The agency announced the civil penalties against five broker-dealers, seven dual registered broker-dealers and investment advisers, and four affiliated investment advisers.

The 16 firms admitted the facts, the SEC said in a statement, and have begun improving compliance policies and procedures.

"The SEC's investigations uncovered pervasive and longstanding uses of unapproved communication methods, known as off-channel communications, at all 16 firms," the SEC statement said.

The firms admitted that employees "communicated through personal text messages about the business of their employers" and "sent and received off-channel communications related to recommendations made or proposed to be made and advice given or proposed to be given," the SEC said.

The firms include Northwestern Mutual Investment Services LLC, Guggehnheim Securities LLC, Oppenheimer & Co. Inc., Cambridge Investment Research Inc., Key Investment Services LLC, Lincoln Financial Advisors Corporation, U.S. Bancorp Investments Inc. and The Huntington Investment Company, the SEC said.

(Reporting by Doina Chiacu; Editing by Susan Heavey and Barbara Lewis)