Source : Webster's Revised Unabridged Dictionary (1913)

Soft-shell \Soft"-shell`\, Soft-shelled \Soft"-shelled`\, a.
   Having a soft or fragile shell.

   {Soft-shell clam} (Zo["o]l.), the long clam. See {Mya}.

   {Soft-shelled crab}. (Zo["o]l.) See the Note under {Crab}, 1.
      

   {Soft-shelled turtle}. (Zo["o]l.) Same as {Soft tortoise},
      under {Soft}.

Source : WordNet®

soft-shelled crab
     n 1: freshly molted crab with new shell still tender and flexible
          [syn: {soft-shell crab}]
     2: edible crab that has recently molted and not yet formed its
        new shell [syn: {soft-shell crab}]