Crabs are crustaceans and to grow they must shed their hard shells, and in a crab’s case its lungs too. This is the time that crabs become a very effective bait. I can only think of a handful of species that don’t regularly eat crabs.
Crabs use hydraulics to shed their shells. They pump the new soft shell that is under the old hard shell up with fluid. This causes the outer shell to start cracking at the sides, eventually lifting the top part of their shell. At this point anglers call them poppers. After the popper stage the crab crawls out. At this stage we anglers call them jellies. These jelly crabs are very fragile but can be a deadly bait. I love to use jellies for close range bass
