Looking at options trading activity among components of the S&P 500 index, there is noteworthy activity today in DexCom Inc (DXCM), where a total volume of 2,860 contracts has been traded thus far today, a contract volume which is representative of approximately 286,000 underlying shares (given that every 1 contract represents 100 underlying shares). That number works out to 60.3% of DXCM's average daily trading volume over the past month, of 474,215 shares..