As mentioned with crabs and other crustaceans, iodine is necessary for most shelled invertebrates to achieve successful molting. Although iodine does not stay in solution very long (weekly water changes cannot come close to keeping a stable level), many foods are rich in this element, and small daily feedings of krill and the like will adequately support ecdysis (shedding the exoskeleton). It's recommended that you leave the shed, nutritious molts of shrimp in the aquarium for them or other creatures to consume. Freshly molted shrimp are quite soft bodied and vulnerable to predation for several days while they swell with water and their new shell hardens. During these brief reclusive periods, remarkabley, missing or damaged parts of a shrimp can be regenerated with enough successive molts.