According to theflyonthewall.com, IBG LLC reported a 6.3% stake in Tradestation. Tradestation was trading up 8.26% to $6.18/sh at the time of this posting.
IBG, according to Bloomberg, IBG LLC operates as an automated global electronic market maker and broker. The company provides broking services for exchange-listed options, stocks, bonds, and futures. IBG LLC was formerly known as Interactive Brokers Group LLC and changed its name in November 2006. The company was founded in 1982 and is based in Greenwich, Connecticut. IBG LLC operates as a subsidiary of Interactive Brokers Group, Inc.
Full disclosure: I own shares in TRAD