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Karl Johan Schuster (Born February 1, 1985), known as Shellback, is an artist from Karlshamn, Sweden.

Before he became part of Max Martin's Maratone production studio in 2007 he was a a drummer in local indie rock bands in Karlshamn. He also was a member of hardcore / metal band Blinded Colony.

He co-wrote Pink's 2008 single So What , and Britney Spears's 2008 single If U Seek Amy.