When purchased, crustaceans must be alive and vigorous, with the exception of shrimp, which are almost never alive when brought to market. Lobster and crayfish (either freshwater or salt water) should feel heavy and full, with their claws and feet intact. The tail of a fresh lobster will curl under strongly when the lobster is lifted from the water. If clawed, the freshest crustaceans will probably be angry enough when touched to make a strong effort to pinch their handler.
Since crustaceans begin to atrophy almost immediately upon being removed from their natural environment, they should be purchased as quickly as possible after being caught. Once dead, the flesh of crustaceans begins to break down immediately. In fact, it actually liquefies so that, when the shell is opened, it runs out as a transparent mucous. The longer they are in captivity, even alive, the more muscle tissue they lose. It is for this reason that the meat seems to have shrunk from the shell when the shell is cracked open. All these reasons make it imperative to purchase only the freshest shellfish at all times.