The Piranha descends from the genetics work of Dr. Lemuel Dorcas, the criminal scientist responsible for empowering Tiger Shark (Todd Ariiss) and Orka. In an underwater complex off the New Jersey coast, Dorcas used radiation and chemical mutagens on aquatic life forms to develop Aquanoids, or Men-Fish, man-sized monstrosities which he planned to use to kill Namor the Sub-Mariner. He trapped Namor (also accidentally involving Spider-Man), but the ensuing battle blew up the lab. The Aquanoids died, but one experimental subject, a yet to be evolved piranha, survived and was bathed in the mutagens. This piranha developed into an enormously strong humanoid, able to survive in the toxic salt water. Consuming the remains of Dorcas's assistants, it unconsciously adapted their intellects and knowledge. It created new Men-Fish by herding other experimental subjects into the lab's ruins. Transporting the damaged mutagenic equipment to South America, the Piranha attempted to mutate other piranhas into beings like himself, but only succeeded in slightly improving the size and intelligence of a school of piranha and enabling them to survive in salt water. The Piranha moved his Men-Fish and mutated piranhas to the Bikini Atoll, hoping its radiation would enable further mutation. Leaving his "subjects" there, the Piranha traveled to Hydrobase to research Dorcas' work. While there, he learned Namor was traveling to the Bikini Atoll, and quickly pursued him. At Bikini, the Piranha and his Men-fish battled Namor, but an explosion destroyed the Men-Fish and trapped the Piranha. Wounded and panicking, he lost control of his mutated piranhas and they devoured him.
The mutagens in the Piranha interacted with the altered school, and over time each piranha evolved into a duplicate of the humanoid Piranha. These Piranhas traveled to the North Atlantic to an undersea amphitheater, where they kidnapped humans from ships and forced them to battle—those who won were returned to their ship and those who lost were devoured. Namor and the Thing (Ben Grimm) stumbled into this mess, and the Piranhas kidnapped Grimm's girlfriend Alicia Masters, forcing the pair to fight for her survival. Their battle "accidentally" damaged the gladiatorial arena, and Namor rescued Alicia while the Thing collapsed the cave, apparently killing all the Piranhas before escaping himself.
The original Piranha eventually regenerated and sought vengeance on Namor. Atlantis was then controlled by the warlord Attuma, and the Piranha joined Attuma's Atlantean defenders alongside Nagala, Orka, Sea Urchin (Jeremy Swimming Bear) and Tiger Shark. When the Sub-Mariner invaded Atlantis with the Defenders, the Piranha attacked him, but the Defenders were teleported away before the fight could conclude. They returned not long after, and the Valkyrie bested the Piranha, and Namor defeated Attuma, freeing Atlantis, and the Piranha fled.