ZURICH, April 19 (Reuters) - Credit Suisse said in an internal memo on Monday that the co-heads of its prime services, John Dabbs and Ryan Nelson, would step down with immediate effect. The Swiss bank said in the memo seen by Reuters that it had appointed Roger Anerella as interim head of prime services “where he will focus on stabilizing the franchise, delivery of remediation actions and repositioning the business to deliver the highest level of service to our clients”. The Swiss bank has been reeling from its exposure to the collapse first of Greensill Capital and then Archegos Capital Management within a month. The bank also appointed Doug Crofton as head of Americas Cash with functional responsibility for Execution and Advisory Sales and Stuart McGuire as head of EMEA Cash with functional responsibility for execution and advisory sales, according to the memo.