A papershell refers to the stage a crab goes through right after it molts but before the new shell fully hardens. This is a process that is similar to that in many insects and other arthropodsand crustaceans. As the crab grows it needs room to expand. At first it fills in the voids in its exoskeleton but since its exoskeleton, or shell cannot grow, it must eventually get rid of old shell and form a new and larger one.
The first stage in this process is to start forming a new shell under the old shell. If you have ever eaten steamed crabs you can often see this new shell under the old as a brown membrane. As time goes on, this shell gets thicker and tougher, but is basicly still a membrane. When the time comes, the crustacean pulls itself out of its old shell and abandons it. At this stage, the crab, now termed a softshell crab is very vulnerable to predators, such as Rockfish and Homo Sapiens, which consider softshell crabs to be a delicacy. If the molting crab is an adolescent female, she will likely be accompanied by a male or Jimmy Crab, who will protect and impreganate the female while she is a softshell. The crabs, known as a doubler, will try to find some cover such as underwater grasses or oyster beds so they can conduct their romantic interlude without becoming a tasty snack.
The second stage of the molting process begins as soon as the crab is free of its old shell. It pumps water inside its still flexible shell and stretches it to its new and larger size. Eventually the new shell starts to harden and the cycle begins all over again. All of this work takes a lot of effort, and the new crab is pretty weak for a while, since the new skin is thin and pliable, and much of its stored energy resources were spent molting. In an adult or nearly adult crab, the final molt actually ends up killing many molting crabs, and it takes quite a while for those that survive to regain their former weight and strength. Their shells, while not expanding anymore are not hard and strong, but thin and somewhat flexible like paper.
What this means for the crabber is that a crab that has not fully recovered from the molt, but too far removed from the molt to be considered a softshell is called a papershell. Papershell crabs are despised by watermen, because they bring little money at market. They are despised by the crab pickers at the packing houses who are paid by the pound of meat picked, since there is little meat inside them. Finally they are despised by crab feasters for the same reason.
A papershell crab will have a whiter underside than its more developed brethren, whose undersides are often stained by mud and time, and they will feel light for their size. If you should catch a papershell while you are crabbing, throw it back and give it a chance to grow into its new shell before turning it into crabcakes.