These guys drop bio toxins, perfect for increasing creature torpidity. If you want to get your hands on these, kill Cnidaras. If you want to kill one, they will spawn in groups of 2+. They are aggressive if you are close to them in water. They usually spawn near shores. Lead them to the shores, and hit them until they die so they don't repetitively electrocute you.
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.