There has been a shake-up at the top of one of the billionaire Ken Griffin's stock-picking units. Rich Schimel, who headed Aptigon, a unit of the $27 billion hedge fund Citadel, and David Bonfili, formerly Aptigon's chief operating officer, have left, a spokesman for Citadel told Business Insider in an email. "Eric Felder has been named Head of Aptigon Capital and Reza Shahi will assume COO responsibilities for the business," the spokesman said, adding that the changes were made "to strengthen the platform and drive investment performance."