I see you are not much of a seafood eater
, Yes, they must be alive to be consumed by either you or the octo. If they do not close up tightly when disturbed, they are no good.
The best way I have found to work with the clams is to find a shallow (1-2 inches high) container and fill it with saltwater so that the clam is almost covered when propped up against the side. Eventually, you will see it open (half hour at the most but usually sooner) about 1/4 inch. You don't have to monitor it as once it opens to feed it will usually remain that way. At supper time (for the octo, unless you are dining too
) gently slide a
dull (butter) knife into the opening. You can do this in advance and leave the knife in place but be sure to prop the clam and knife securely or it will manage to spit it out.
There are two strong mussels on each side of the clam back toward the hinge (one on each side). Holding the clam cupped in your hand
over the sink with the hinge in the palm work the knife down the toward the hinge (this is why you need a dull knife, use a cutting board if the knife is at all sharp but it is harder to hold this way). It is usually easiest to cut one mussel and then work on the other side (after one side is cut, the other is easier). Don't let the knife come out of the shell until you have cut one side or you will need to try one of the other clams and come back to this one another time. If you are using an old knife (or a proper clam/oyster knife) that is not table wear, you can twist it to help break the mussle (you will likely bend a normal knife if you twist and that cost more than the clam to replace, not to mention the ire of other family members).
Alternately, a quick wack with a hammer on concrete (not the counter top and not the tile floor) works to crack the shell but it comes apart in lots of pieces and it is hard to get the meat out cleanly.
You can keep them in the bowl for a couple of days in the frig but I usually just put them in an aquarium after a few hours (to be sure they stay alive and flush them a bit) if I am not using them within 24.
SueNami only eats the outer slimy part (clams are tougher than oysters) but you can offer all of it and see what Fletcher will take. If you split the clam successfully, just scrape everything into one side of the shell and place it near the den. If you chose the smash method, put all the soft parts, minus shell, in a small plastic bowl (one that will sink) filled with tank water and carefully put the bowl near the den. Clams, Oysters and mussels are messy to feed but keeping it on the half shell is less of a mess getting it into the tank. Of the three mussels are the easiest to open.
I suggest disgarding the shells in a zip lock bag to avoid another ill smell the next morning.