Bivalve shell
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A bivalve shell is part of the body, the
exoskeleton An exoskeleton (from Greek ''éxō'' "outer" and ''skeletós'' "skeleton") is an external skeleton that supports and protects an animal's body, in contrast to an internal skeleton (endoskeleton) in for example, a human. In usage, some of the ...
or shell, of a
bivalve Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. As a group, bival ...
mollusk Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000  extant species of molluscs are recognized. The number of fossil species is e ...
. In life, the shell of this class of mollusks is composed of two hinged parts or ''
valves A valve is a device or natural object that regulates, directs or controls the flow of a fluid (gases, liquids, fluidized solids, or slurries) by opening, closing, or partially obstructing various passageways. Valves are technically fitting ...
''. Bivalves are very common in essentially all aquatic locales, including saltwater,
brackish Brackish water, sometimes termed brack water, is water occurring in a natural environment that has more salinity than freshwater, but not as much as seawater. It may result from mixing seawater (salt water) and fresh water together, as in estuari ...
water, and freshwater. The shells of bivalves commonly wash up on beaches (often as separate valves) and along the edges of lakes, rivers, and streams. Bivalves by definition possess two shells or ''valves'', a "right valve" and a "left valve", that are joined by a
ligament A ligament is the fibrous connective tissue that connects bones to other bones. It is also known as ''articular ligament'', ''articular larua'', ''fibrous ligament'', or ''true ligament''. Other ligaments in the body include the: * Peritoneal li ...
. The two valves usually articulate with one another using structures known as "teeth" which are situated along the
hinge line A hinge line is an imaginary longitudinal line along the dorsal edge of the shell of a bivalve mollusk where the two valves hinge or articulate. The hinge line can easily be perceived in these images of a mussel shell and an ark shell Ark cla ...
. In many bivalve shells, the two valves are symmetrical along the hinge line—when truly symmetrical, such an animal is said to be ''equivalved''; if the valves vary from each other in size or shape, ''inequivalved''. If symmetrical front-to-back, the valves are said to be ''equilateral'', and are otherwise considered ''inequilateral''. This exoskeleton serves not only for muscle attachment, but also for protection from predators and from mechanical damage. The shell has several layers, and is typically made of calcium carbonate precipitated out into an organic matrix. It is secreted by a part of the molluscan body known as the mantle. The shells of bivalves are equal sides connected by a hinge. Bivalve shells are collected by professional and amateur conchologists and are sometimes harvested for commercial sale in the international shell trade or for use in glue, chalk, or varnish, occasionally to the detriment of the local ecology.


Shell anatomy, structure and composition

The bivalve shell is composed of two
calcareous Calcareous () is an adjective meaning "mostly or partly composed of calcium carbonate", in other words, containing lime or being chalky. The term is used in a wide variety of scientific disciplines. In zoology ''Calcareous'' is used as an adje ...
valves. The mantle, a thin
membrane A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Membranes can be generally classified into synthetic membranes and biological membranes. B ...
surrounding the body, secretes the shell valves,
ligament A ligament is the fibrous connective tissue that connects bones to other bones. It is also known as ''articular ligament'', ''articular larua'', ''fibrous ligament'', or ''true ligament''. Other ligaments in the body include the: * Peritoneal li ...
, and
hinge teeth Hinge teeth are part of the anatomical structure of the inner surface of a bivalve shell, i.e. the shell of a bivalve mollusk. Bivalves by definition have two valves, which are joined together by a strong and flexible ligament situated on the hing ...
. The mantle lobes secrete the valves, and the mantle crest creates the other parts. The mantle itself is attached to the shell by numerous small mantle retractor muscles, which are arranged in a narrow line along the length of the interior of the shell. The position of this line is often quite clearly visible on the inside of each valve of a bivalve shell, as a shiny line, the
pallial line The pallial line is a mark (a line) on the interior of each valve of the shell of a bivalve mollusk. This line shows where all of the mantle A mantle is a piece of clothing, a type of cloak. Several other meanings are derived from that. Mantle m ...
, which runs along a small distance in from the outer edge of each valve, usually joining the anterior adductor muscle scar to the posterior adductor muscle scar. The adductor muscles are what allow the bivalve to close the shell tightly. In some bivalves the mantle edges fuse to form
siphon A siphon (from grc, σίφων, síphōn, "pipe, tube", also spelled nonetymologically syphon) is any of a wide variety of devices that involve the flow of liquids through tubes. In a narrower sense, the word refers particularly to a tube in a ...
s, which take in and expel water during
suspension feeding Filter feeders are a sub-group of suspension feeding animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized filtering structure. Some animals that use this method of feedin ...
. Species which live buried in sediment usually have long siphons, and when the bivalve needs to close its shell, these siphons retract into a pocket-like space in the mantle. This feature of the internal anatomy of a bivalve is clearly indicated on the interior of the shell surface as a
pallial sinus The pallial sinus is an indentation or inward bending in the pallial line on the interior of a bivalve mollusk Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as mollusc ...
, an indentation in the pallial line. In addition, the water flows through incurrent siphon ventrally and exit out of the body through excurrent dorsally to the body. The valves of the shell are made of either
calcite Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...
(as with, e.g. oysters) or both calcite and
aragonite Aragonite is a carbonate mineral, one of the three most common naturally occurring crystal forms of calcium carbonate, (the other forms being the minerals calcite and vaterite). It is formed by biological and physical processes, including prec ...
, usually with the aragonite forming an inner layer, as is the case with the
Pteriida The Pteriida are an order of large and medium-sized marine bivalve mollusks. It includes five families, among them the Pteriidae (pearl oysters and winged oysters). 2010 taxonomy In 2010, a new proposed classification system for the Bivalvia ...
which have this layer in the form of
nacre Nacre ( , ), also known as mother of pearl, is an organicinorganic composite material produced by some molluscs as an inner shell layer; it is also the material of which pearls are composed. It is strong, resilient, and iridescent. Nacre is f ...
or mother of pearl. The outermost layer of the shell is known as the periostracum and is composed of a horny organic substance. This sometimes forms a yellowish or brownish "skin" on the outside of the shell. The periostracum may start to peel off of a shell when the shell is allowed to dry out for long periods. The shell is added to, and increases in size, in two ways—by increments added to the open edge of the shell, and by a gradual thickening throughout the animal's life. The two shell valves are held together at the animal's dorsum by the
ligament A ligament is the fibrous connective tissue that connects bones to other bones. It is also known as ''articular ligament'', ''articular larua'', ''fibrous ligament'', or ''true ligament''. Other ligaments in the body include the: * Peritoneal li ...
, which is composed of the tensilium and resilium. In life the ligament opens the shell (like a bent eraser in a door hinge), and the adductor muscle or muscles close the shell (like a person pulling the door closed by the handle). When a bivalve dies, its adductor muscle(s) relax and the resilium pushes the valves open.


Cementation

A few groups of bivalves are active swimmers like the
scallop Scallop () is a common name that encompasses various species of marine bivalve mollusks in the taxonomic family Pectinidae, the scallops. However, the common name "scallop" is also sometimes applied to species in other closely related families ...
s; many bivalves live buried in soft sediments (are
infauna Fauna is all of the animal life present in a particular region or time. The corresponding term for plants is ''flora'', and for fungi, it is '' funga''. Flora, fauna, funga and other forms of life are collectively referred to as '' biota''. Zoo ...
l) and can actively move around using their muscular foot; some bivalves such as
blue mussel The blue mussel (''Mytilus edulis''), also known as the common mussel, is a medium-sized edible marine bivalve mollusc in the family Mytilidae, the mussels. Blue mussels are subject to commercial use and intensive aquaculture. A species with a l ...
s attach themselves to hard substrates using a
byssus A byssus () is a bundle of filaments secreted by many species of bivalve mollusc that function to attach the mollusc to a solid surface. Species from several families of clams have a byssus, including pen shells (Pinnidae), true mussels (Mytilid ...
; other groups of bivalves (such as
oyster Oyster is the common name for a number of different families of salt-water bivalve molluscs that live in marine or brackish habitats. In some species, the valves are highly calcified, and many are somewhat irregular in shape. Many, but not al ...
s,
thorny oyster ''Spondylus'' is a genus of bivalve molluscs, the only genus in the family Spondylidae.MolluscaBase (2019). MolluscaBase. Spondylus Linnaeus, 1758. Accessed through: World Register of Marine Species at: http://www.marinespecies.org/aphia.php?p=t ...
s, jewel boxes,
kitten's paw ''Plicatula gibbosa'', or the Atlantic kitten's paw, is a species of bivalve mollusc in the family (biology), family Plicatulidae. Description The size of the shell attains 38.6 mm. Distribution It can be found along the Atlantic coast of Nor ...
s, jingle shells, etc.) cement their lower valve to a hard substrate (using shell material as cement) and this fixes them permanently in place. In many species of cemented bivalves (for example the jewel boxes), the lower valve is more deeply cupped than the upper valve, which tends to be rather flat. In some groups of cemented bivalves the lower or cemented valve is the left valve, in others it is the right valve.


Orientation

The oldest point of a bivalve shell is called the ''beak'', and the raised area around it is known as the '' umbo'' (plural ''umbones''). The hinge area is the ''dorsum'' or back of the shell. The lower, curved margin is the ''ventral'' side. The ''anterior'' or front of the shell is where the byssus and foot are located (if the animal has these structures) and the ''posterior'' or back of the shell is where the siphon is located (again, if present— the
scallop Scallop () is a common name that encompasses various species of marine bivalve mollusks in the taxonomic family Pectinidae, the scallops. However, the common name "scallop" is also sometimes applied to species in other closely related families ...
s, for example, do not have siphons). Without being able to view these organs, however, determining anterior and posterior can be rather more difficult. In those animals with a siphon, the pallial sinus of the siphon, which will be present on both the left and right valves, will point towards the animal's ''posterior''— such valves are called ''sinopalliate''. Shells without a pallial sinus are termed ''integripalliate''— such animals (as mentioned, the scallops as well as some other groups) often have a byssal notch present on the ''anterior'' end of the ''right'' valve (only), and the anterior auricles or "wings" of both valves will be either larger than, or equal to, the posterior ones. Such valves may also have a distinctive "comb" or ''ctinoleum'' within the byssal notch on the right valve. If a valve has neither notch nor comb nor sinus, and the auricles are of the same size, it is likely to be a ''left'' valve. In those animals whose valves have an umbo that seems to "point", that point is most often towards the ''anterior'' part of the valve (though there are some exceptions to this rule). Also, in those bivalves with two adductor muscle scars of different sizes, the ''posterior'' scar will be the larger of the two and will be visible on both valves— this condition is referred to as being ''anisomyarian''; if the scars are of equal size, this is termed ''isomyarian''; if the valve has only one muscle scar, this is termed ''monomyarian''. Furthermore, in those animals with a distinct external ligament, the ligament is usually to the ''posterior'' side of the umbo of both valves. Using one or more of these guidelines should strongly suggest the anterior/ posterior orientation of any given bivalve shell, and therefore whether any particular shell belongs to the right side or the left.


Age estimation

The age of bivalve molluscs can be estimated in several ways. The Noah's Ark clam ''
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'' has been used to compare these methods: the annual growth rings on the exterior of the valves can be counted at one per year and give a satisfactory result, but sometimes spurts of growth occur which may create an extra ring and cause confusion. Early rings may get worn away near the umbones and the narrow rings near the margin may be difficult to interpret in fully grown individuals. Similar annual pallial line scars on the interior of the valves are more easily seen in dark colored shells, but these may be overgrown and obscured by further deposition of hard material. Another method is the examination of the growth lines and bands seen in acetate peel replicas taken in the region of the umbones. The most accurate but most time-consuming method is the microscopic examination of sections through the outer prismatic layer of the shell. Using more than one of these methods should increase the accuracy of the result.


Hinge teeth

The
hinge teeth Hinge teeth are part of the anatomical structure of the inner surface of a bivalve shell, i.e. the shell of a bivalve mollusk. Bivalves by definition have two valves, which are joined together by a strong and flexible ligament situated on the hing ...
(dentition) or lack of them is an important feature of bivalve shells. They are generally conservative within major groups, and have historically provided a convenient means upon which to base classification schemes and the phylogenetic order. Some of the various hinge tooth arrangements are as follows: * Taxodont; rows of similar interlocking teeth on either side of the umbones, as in the arc clams. * Dysodont; weak teeth near the umbones, as in the marine mussels. * Isodont; lateral tubercles and sockets on either side of a thick ligament referred to as a resilifer, typical of the oysters and
scallops Scallop () is a common name that encompasses various species of marine bivalve mollusks in the taxonomic family Pectinidae, the scallops. However, the common name "scallop" is also sometimes applied to species in other closely related famil ...
. * Heterodont; with several wedge-shaped cardinal teeth set within the umbones, may or may not have elongated lateral teeth on either side. This arrangement is characteristic of the
venus clams The Veneridae or venerids, common name: Venus clams, are a very large family of minute to large, saltwater clams, marine bivalve molluscs. Over 500 living species of venerid bivalves are known, most of which are edible, and many of which are ex ...
, cockles and several other important groups. * Asthenodont; cardinal teeth replaced by a large chondrophore or resilifer, as in the soft-shell clams * Anodont; true teeth absent in adults as in razor clams, and some freshwater mussels such as '' Anodonta'' and ''
Anodontites ''Anodontites'' is a genus of freshwater mussels, aquatic bivalve mollusks in the family Mycetopodidae The Mycetopodidae are a family of freshwater pearly mussels in the order Unionida restricted to South America. They are named for the mu ...
''


Uses

Bivalve shells have many uses, leading international trade in bivalves and their shells. These uses include: * Aesthetic **
Jewellery Jewellery ( UK) or jewelry (U.S.) consists of decorative items worn for personal adornment, such as brooches, rings, necklaces, earrings, pendants, bracelets, and cufflinks. Jewellery may be attached to the body or the clothes. From a wester ...
, in particular
nacre Nacre ( , ), also known as mother of pearl, is an organicinorganic composite material produced by some molluscs as an inner shell layer; it is also the material of which pearls are composed. It is strong, resilient, and iridescent. Nacre is f ...
(mother of pearl) ** High-quality Go stones **
Conchology Conchology () is the study of mollusc shells. Conchology is one aspect of malacology, the study of molluscs; however, malacology is the study of molluscs as whole organisms, whereas conchology is confined to the study of their shells. It includ ...
(in the sense of collection rather than research) * Raw material **
Chalk Chalk is a soft, white, porous, sedimentary carbonate rock. It is a form of limestone composed of the mineral calcite and originally formed deep under the sea by the compression of microscopic plankton that had settled to the sea floor. Chalk ...
**
Glue Adhesive, also known as glue, cement, mucilage, or paste, is any non-metallic substance applied to one or both surfaces of two separate items that binds them together and resists their separation. The use of adhesives offers certain advant ...
**
Varnish Varnish is a clear transparent hard protective coating or film. It is not a stain. It usually has a yellowish shade from the manufacturing process and materials used, but it may also be pigmented as desired, and is sold commercially in various ...
* Food ** ( Greenshell mussel) As a supplement for use against inflammation-associated
arthritis Arthritis is a term often used to mean any disorder that affects joints. Symptoms generally include joint pain and stiffness. Other symptoms may include redness, warmth, swelling, and decreased range of motion of the affected joints. In som ...
in humans and animals, though a 2006 review suggested a lack of compelling evidence in human cases. * Other **
Shell money Shell money is a medium of trade, exchange similar to coin money and other forms of commodity money, and was once commonly used in many parts of the world. Shell money usually consisted of whole or partial sea shells, often worked into beads or o ...
, a medium of
exchange Exchange may refer to: Physics *Gas exchange is the movement of oxygen and carbon dioxide molecules from a region of higher concentration to a region of lower concentration. Places United States * Exchange, Indiana, an unincorporated community * ...


See also

* Aspein * Molluscs in culture


References


External links

A glossary of terms used to describe bivalves

{{DEFAULTSORT:Bivalve Shell Bivalve anatomy Mollusc shells