Yahoo! Dismisses Chief Operating Officer

Over decline in ad revenue, first Marissa Mayer executive to get the axe

Yahoo! Inc. retreated in morning trading after the largest U.S. Web portal  dismissed  Chief Operating Officer Henrique de Castro amid disappointment with his efforts to revive its long-declining advertising sales.

Yahoo shares dropped 1.6 percent to $40.43 at 9:34 a.m. in New York, set to erase this year's gains.