Stocks open lower on Wal-Mart and retailers
Highlights:
* Many retailers posted disappointing December results; Wal-Mart Stores Inc falls more than 8 percent at the open.
* The Dow Jones industrial average fell 58.78 points, or 0.67 percent, at 8,710.92. The S&P 500 Index slid 4.44 points, or 0.49 percent, at 902.21. The Nasdaq Composite dipped 6.22 points, or 0.39 percent, at 1,592.84.
(Reporting by Rodrigo Campos; Editing by James Dalgleish)