Quite brilliant actually, the crab already has the genetic machinery to moult. It seems to me an fascinating evolutionary story to visualize. Crabs that were able to make physiological adaptations to concentrate damaging heavy metals in their cuticles or exoskeletons had more viable offspring than those who sequestered the metals and let it damage their tissues and reproductive system. Of course zinc and copper are enzymatically important metals and are in much higher demand, which may be why the authors disn’t see much elimination of these two elements. Lead has no known vital function in fiddler crabs