Sorry to hear that the clam didn't make it. How long did you had it for?
I'm a clam newbie myself, but I'll ask anyway since someone more experienced may want to know. What kind of snails are those? You have pic? Certain species of snails are known to eat clams. If the snails are considered harmless, it could be possible that the snail may have detected that the clam was going kaput the day before to scavenge. From what I read, a dead or dying clam may still have reflexes. If it was a recent purchase, let's say a few days or less, maybe the clam was already stressed big time at the LFS and the extra stress from the LFS to your tank ended it. Well, I need a little more info (date or purchase, type of snail), that is all I can come up with. Anyway, let's wait and see what else can be said to solve the mystery.
Caclium should be at least 400ppm. I keep mine at 450ppm.
BTW, you're really breeding octopuses? If so, very cool! Wouldn't mind seeing some images, if you have some!
Thanks,
James