Before its retreat, GameStop briefly became the biggest stock on the Russell 2000 index of small caps, according to Zerohedge. Dramatic jumps in GameStop, BlackBerry Ltd and AMC drew calls for regulatory scrutiny. "In terms of short interest being monitored, the U.S. markets are probably the most transparent, but there's always room for improvement," former SEC chairman Jay Clayton told CNBC. U.S. Representative Alexandria Ocasio-Cortez criticized the decision by Robinhood to restrict retail trading. Robinhood did not respond to requests for comment. Silver industry shares also caught traders' attention. Canada's First Majestic Silver was halted briefly in New York after shares rose more than 30%.