After the Piranha was created as a result of the experiments of Dr. Lemuel Dorcas, having been exposed to remaining radiation from Dorcas's previous work. After devouring Dorcas's deceased assistants and evolving from a normal fish into a humanoid fish, he ordered other fish into the radiation, creating more Men-Fish. Piranha and his Men-Fish travelled to Hydro-Base in the hopes of researching more of Dorcas' work there. They followed Namor and attacked him. During the fight, Piranha was wounded, and his blood attracted his piranhas, which turned on him and devoured him.
These piranhas who ate the original Piranha evolved into identical versions of the original, eventually numbering in the hundreds. They later captured Namor and the Thing, forcing them to fight to the death. The two combatants instead managed to crush the underwater stadium they fought in and crushed all the Piranhas.
After escaping A.R.M.O.R.'s headquarters, the zombie Deadpool's head was teleported to the bottom of the ocean along with the Zombie. The Deadpool head infected Piranha and all of the Men-Fish with the zombie plague. When the Men-Fish attacked a cruise ship, the new Midnight Sons arrived and destroyed them.
Oxygen/nitrogen rich liquid atmosphere