As a christian I strongly advise you to reconsider eating any shellfish. The Bible states in no uncertain terms that the eating of shellfish is an abomination in the eyes of God. "And all that have not fins and scales in the seas, and in the rivers, of all that move in the waters, and of any living thing which is in the waters, they shall be an abomination unto you: They shall be even an abomination unto you; ye shall not eat of their flesh, but ye shall have their carcases in abomination. Whatsoever hath no fins nor scales in the waters, that shall be an abomination unto you." (Lev. 11:10-12). No I am not trying to start a flame war, or anything like that I'm trying to educate.
Okay I'm kidding but I still kind of smirk any time I read that passage.
I DO NOT eat oysters, I've only committed the "sin" to post this article ;-))
I love raw oysters. That being said, we could see that the ancient Hebrew dietary restrictions could help VERY much to avoid disease and poisionous contamination. If a fin-fish is sour enough to make one sick, it most likely won't make it past the teeth...
Clams, on the otherhand, can be fresh, taste ok, but have worse things than botulism contained in them. Like pathogens that can cause respiratory paralysis...
Shrimp and crabs can "go bad", but it not be evident enough from the smell, if one were to try to "gauge" the way change in smell (as the shellfish ages a bit), compared to how meat changes from "fresh", to "aged".