Sotheby’s (BID) shares plunged as much as 20 percent today amid concern that the art market rally since the 2008 financial crisis will fizzle.

Shares of the publicly traded auctioneer closed down $5.17, or 13 percent, to $33.44, and are off 39 percent since their April 5 high this year. In the past three sessions, they’re down 16 percent.

Stocks around the world have tumbled as investors fretted about a global relapse into recession. Late Friday, Standard & Poor’s cut the U.S. credit rating.

“Global financial equity markets are a proxy for global wealth,” said Rommel Dionisio, an analyst with Wedbush Securities, who has an “outperform” rating on the shares. “There’s reason for concern. It depends on how pronounced or severe this downturn is.”

On Aug. 3, Sotheby’s reported its best-ever quarterly profit of $127 million, with record sales of $3.4 billion for the first half of 2011. In a conference call after the earnings release, Chief Executive Officer William Ruprecht said he believed “market volatility” around the world “in other arenas” has encouraged participation in the art market.

Ruprecht also referred to economic volatility “that the media is awfully good at frightening people with on a daily basis.”