Meranno is a member of the aquatic race of beings known as Home Mermanus. As a boy Meranno was a playmate Prince Namor, the Sub Mariner, but actually despised him. In the late 1930s Meranno worked as a research scientist in the Atlanteans' Antarctic city. Allying himself with Nazi Germany, Meranno betrayed the city's location to the Nazis, who launched an attack that severely damaged the Atlantean fleet, destroyed the Atlantean Hall of Science, and left Atlantis's Emperor Thakorr in a coma. Namer, as acting emperor, banished Meranno for treason. Meranno then joined the Nazis in their mad quest for world domination, expecting a Nazi victory to bring him enough personal power to wreak his vengeance against Namor. Using a combination of his own advanced Atlantean scientific knowledge and Nazi science, he developed an as-yet-unknown means of increasing his size and strength to a point where he rivaled; the Sub-Mariner himself for might. Dubbing himself U-Man; Meranno went on to fight alongside the Axis in World War II, often coming into conflict with his hated enemy Namor and Namor's allies, the small band of adventurers known as the Invaders. In the end, however, Meranno found himself defeated time and again, encountering the final end to his plans of vengeance with the defeat of Germany in 1945, although he was never captured and tried as a war criminal.
Although today Meranno would be aged by human standards, Atlanteans enjoy a lifespan considerably longer than that of surface men; therefore, Meranno is considered to be in the prime of his life by Atlantean standards. Meranno participated in the Atlantean warlord Attuma's failed attempt to conquer the surface world. Later, U-man was seen with other Atlantean warriors who attacked a stolen Soviet nuclear submarine and a large group of international heroes during the so-called "Crossing Line" event, believing that their realm was being invaded. His present whereabouts are unknown. His present whereabouts are unknown.
