Nothing uncommon caused it, as far as I can see, and after the spawning, the clam looks exactly as before. Activated carbon was added - 0,5 gram/liter of water tank and a 10% water change performed, to avoid spem toxicity. The skimmer stopped working during the event, but the next day it was working again. After the water change, skimmer perfomance improved. We're doing pretty well, after the "hapening".