Shells

Shells are external coverings that are produced by an organism. As such, they require a process by which the constituents of the shell are deposited in a site-directed fashion. This means that calcium and carbonate cannot come in contact with one another when their concentrations exceed their solubility product; otherwise, they will precipitate (form a solid). Thus, these ions must be directed to the area where the preferred precipitation is to take place. For this reason, a matrix is needed to provide a negative attractive force for calcium or other bivalent ions, such as magnesium. Other parts of the matrix may be composed of or house the enzyme carbonic anhydrase for the conversion of carbon dioxide reversibly to bicarbonate. The bicarbonate will degrade to a carbonate ion that can then react with the positively charged calcium ion. Calcium can be taken out of the seawater or diet and concentrated; likewise, bicarbonate ions can be formed in the gills of some of these organisms and transported to deposition sites. However, having a matrix containing the enzyme carbonic anhydrase ensures that the calcium will only precipitate at that matrix site and nowhere else. In this way, the organism can control the shape of the shell by laying down a fiber matrix, usually composed of protein, as in mollusks, or a protein-chitin mixture, as is found in arthropods. The dumping of bicarbonate outside of a tissue where calcium is present will cause precipitation on the tissue membranes, a consequence with detrimental effects on the tissue cells. How calcium and carbonate ions are brought together varies from phylum to phylum.

The Protozoa

Among protists, mineralization is usually accomplished within a membrane-bound vacuole. The precipitated calcium carbonate is then exported by exocytosis to reside on the external surface of the cell. A matrix is secreted in the vacuole to facilitate the mineralization process. External calcium carbonate shells are found among the group Sarcodina of the phylum Sarcomastigophora (these classifications are debated and not universally used). The shells of these amoebas become chambered with growth and are perforated by small openings, through which protrude slender pseudopodia that may be interconnected to form reticulopodia. These include single-celled organisms like Foraminifera, which has a calcium shell resembling a snail shell. Other sarcodines in the subphylum radiolaria (sometimes called Radiozoa) and phylum heliozoa (often called sun-animalcules) use silica as their shell material, forming shells called tests. In this case, the shell or test is also perforated with puncta, or small openings, which extend slender axopodia that will collect food.

Shells of Arthropods

The mineralized shell makes up the exoskeletons of the subphylum Crustacea and the class Merostomata, which includes the horseshoe crabs. The mineralization process in the Crustacea is more complex than that of mollusks because the exoskeleton must be periodically shed, and a new exoskeleton must be formed in the growth process. A percentage of the old exoskeleton is reabsorbed before molting and then redeposited in the new exoskeleton. Like mollusks, an epithelial, sheetlike tissue, the hypodermis, is responsible for forming the fiber matrix upon which is deposited calcium and carbonate. Unlike mollusks, the hypodermis forms cellular extensions that elongate as the layered exoskeleton is constructed. These cellular processes come to lie in pore canals as mineralization proceeds. The first layers laid down are the first to be mineralized, so new matrix layers are being formed while the mineralization of earlier deposited layers is proceeding.

Other differences found between the mollusks and crustaceans are the amount of protein-chitin matrix of Crustacea and the main protein matrix of the Mollusca. The fiber matrix of crustaceans averages about 40 percent by volume, a volume as high as the protein matrix of bone. Because the enzyme becomes entombed in the matrix during the mineralization process, a new matrix must be laid down in a stepwise fashion. This gives the exoskeleton a laminate structure divided into an outer epicuticle, a middle exocuticle, and an inner endocuticle. A noncalcified membrane lies between the hypodermis and the endocuticle.

Shells of arthropods are quite variable, but most have a carapace covering the dorsal part of the body (the head and the thorax). The jointed plates of the abdomen are also calcified, as are the legs and antennae. Ostracod arthropods have a clamlike shell that is hinged dorsally. Barnacles are the other crustacean group that has a different sort of exoskeleton. Their exoskeleton comprises plates called parietes that form a wall around the organism strengthened by calcium carbonate. These mineralized exoskeletons are theorized to have been a crucial development for animals in the Cambrian period.

Shells of Mollusks

The shells of mollusks are usually arranged into two major layers: an outer prismatic layer and an inner nacreous or pearly layer. The prismatic layer is composed of vertically oriented prisms that are bounded by a matrix to separate each prism from one another. The prisms are elongated and extend upward to the organic layer of the shell, termed the periostracum. The nacreous layer is composed of cross-laminated lamellae. The lamellae are oriented in different directions, much like plywood layering. This serves to increase the strength of the shell in its resistance to cracking. The amount of matrix in the shells of mollusks is quite low and may be less than 1 percent of the shell volume. The mantle tissue that lines the shell is responsible for exporting the matrix, calcium, bicarbonate, and carbonic anhydrase for shell mineralization. A space, the extrapallial space, lies between the mantle and the shell and is filled with fluid to facilitate the transfer of ions to the shell. Whereas the shell is deposited along the area of the mantle, the new shell is deposited along the edge of the mantle.

The shells of mollusks are found in many members of this taxon. Animals in the superclass Monoplacophora have a single cap-shaped shell covering a muscular foot. The marine molluscs called chitons in the class Polyplacophora have an elongated body and eight shell plates containing nerves covering their dorsal surface. More familiar shells of mollusks are found among the classes Gastropoda and Bivalvia. Gastropods, including snails and slugs, which live in saltwater, freshwater, and on land, have a single coiled shell that may be twisted, with an opening for the protrusion of the foot and head. The opening may be covered by an operculum that, in some groups, is calcified. The coiling of the shell may keep the weight of the shell over the foot for balance. The shell is used for protection against predation and to prevent desiccation in landforms. The animal can retreat into the shell when frightened or if poor water conditions occur. Many groups lose the shell in both terrestrial and marine forms and are sluglike. Some mollusks have shells with specialized features to allow certain behaviors. The shells of those gastropod mollusks in the family Haliotidae, commonly called Abalone, have openings to release sperm or eggs and another to breathe.

Bivalves, as the name implies, have two shells or valves that enclose the animal and are hinged dorsally with an elastic ligament. Some species of both gastropods and bivalves grow spines by the evagination and elongation of the mantle to export a matrix for mineralization. This is carried out along the edge of the mantle; a ventral slit in the spine often remains to show where the mantle was during spine formation. Spine formation appears to be a device used by gastropods and bivalves to thwart predation, although this idea has not been fully tested. Spines may make these animals difficult to grasp, for example, by the claws of crabs or lobsters. In addition, the spines may make the animal difficult to swallow or crush in the mouths of fish. Another strategy seen by these mollusks is the thickening of the shell, especially around its lip or edge. This makes it difficult for a crab to start breaking the shell from the edge, which is usually the thinnest part of the shell, as this area represents new shell deposition.

Other molluscan groups with external shells are the nautiloids and extinct ammonites. These cephalopods often have coiled shells that resemble those shells of gastropods in their external appearance. Their shells are chambered, with the animal living in the endmost chamber. Gas can be secreted into the old chambers to act as a buoyancy device. The chamber partitions strengthen the shell to withstand ocean pressure at great depths. Nautiloid shells lack a periostracum. Ammonites were a similar cephalopod that occurred in great numbers during the Mesozoic era. Their shells were coiled or straight and developed complex sutures between the separate chambers. Rib patterns developed as well.

The class Scaphopoda has tusklike shells, sometimes called tooth shells, that are made of four layers and are open at both ends. The animal extends out the lower opening, and the upper opening extends above the sediment in this burrowing form, allowing water to flow into and out of the mantle cavity. Most shells in this class are 3 to 6 centimeters (1 to 2 inches) long, but some reach 15 centimeters (6 inches). The horn-shaped shells of the small scaphopod molluscs in the order Gadilida are smooth and shiny. The middle of their shell is usually larger in diameter than its ends. The shells of the larger animals in the order Dentaliida are rougher and wider than those of the Gadilida. To take in oxygen, these animals have cilia that regulate the flow of water around their mantle cavity. These shells sometimes wash onto shore, but are usually found at depths of 7,000 meters (23,000 feet).

Principal Terms

Carapace: the exoskeleton of arthropods

Carbonic Anhydrase: an enzyme used in the mineralization process to convert carbon dioxide to bicarbonate

Laminate Structure: having a layered shell, as in the exoskeletons of crustaceans and the valves of clams

Matrix: composed of proteins or protein-chitin polymers that act as nucleation sites for mineralization

Nacreous Layer: the pearl-like inner layer of molluscan shells

Pore Canals: sites that house the cytoplasmic extensions of the crustacean hypodermis

Prismatic Layer: the outer crystalline layer of the molluscan shell

Bibliography

Carter, J. G., ed. Skeletal Biomineralization: Patterns, Processes, and Evolutionary Trends. Vol. 1. New York, Van Nostrand Reinhold, 1990.

Compére, P., and G. Geoffinet. “Elaboration and Ultrastructural Changes in the Pore Canal Systems of the Mineralized Cuticle of Carcinus maenas During the Molt Cycle.” Tissue and Cell vol. 19, 1987, pp. 859-875.

Kamat, S., X. Su, R. Ballarini, and A. H. Heuer. “Structural Basis for the Fracture Toughness of the Shell of the Conch, Strombus gigas.” Nature vol. 405, June 2000, pp. 1036-1040.

Mulcrone, R. "Scaphopoda." Animal Diversity Web, animaldiversity.org/accounts/Scaphopoda. Accessed 10 Sept. 2024.

“9 Examples of Gastropods (Interesting Facts).” Wildlife Informer, 2023, wildlifeinformer.com/examples-of-gastropods. Accessed 8 July 2023.

Roer, R., and R. Dillaman. “The Structure and Calcification of the Crustacean Cuticle.” American Zoologist vol., 24, 1984, pp. 893-909.

Simkiss, K., and K. M. Wilbur. Biomineralization. New York: Academic Press, 1989.

Vermeij, G. J. Evolution and Escalation. Princeton, N.J.: Princeton University Press, 1987.