How long after purchase will shellfish remain good to eat?

Processed, shucked shellfish should be eaten or frozen within two days of purchase.  Whole crab with its shell intact should be cooked within 24 hours or frozen.  Maintaining a temperature of 35°F or lower may extend shelf life of processed shellfish.
 
Frozen shellfish should be consumed within three to six months for  purposes of quality.
 
Live lobster and crab should be cooked the day they are purchased. Clams and oysters will stay alive for up to a week if properly refrigerated. If shellfish die before they are cooked, they undergo chemical changes that quickly affect their flavor.  They also may contain sufficient levels of bacteria to make you ill. Check for signs of life--moving legs, shells that close after lightly tapping them--before preparing  live shellfish. Discard dead specimens. Never shuck and freeze dead shellfish.
 
Source(s):
Nova Scotia Agriculture--Food Protection and Enforcement Division "Shellfish Food Safety"
University of California Cooperative Extension "Meat, Poultry and Shellfish Safety"
Safeoysters.org "Safely Storing Oysters and Other Molluscan Shellfish"