I don't know what happened, but yesterday morning my big Derasa was happy and looked healthy. Last night when I got home, he was gaping a little and his mantle was shrunken in slightly. This morning he is gaping wide open, and is almost completely unresponsive. I didn't make any changes in the last week or so except that I threw some of my old shells in the sump two days ago. I hope I didn't reintroduce a clam disease that wiped out my other ones.