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There are several stocks trading more heavily than usual this morning, and also experiencing large gains or drops in share prices. These include DemandTec Inc. (NASDAQ: DMAN), Affymax Inc. (NASDAQ: AFFY), Pacific Sunwear of California, Inc. (NASDAQ: PSUN), Clearwire Corp. (NASDAQ: CLWR), and AMR Corp. (NYSE: AMR), parent of American Airlines. Here the details after the first half-hour of trading.

DemandTec is up more than 55.5% at $13.11 in a 52-week range of $5.41-$14.08. Volume is already equal to the daily average. The company has accepted a takeover bid from IBM Corp. (NYSE: IBM) of $13.50/share, a 57% premium over yesterday’s closing price.

Affymax is up nearly 31% at $7.40 in a 52-week range of $3.93-$8.50. Volume is already about 10X the daily average. The US FDA has approved the company’s anemia drug for dialysis patients suffering from chronic kidney failure.

Pacific Sunwear is up more than 22% at $1.65 in a 52-week range of $1.11-$6.02. Volume is already about 3X the daily average. The company has announced a restructuring plan that would close up to 200 of its more than 800 stores. The retailer also said that it had secured $60 million five-year term loan and a $100 million revolving credit facility.

Clearwire is down more than -6.5% at $2.13 in a 52-week range of $1.24-$6.28. Volume is about 5X the daily average already this morning. The company has increased a secondary share offering to raise $350 million at a price of $2/share.

AMR Corp is down nearly -30% at $0.79 in a 52-week range of $0.20-$8.89. Volume is nearly double the daily average of 25.5 million shares traded. The company which has already filed for bankruptcy protection is being hit hard by reports of expected weakness in airline companies’ profits for the current quarter and next year.