Like hermiesguardian said offer plenty of healthy foods as part of their normal diet. I don't like to refer to any one type of food as healing since crabs will eat what they feel their body needs. Offering all food parts of the food pyramid daily ensures your crabs are getting the proper nutrition at all times.
Our nutrition guide will assist you in knowing everything crabs need to stay healthy.
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The missing limb could have happened due to the stresses of being taken from the wild, shipped, deprived proper foods, waters, and being in improper conditions. With any luck it was a one time occurrence and won't happen again. Just monitor this crab to make sure it doesn't drop any more limbs. Also with being a new crab recently purchased it will probably dig and molt right away. Sometimes this can be good or bad depending on the current health of the crab. Your crab should eventually regrow this leg over it's next few molts.
Here is our guide for molting for more information.
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Here is our guide for assisting with PPS, post purchase stress. There are two ways you can introduce new crabs into your crabitat. I feel the second approach is the best but requires isolation for about a month and more care. It allows the crab to slowly adjust to proper conditions and allows them to regain strength before being introduced into your regular crabitat to molt.
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