jelly fish is NOT agroed on you then you can swim away very slowly as the jelly fish will stun you about .2 seconds after your not stuned but if its agroed on you it will follow you and stun you again.
If a cnidaria killed my spino, boy i would be mad. i lost my sarco to cnidarias, a scare with my icthy with cnidarias, and almost got my megalceros killed by some. ( he is dead now D:) I kill any of these that i can. The dossier says that they are passive, but aggresive when approched, NO! as soon as i step in the water these attack, and they litterally don't move at all, they just sit there and chill until you or a tame steps in the water. one like=one prayer for my deceased sarco, Gatorade (you can find his story in sarco) and one member of the cnidaria hate club.
I think that the devs need to reconsider some of the creatures in ark. With the jellies and electrophorus, it's too much. Especially on fjordur. There are hundreds of eels EVERYWHERE in the shallows! I don't mean reconsider as removal, but they need to be nerfed. No person I know tames mosas, plessies, or dunkies anymore because of those two creatures. They've made a lot of ocean creatures irrelevant. My first water creature is always a basilo. And that's all I need, because what's the point of anything else.
Ummm funny story so I just tamed a titan ok and I decide to swim back to my base with it and I see these things an I thought they wouldn't do anything but.... it got my titan and after 5 hours of trying to escape it killed my titan his name was Billy