Nasdaq CEO says exchanges need to be vigilant against bad actors

By John McCrank NEW YORK (Reuters) - Exchanges need to constantly be on the lookout for abusive trading behavior in order to protect the resiliency of the marketplace, Robert Greifeld, chief executive officer of Nasdaq OMX Group said on Thursday. "We have to have the assumption that bad actors will try to do bad things within the market," Greifeld said in an interview.