About 22 percent of all share volume is done off the main exchanges; this volume includes dark pool volume. Such services have, of course, grown dramatically over the past few years. But the growth has now caught the eye of the SEC, which is showing lots of new interest in the executions arena.

While high-frequency trading--and flash orders and sponsored access--has eaten up most of the media ink. Traders notes that dark pool trading is up for review. The SEC may issue "both an outright rule proposal as well as a 'concept release' outlining possible changes to the way broker-dealers operate their dark pools. The SEC's concern is that too much volume is being done away from exchanges and electronic communication networks, and within the four walls of individual broker-dealers." One issue is whether there is a threshold past which too much dark trading unduly affects the public markets. Broadly speaking, this is all about fragmentation.

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