The Types of Crab Shells

• The shell of this Dungeness crab can reach up to 9 inches in size. dungeness crab image by Mat Hayward from Fotolia.com

  Crabs are members of the Crustacean subphylum and are characterized by an enlarged, flattened cephalothorax, or body, which is covered by a carapace, or shell. The shell is part of the animal's exoskeleton, which covers its entire body. The exoskeleton is comprised of a cuticle -- a body covering outside the epidermis -- which is reinforced with calcium carbonate as well as with a polysaccharide known as chitin.

True Crab Shells

• The true crabs are members of class Malacostraca, which also includes shrimp, crayfish and lobsters. The blue crabs, fiddler crabs, king crabs, ghost crabs and giant spider crabs are all examples of true crab species. True crabs can have shells as large as one foot across in the case of the giant spider crabs of Japan, and can weigh as much as 20 pounds in the case of the king crab of the North Pacific. The shell can be blue to blue-green, red, brown or gold in color and can be smooth or covered in tiny projections, giving the shell a rough texture.

Soft-Shelled Crabs

• The blue crab, whose scientific name Callinectes sapidus means "beautiful swimmer that tastes good," is often sold for food as a soft-shelled crab. This crab's shell is actually quite similar to those of the other true shells. The soft-shelled blue crab is sold just after molting when it has shed its previous shell and its new shell has not yet hardened. Because crabs are especially vulnerable just after molting, they tend to hide during this time and are difficult for fishermen to catch. Instead, they catch the crab when it is at the "peeler" stage just before molting and hold the crabs in a tank until after molting has occurred. Hard-shelled blue crabs are also available.

Male vs. Female shells

• The male and female members of most crab species can be distinguished from one another on the basis of the shape of their shell. In the case of the Dungeness, red rock and blue crabs, when looking at the lower abdomen of a crab, a male will have a narrow triangular abdomen and a female will have a wider, more oval-shaped abdomen.

Hermit Crab Shells

• Hermit crabs are also Crustaceans, but as they are not members of class Malacostraca, they are not considered to be true crabs. Hermit crabs are distinguishable from true crabs by their long, soft, spirally coiled abdomens, which they protect by inhabiting shells that have been cast off by other creatures. A hermit crab will examine any object that releases calcium but does not release other chemicals that may indicate that the shell is already occupied. As a result, a hermit crab can end up wearing any of a number of different shell types, but most often makes use of the shells of molluscs, such as periwinkles and whelks, although they are also known to inhabit other homes, including bamboo cane.