If the middle is already "wilted" the clam is dead. Problem is, clam could have already been well on it's way to death before you bought it. They die from the inside out. By the time they show any symptoms it's usually too late.
Why it died, who knows. Could have been anything.
Next time, if you're going to buy a clam, make sure it reacts to shadows quickly. If a hand passed over the clam blocking the light doesn't result in a "flinch", pass on the clam. Now, as clams get used to being in captivity this won't happen much anymore. My gigas will not react to most shadows. It's been used to all the activity in and around the tank for years and years so much so that fish can swim in and out of it's inhalant without much of a reaction. But a clam new to the store or your tank should definitely have a reaction. Also turn it upside down to make sure the byssal gland is present and intact.
Finally, there are few instances and even fewer clams I would try keeping with PC's.
HTH,
JP |