Severe stress due to temperature, salinity or oxygen problems or toxic chemicals can cause the raptorial appendages to fail and ultimately to be lost. Usually when this occurs they are physically ripped off. Loss during molting is a different issue and the bottom line is that we just don't know why this happens. It is more common in large smashers like O. scyllarus than in smaller ones, and it seems rare in spearers, although I have seen it fairly frequently in Lysiosquillina. I suspect that the problem is a combination of lack of excercise and nutrition, but I don't have a magic preventative except the old saw of good water quality and a varied, natural diet that for smashers includes items that must be broken up.
Roy