As I guess you only need the shell - Boil the crab for an extended period then remove all the flesh - The remaining shell can be wired together as it originally was.
Alternatively - Dig a shallow pit in the garden and fill with dry sand - Bury the crab for 4 to 6 months and allow insects to do all the work for you. Dig up and reassemble. this works well for small mammals as well.
Make sure the local wildlife don't dig up your crab etc.
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