The Collaborative International Dictionary of English v.0.48:

Soft-shell \Soft"-shell`\, Soft-shelled \Soft"-shelled`\, a.
   Having a soft or fragile shell.
   [1913 Webster]

   Soft-shell clam (Zool.), the long clam. See Mya.

   Soft-shelled crab. (Zool.) See the Note under Crab, 1.

   Soft-shelled turtle. (Zool.) Same as Soft tortoise, under
      Soft.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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