I’m not sure what you’re describing. When I saw the title I was wondering if you were talking about a rhythmic spasm like perhaps repeated electrical shock- stimulus like I’ve seen in clams after adding assorted chemicals (alk, calc, nitrates,) and other things that are supposed to be safe? I have repeatedly found that soon after the clam fades into death. Spreading? (this is text discussion is getting a bit squalid -perhaps you should post a quick video clip if you can?
Family first, I hope you’re not seeing what I saw.