If the piranhas they say are some of the most annoying, these bastards are deadly af. Mantas pierce through armour, shred it, while these guys stun you. Include the piranhas, and omae wa mou shindeiru. Best bet is Basilosaurus, or decent levelled mosa ( not recommended as shock attacks are the weakness of the mosa) and tusoteuthis, because of the attack range. I hunt these down to total extinction, then gladly save up the bio toxin they drop for giga, royal griffin ( I play on mobile so...) or taming of other whatnot thingys. If you want to kill these early game, as they are seen on the shore quite easily, just grab a few spears and/or pikes then slaughter these bastards. Lure them by walking until swim mode into the water, then depending on your movement speed go backpedal to the shoreline. This will successfully lure them near you and you can slaughter them as you please without endangering you or your mount.
These things spawn on The Island at the bottom right of the map near the Lava Cave. These cheeky little buggers are more than happy to ruin your day by swarming and stunlocking you and your tames nonstop unless you are on a Basilosaurus, which is the only creature that is immune to their attacks. If you don't have a Basilosaurus, lure them close to shore and cheese it with a spear or pike, better yet pray that they beach themselves and die.
Went on a killing spree with my spino to pass time to breed aga. Decided to go kill piranhas and my spino legit got stunlocked permanently until I finally managed to kill all of them with my spino hanging on by a thread because of these godforsaken things. Hunt these things into extinction or they'll murder everything you love and the neighbors.
Once I saw this thing in the water and I was like “ oOh I Didn’T KnOw TherE WeRe TreASure chEstS In tHIs gaMe “ and just ran like Usain bolt into them. I’d say it was more insane Bolt. And Bolted to the spot was I! Yep, my discovery of the Cnidaria!