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.
Kill on sight. VERY NOT FRIENDLY. They will totally obliterate you, your tame, your tames tame, your tames babies... they will even float up out of the water to attack your flyers. Ok that's not real but they're freaking ridiculous.