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The mollusc shell or mollusk shell[spelling 1] is typically a calcareous exoskeleton which encloses, supports and protects the soft parts of an animal in the phylum Mollusca. In everyday language, mollusc shells are often simply known as "shells", or in the case of marine species, "seashells". The ancestral mollusc is thought to have had a shell, but this has subsequently been lost or reduced on some families, such as the squid, octopus, and some smaller groups such as the caudofoveata and solenogastres,[1] and the highly derived Xenoturbella.[2] Malacology, the scientific study of molluscs as living organisms, has a branch devoted to the study of shells, and this is called conchology - although these terms used to be, and to a minor extent still are, used interchangeably, even by scientists (this is more common in Europe). Within some species of molluscs there is often a surprising degree of variation in the exact shape, pattern, ornamentation, and color of the shell.
[edit] Shell formation A mollusc shell is formed, repaired and maintained by a part of the anatomy called the mantle. Any injuries to or abnormal conditions of the mantle are usually reflected in the shape and form and even color of the shell. When the animal encounters harsh conditions that limit its food supply, or otherwise cause it to become dormant for a while, the mantle often ceases to produce the shell substance. When conditions improve again and the mantle resumes its task, a "growth line" is produced. In shelled molluscs, the mantle is what forms the shell, and what adds to the shell to increase its size and strength as the animal grows. Shell material is secreted by the ectodermic (epithelial) cells of the mantle tissue.[3] Mollusc blood is rich in a soluble form of calcium, and this calcium is concentrated and crystallized as calcium carbonate (CaCO3). In those shelled molluscs that have indeterminate growth, the shell grows steadily over the lifetime of the mollusc by the addition of calcium carbonate to the leading edge or opening. Thus the shell gradually becomes longer and wider, in an increasing spiral shape, to better accommodate the growing animal inside. The animal also thickens the shell as it grows, so that the shell stays proportionately strong for its size. [edit] Shell structureThe shell of a mollusc is formed of two or three layers. The outermost shell layer in many molluscs is composed solely of organic material, and is known as the periostracum. The inner layers of the shell are formed of calcium carbonate crystalized into an organic matrix; these are the nacreous and prismatic layers. The individual crystals of each shell layer differ in shape and orientation, such that one layer is calcite and another aragonite. Thin new layers of shell are continually deposited onto the inner surface of the animal's shell. In some mollusc shells the inner layer is especially strong, and is known for its colorful, iridescent appearance. This is known as the nacreous layer, nacre, or mother of pearl. The calcium carbonate layers in a shell are generally of two types: an outer, chalk-like prismatic layer and an inner pearly, lamellar or nacreous layer. The layers usually incorporate a substance called conchiolin, often in order to help bind the calcium carbonate crystals together. Conchiolin is composed largely of quinone-tanned proteins. The periostracum and prismatic layer are secreted by a marginal band of cells, so that the shell grows at its outer edge. Conversely, the nacreous layer is derived from the main surface of the mantle.[4] Some shells contain pigments which are incorporated into the structure. This is what accounts for the striking colors and patterns that can be seen in some species of seashells, and the shells of some tropical land snails. These shell pigments sometimes include compounds such as pyrroles and porphyrins. Shells are almost always composed of polymorphs of calcium carbonate - either calcite or aragonite. In many cases, different layers of the shell are composed of different minerals. In a few species which dwell near hydrothermal vents, iron sulfide is used to construct the shell. Phosphate is never utilised by molluscs,[5] with the exception of Cobcrephora, whose molluscan affinity is uncertain.[6] Shells are composite materials of calcium carbonate (found either as calcite or aragonite) and organic macromolecules (mainly proteins and polysaccharides.) Shells can have numerous ultrastructural motifs, the most common being crossed-lamellar (aragonite), prismatic (aragonite or calcite), homogeneous (aragonite), foliated (aragonite) and nacre (aragonite). Although not the most common, the nacre is the most studied layer. Shells of the class Polyplacophora are made of aragonite. [edit] Shells of chitonsMain article: Chiton#Shell  Shells of chitons are made up of eight overlapping calcareous valves, surrounded by a girdle. [edit] Gastropod shell Main article: gastropod shell In some marine genera, during the course of normal growth the animal undergoes periodic resting stages where the shell does not increase in overall size, but a greatly thickened and strengthened lip is produced instead. When these structures are formed repeatedly with normal growth between the stages, evidence of this pattern of growth is visible on the outside of the shell, and these unusual thickened vertical areas are called varices, singular "varix". Varices are typical in some marine gastropod families, including the Bursidae, Muricidae, and Ranellidae. Finally, gastropods with a determinate growth pattern may create a single and terminal lip structure when approaching maturity, after which growth ceases. These include the cowries (Cypraeidae) and helmet shells (Cassidae), both with in-turned lips, the true conchs (Strombidae) that develop flaring lips, and many land snails that develop tooth structures or constricted apertures upon reaching full size. [edit] Shells of cephalopods Main article: Cephalopod#Shell Nautiluses are the only extant cephalopods which have an external shell. (For information on a very large extinct subclass of shelled cephalopods, please see Ammonites.) Cuttlefish, squid, spirula, and cirrate octopuses have small internal shells. Females of the octopus genus Argonauta secrete a specialised paper-thin eggcase in which they partially reside, and this is popularly regarded as a "shell", although it is not attached to the body of the animal. [edit] Bivalve shellMain article: bivalve shell The shell of the Bivalvia is composed of two parts, two valves which are hinged together and joined by a ligament. [edit] Tusk shellMain article: tusk shell The shell of many of the scaphopods resembles a miniature elephant's tusk in overall shape, except that it is hollow, and is open at both ends. [edit] Relative size of the shellIn most shelled molluscs, the shell is large enough for all of the soft parts to be retracted inside when necessary, for protection from predation or from desiccation. However there are many species of gastropod mollusc in which the shell is somewhat reduced or considerably reduced, such that it offers some degree of protection only to the visceral mass, but is not large enough to allow the retraction of the other soft parts. This is particularly common in the opisthobranchs and in some of the pulmonates. Some gastropods have no shell at all, or only an internal shell or internal calcareous granules, and these species are often known as slugs. Semislugs are pulmonate slugs with a greatly reduced external shell which is in some cases partly covered by the mantle. [edit] Mother of pearl and pearl formationMain article: pearl Mollusc blood is rich in dissolved calcium, and during shell deposition, the calcium is concentrated out from the blood and crystallized as calcium carbonate. Layers of calcium carbonate are continually being deposited onto the inner surface of the animal's shell, as a means to thicken, strengthen and smooth the inner surface of the shell itself, and as a defense against parasitic organisms and damaging detritus. Nacre, commonly known as mother of pearl, forms the inner layer of the shell structure in some groups of gastropod and bivalve molluscs, mostly in the more ancient families such as top snails (Trochidae), and pearl oysters (Pteriidae). Like the other calcareous layers of the shell, the nacre is created by the epithelial cells (formed by the germ layer ectoderm) of the mantle tissue.) When a mollusc is invaded by a parasite, or is irritated by a foreign object that the animal cannot eject, a process known as encystation entombs the offending entity in successive, concentric layers of the inner shell material, which in some cases is composed of nacre. This process eventually forms a pearl, and this process continues, such that the pearl keeps increasing in size, as long as the animal lives. Almost any species of bivalve or gastropod is capable of producing "pearls", even molluscs whose inner shell layer is not nacreous but porcellaneous. However, only a few species, including pearl oysters, can create pearls which are highly prized. Although the best-known examples of these are nacreous (e.g. oyster pearls), some non-nacreous pearls are also highly valued commercially (e.g. conch pearls). [edit] Fossil recordMollusc shells (especially those formed by marine species) are very durable and outlast the otherwise soft-bodied animals that produce them by a very long time (sometimes thousands of years even without being fossilized). Most shells of marine mollusks fossilize rather easily, and fossil mollusc shells date all the way back to the Cambrian period. Large amounts of shells sometimes form sediment, and over a geological time span can become compressed into limestone deposits. Most of the fossil record of molluscs consists of their shells, since the shell is often the only mineralised part of a mollusc (however also see Aptychus and operculum). The shells are usually preserved as calcium carbonate - usually any aragonite is pseudomorphed with calcite - but it may also be replaced with phosphate.[7] [edit] See also
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