ABSTRACT
Site-directed mineralization is necessary to build complicated shapes seen in the skeletons of arthropods and other animals. Crustaceans may offer an insight into the process of mineralization, because they use a single tissue to mineralize the skeleton, and must do so periodically in order to grow. Along with chitin, proteins are also deposited with the fibrous portion of the exoskeleton. We found, using polyacrylamide gel electrophoresis (PAGE) and immunohistological techniques that carbonic anhydrase is deposited in the exoskeletons of slipper lobsters, particularly in the endocuticle. Carbonic anhydrase is capable of raising the bicarbonate concentrations in specific layers and areas of the exoskeleton, thus raising carbonate concentrations in the newly forming exoskeleton during molting. The biopolymers may act as a nucleating surface for calcium carbonate crystallization and, with a supply of calcium from seawater, gastroliths, and the carbonate via the action of carbonic anhydrase, crystallization can proceed until the enzyme becomes entombed in the crystalline phase. For this reason, skeletal layering becomes an inevitable consequence of such enzyme-mediated supply of the anion. Membranes or nonmineralized portions of the exoskeleton do not stain for carbonic anhydrase. Because slipper lobsters have the thickest shells and a complicated internal pit system, they have the strongest shells among lobsters. Punch test data showed that slipper lobsters resisted shell fracture better than clawed and spiny lobsters.
