In life, the shell of this class of mollusks is composed of two hinged parts or valves. Relative to the outer layer of the mollusc shell, the position of the periostracum. The eyes (blue dots) and tentacles are positioned on the mantle. Bivalve mollusc culture is an important and rapidly expanding sector of world aquaculture production, representing approximately 20% of this output at 14 million tonnes in 2000. It can also function as an escape organ for some epifaunal forms (e.g., cockles, Cardium spp., family Cardiidae). In most bivalves (including Mercenaria), the umbo points towards the anterior side and the ligament and escutcheon towards the posterior side. (1895; Molluscs) (Wikimedia Commons; public domain). The shell. Most bivalves have a large foot, which is a muscular and expandable structure in the middle of the mantle cavity. They live today throughout the world’s oceans and fresh waters, where they are of major ecological importance as a food source for other organisms and for their water-filtering capabilities. One of the main traits of all bivalves is the absence of a head and its associated organs such as eyes, head tentacles, and mouthparts. Image by "Shellnut" (Wikimedia Commons; Creative Commons Attribution-Share Alike 3.0 Unported license; image cropped and label added). The shell of Mercenaria has two crossed-lamellar layers and an outermost prismatic layer. 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. The shiny layer in some bivalve mollusks is known as mother-of-pearl. 1 from Gao et al. Image by Jonathan R. Hendricks (Redrawn from Pojeta, 1987). 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. The incurrent siphon brings water and suspended food into the mantle cavity, and the excurrent siphon carries out water and waste. Width of specimen is approximately 10 cm. A hinge ligament is a crucial part of the anatomical structure of a bivalve shell, i.e. When oysters are “shucked” for human consumption, the first step is to break the hinge, followed by a quick slice of the single adductor muscle. There may be as many as 50,000 described living species of bivalves (estimates of the diversity of living species range from 7500 to about 50,000; see Huber, 2010; Ponder et al., 2020) and over 42,000 described fossil species (Pojeta, 1987). Diagrams showing how mantle fusion forms siphons in many infaunal bivalves (shell removed). In some bivalves the mantle edges fuse to form siphons, which take in and expel water during suspension feeding. Fig. CaCO3 precipitated in warmer water is relatively high in 16O and depleted in 18O (“isotopically light”); CaCO3 precipitated in cooler water is relatively high in 18O and depleted in 16O (“isotopically heavy”). Right image from Cooke et al. "Clam Digs into Sand" by DailyPicksandFlicks (YouTube). In addition, the water flows through incurrent siphon ventrally and exit out of the body through excurrent dorsally to the body. The Digital Atlas of Ancient Life project is managed by the Paleontological Research Institution, Ithaca, New York. It is secreted by a part of the molluscan body known as the mantle. 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. cement their lower valve to a hard substrate (using shell material as cement) and this fixes them permanently in place. Various arrangements of hinge teeth have been named (see images below). Bivalves by definition possess two shells or valves, a "right valve" and a "left valve", that are joined by a ligament. Published caption: "δ13Cshell and δ18Oshell profiles of a Tridacna derasa shell (KTd-1). Palaeontology , 14 , 571 – 588 . In species with a large siphon the pallial line shows an embayment (the pallial sinus) which allows for connection of larger retractor muscles. Composition is group-specific, but the major categories include: 1) nacreous layers (nacre or mother-of-pearl), always innermost, composed of tablet-shaped crystals of aragonite; 2) crossed-lamellar layers, the most common type, composed of crystals of aragonite obliquely positioned at angles to one another; 3) prismatic layers of columnar-oriented crystals of calcite or aragonite; 4) foliate layers, composed of sheet-like crystals of calcite, similar to nacre but differing in composition; and 5) homogenous layers of granular calcite or aragonite, lacking obvious crystalline structure. In most bivalves, the plane of symmetry is between the two valves (that is, the shells lie laterally on the body, and the right and left sides are nearly symmetrical, whereas in most brachiopods it is across the valves (i.e., the valves lie dorsally and ventrally on the body). Notice that this pink line is perpendicular to the feint lines of the shell, which show grow within each year. The shells of most molluscans (including all gastropods and bivalves) have a thin, outer organic layer (the periostracum); a thin, innermost calcareous layer (the nacreous layer); and a thick, calcareous middle layer (the prismatic layer). In most species the valves are approximately the same size, but in some they are unequal, a condition called inequivalve. The two valves usually articulate with one another using structures known as "teeth" which are situated along the hinge line. 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 oldest point of a bivalve shell is called the beak, and the raised area around it is known as the umbo (plural umbones). Mollusk shells are made of a chalky material called calcium carbonate. Photo by Chandler Olson, courtesy of Kathryn Estes-Smargiassi, Academy of Natural Sciences of Drexel University. (2000) for more information. Drawings by Christi Sobel, based on Beerbower (1968) and Ponder et al. The periostracum may start to peel off of a shell when the shell is allowed to dry out for long periods.[1]. Mollusk shells are made of a chalky material called calcium carbonate. Specimen is from the collections of the Paleontological Research Institution, Ithaca, New York. These scars indicate the position of the muscles in the bivalve's body, as well as the overall sizes of the muscles (see additional discussion of the muscles themselves below). Species with two approximately equal-sized adductor muscles are called isomyarian; those with two unequal muscles are anisomyarian; those with only one muscle, including oysters and scallops, are monomyarian. Autobranch gills are much longer and usually folded in a W-shape, forming ascending and descending branches on each side of the foot – each pair called a demibranch (see image above). 2020. Specimen is from the collections of the Paleontological Research Institution, Ithaca, New York. Specimen is from the Quaternary of St. Mary’s County, Maryland (PRI 76728), with external shell features annotated. 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). They are generally conservative within major groups, and have historically provided a convenient means upon which to base classification schemes and the phylogenetic order. The majority of production is from natural populations although increasingly stocks are approaching or have exceeded maximum sustainable yields. The middle fold is sensory, which is particularly important for bivalves, given their lack of a head. Allmon, W. D., and P. M. Mikkelsen. The meeting of the two valves is sometimes called the commissure or shell margin. Left image by Brocken Inaglory (Wikimedia Commons; Creative Commons Attribution-Share Alike 3.0 Unported license). Dissected specimens of the clam Mercenaria mercenaria (left) and the oyster Crassostrea virginica (right). For most of their history, and especially for the past 250 million years, bivalves have been among the most taxonomically diverse and ecologically important groups of animals in the oceans. The outermost layer of the shell is organic and is called the periostracum. For comments on the text we thank Chris McRoberts. Some of the external shell features of the bivalve Mercenaria mercenaria, including the valves, commissure, and umbones. Left: Bivalve with completely unfused mantle edge. Molluscan shells are known to be composed of CaCO 3 crystals embedded in a thin organic cell-free matrix layer that is essential for controlling the shell biomineral deposition. The mantle contains pallial muscles, which retract its edge and control the flow of water into and out of the mantle cavity. (Suspension-feeding gastropods, such as turritellids, also have a crystalline style.) Some bivalves are symetrical, like clams, while others have different shaped sides, but most have a hinge that connects the two valves. 2006., Creative Commons Attribution-ShareAlike License, This page was last edited on 17 November 2020, at 21:18. The larval shell, which is frequently retained on the umbo in older individuals, is called the prodissoconch. Carnivorous bivalves (e.g., Poromyidae) have septibranch gills, comprised of a muscular septum (for sucking small prey into the mantle cavity) with small windows (ostia) containing respiratory gill filaments. The knob-like, sometimes-pointed (and earliest-formed) part of a valve is the umbo (or “beak”, pl. This allows investigation of a wide variety of topics in extinct species, from growth rates to heterochrony in evolution. Fossil and modern bivalve shells can yield information about their chronological age at death through the methods of sclerochronology. Periostracum on a live individual of the Banded Ark photographed in a tide pool at Mullaway Headland, New South Wales, Australia. Interior features of a valve of Mercenaria mercenaria. Protobranch gills are typically rather small and simple, and used primarily for respiration. This specimen shows increments of annual growth. Bivalvia. Particular strengths conferred by different crystal layer arrangements help bivalves adapt to a variety of environments. Thick fold of tissue forming two lateral lobes; it envelops the organic mass of the mollusk and secretes its shell. Pink line indicates the sampling transect." 2) in Geophysical Research Letters; image provided courtesy of Linda Ivany. For example, this is where the tentacles and eyes of scallops are located. Many species of animals have lost body parts during the course of their evolution. (2016) in PLoS ONE; Creative Commons Attribution 4.0 International license. Most bivalves have a byssus as larvae, and many lose it as adults. The areas where the muscles connected with the inner surfaces of the shell are represented by muscle scars. This feature of the internal anatomy of a bivalve is clearly indicated on the interior of the shell surface as a pallial sinus, an indentation in the pallial line. The shells of bivalves commonly wash up on beaches (often as separate valves) and along the edges of lakes, rivers, and streams. 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. Among molluscs it is primarily seen in snails and clams, i.e. Whereas the main shell layer consisting of the hard mineral aragonite (chemically CaCO 3) is mechanically very hard, on the other hand it is very susceptible to chemical corrosion.The shell skin, on the other hand, may be mechanically very weak, but it is quite unsusceptible to chemical corrosion and thus protects the shell layer below. 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. Width of specimen is approximately 10 cm. They are the reason a bivalve shell keeps on growing with its inhabitant. Access here. Width of specimen is approximately 10 cm. The shell has several layers, and is typically made of calcium carbonate precipitated out into an organic matrix. The ligament is made of a strong, flexible and elastic, fibrous, proteinaceous material which is usually pale brown, dark brown or black in color. The bivalve shell is usually composed of several layers of crystals of calcium carbonate (CaCO3). The periostracum is a thin organic coating or "skin" which is the outermost layer of the shell of many shelled animals, including molluscs and brachiopods. The periostracum provides protection against abrasion, and/or camouflage by attached sponges, algae, and other epibionts. The shells of bivalves are equal sides connected by a hinge. A virtual collection of interactive 3D models of bivalve specimens is associated with this chapter. The Noah's Ark clam Arca noae 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. Furthermore, in those animals with a distinct external ligament, the ligament is usually to the posterior side of the umbo of both valves. In life, the shell of this class of mollusks is composed of two hinged parts or valves. Left image source: "Pallbo" (Wikimedia Commons; public domain). Image by Harry Rose (flickr; Creative Commons Attribution 2.0 Generic license; label identifying periostracum added). Photographs by Warren D. Allmon. umbones). The representative combinations can be summarized from the outer to the inner surface are as follows. Sheet nacre Lenticular nacre Taylor, John D., and M. Layman. Nacreous Layer The nacreous layer of the clam shell is the innermost layer of the shell. References and Further Reading. The valve structure and soft anatomy of bivalves varies among species. A bivalve's two valves are connected by a hinge with a series of interlocking teeth and sockets that keep the valves aligned, especially when opening and closing. A shell of one species consists of the same combination and layer arrangement (layer structure) of shell morphological types. Nacreous and prismatic layer-specific matrix proteins have been reported in Pteriidae bivalves, but remain unclear in Pinnidae. The mantle itself is attached to the shell by a series of small muscles. The resilium (internal ligament) of a scallop shell. on Sketchfab. Bivalves are very common in essentially all aquatic locales, including saltwater, brackish water, and freshwater. Specimen is from the Quaternary of St. Mary’s County, Maryland (PRI 76728), with external shell features annotated. The cellular epithelium that lines the internal surface of both halves of the shell has two layers, the inner layer is called the mantle. Photograph by "Shellnut" (Wikimedia Commons; Creative Commons Attribution-Share Alike 3.0 Unported license). oysters) or both calcite and aragonite, usually with the aragonite forming an inner layer, as is the case with the Pteriida which have this layer in the form of nacre or mother of pearl. The bivalves, like clams, are specialists at this type of feeding. Veliger. Siphons in several burrowing infaunal modern species. bivalves In bivalve: The shell The periostracum, the outermost organic layer, is secreted by the inner surface of the outer mantle fold at the mantle margin. Scientists believe that early ancestors of bivalves had heads, like their close relatives the snails, but that bivalves have lost the feature. Cross-section through the shell of Cucullaea raea (Superfamily Arcoidea; Eocene, Seymour Island, Antarctica). (Drawing by Christi Sobel; modified from Barnes, 1980.). In fact, the only major feature that all mollusks possess is a sheet of tissue covering the body called the mantle, which (in most) covers the viscera and gills, and secretes the shell. Bivalves are members of the phylum Mollusca, which also includes cephalopods (squids, octopuses, nautiloids, and ammonoids), gastropods (snails, slugs, and nudibranchs), scaphopods (tusk shells), polyplacophorans (chitons), the extinct rostroconchs, and three “minor” groups, the monoplacophorans, Caudofoveata, and Solenogastres (the latter two are generally collectively called “aplacophorans”). Usually brown or black in color, the ligament includes two distinct components: an inner portion that is partly mineralized with aragonite fibers, and an outer organic region composed of fibrous glycoprotein (conchiolin). Right: Ruditapes philippinarum with separate incurrent and excurrent siphons extended; image by Stefan Didam (Wikimedia Commons; Creative Commons Attribution-Share Alike 3.0 Unported license). Without being able to view these organs, however, determining anterior and posterior can be rather more difficult. A few groups of bivalves are active swimmers like the scallops; many bivalves live buried in soft sediments (are infaunal) and can actively move around using their muscular foot; some bivalves such as blue mussels attach themselves to hard substrates using a byssus; other groups of bivalves (such as oysters, thorny oysters, jewel boxes, kitten's paws, jingle shells, etc.) The shell ends up with three layers that support and protect the clam inside the shell. Those rings are also called umbones. This technique uses the temperature-dependence of the ratio between two stable (i.e., non-radioactive) isotopes of oxygen when it is incorporated into the calcium carbonate (CaCO3) of the bivalve shell. Drawing by Christi Sobel. The age of bivalve molluscs can be estimated in several ways. "Cockle Jumping" by Fritz Rivera (YouTube). Image above: Examples of different types of modern and fossil bivalves. The innermost shell layer, which is often shiny and iridescent, is called mother of pearl. When a bivalve dies, its adductor muscle(s) relax and the resilium pushes the valves open. Specimen is from the collections of the Paleontological Research Institution, Ithaca, New York. Within the shell is a fleshy layer of tissue called the mantle; there is a cavity (the mantle cavity) between the mantle and the body wall proper. The byssus is a set of elastic or calcified fibers secreted by a gland in the foot, used to anchor the bivalve to a hard substrate. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. 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. Some of the external shell features of the bivalve Mercenaria mercenaria, including the lunule, escutcheon, and umbo. The lunule and escutcheon are impressed external features anterior or posterior, respectively, to the umbo. There are many kinds of combinations of the types in shell structure of bivalve mollusks. Torreites sanchezi, the prehistoric bivalve, formed sedimentary bands along its shell—similar to the rings we use to age trees. Insoluble layers in some bivalve shells resisted the chemical attack of shell-boring gastropods. Browse. Class Bivalvia: Introduction and Morphology ←– 1. • in middle and inner layers of shells • strongest form of microstructure in tension, compression and bending. Along with the hinge teeth, the ligament forms the hinge system that holds the two shell valves together. Four types of bivalve gills: protobranch, filibranch, eulamellibranch, and septibranch (the latter three are all forms of autobranch gills). One or two adductor muscles hold the shell tightly closed when they contract. Bivalves are also of major economic importance to humans, as sources of food and other products, and as damaging invasive species. The two siphonal tubes can be separate or fused together. Bivalves by definition possess two shells or valves, a "right valve" and a "left valve", that are joined by a lig… Some of the various hinge tooth arrangements are as follows:[4]. The mantle is a complex sheet of tissue consisting of soft exterior membranes that secrete the shell and usually form a cavity – the mantle cavity – that encloses the gills and other organs, and which produces and enlarges the shell by secreting new shell material at the free edge. 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). 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. Shell. By taking samples across the shell, a profile of isotopic measurements can determine how many annual temperature cycles the bivalve lived through, and therefore how many years old it was when it died. The mantle covers the body and secretes the shell. Bivalves have two shells or valves connected by a hinge with hinge teeth.They are made of a calcareous mineral, calcite or aragonite.The valves are covered by a periostracum, which is an organic horny substance. The two halves of the shell are secreted by the two lobes of the body wall (the mantle), and consist of layers of calcium carbonatecrystals embedded in a protein matrix. If you hold a Mercenaria with both valves with the umbo pointing away from you and the shell margins down, the valve on the right is conventionally referred to as the right valve, and the valve on the left is the left valve. Farrow , G. E. , 1972 . A bivalve shell is part of the body, the exoskeleton or shell, of a bivalve mollusk. Image from Ivany et al., 2011 (fig. For assistance with illustrations, we are grateful to Alexandra Allmon, James Crampton, Katy Estes-Smargiassi, Liz Harper, Jon Hendricks, Carole Hickman, Linda Ivany, Claudia Johnson, Carlie Pietsch, Elizabeth Petsios, Gary Rosenberg, Leslie Skibinski, Christi Sobel, Chelsea Steffes, Jennifer Tegan, Vicky Wang, and Alex Zimmerman. Dorsally (toward the hinge) the mantle is fused to the visceral mass and forms its integument; ventrally (toward the commissure) the mantle surrounds the open space of the mantle cavity. Mercenaria is isomyarian; Crassostrea is monomyarian. The mantle lobes secrete the valves, and the mantle crest creates the other parts. In some groups of cemented bivalves the lower or cemented valve is the left valve, in others it is the right valve. 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 scallops, for example, do not have siphons). The shiny layer in some bivalve mollusks is known as mother-of-pearl. This matrix contains several macromolecules, including polysaccharides (e.g., chitin), proteins and glycoproteins that are present both in inter- and intracrystalline locations –. Common names of representatives: clams, scallops, oysters, mussels. by Digital Atlas of Ancient Life This and two other associated specimens were first described in 1848, and are the first specimens of fossil mollusks—and apparently the first fossils of any sort—found to have preserved most of their soft anatomy. The valves are composed of three layers, similar to mollusk shells; the outer layer is composed of proteins, the middle layer is comprised of calcium carbonate, and the inner layer consists of a mixture of calcium and protein. 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. Specimen is from the collections of the Paleontological Research Institution, Ithaca, New York. Width of specimen is approximately 10 cm. The shell is made of calcium carbonate and is secreted by the mantle (soft body wall). One of the other larval stages common in molluscs. Development of this project was supported by the National Science Foundation. As their name implies, bivalves have two shells that are called valves. On an empty shell these are seen as the pallial line which runs from the … The periostracum, the outermost organic layer, is secreted by the inner surface of the outer mantle fold at the mantle margin. Periostracum is an integral part of the shell, and it forms as the shell forms, along with … in gastropods and bivalves, but it is also found in cephalopods such as Allonautilus scrobiculatus. The adductor muscles are what allow the bivalve to close the shell tightly. Interactive 3D model of a fossil specimen of the bivalve Mercenaria mercenaria from the Quaternary of St. Mary’s County, Maryland (PRI 76728), with external shell features annotated. Gill structure is the morphological basis for dividing bivalves into two major groups: the protobranchs and autobranchs (see section on Bivalve Phylogeny and Classification). Palaeontology 15.7 (1972): 5. The bivalve shell is secreted by various parts of the mantle. 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, 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. Magnified view shows annual growth patterns (one year spanned by black arrow); the dark bands identified by white arrows represent the periods of slowest growth, in this case, summer. Warren D. Allmon (Paleontological Research Institution, Ithaca, New York) and Paula M. Mikkelsen (Integrative Research Center, Field Museum of Natural History, Chicago, Illinois). Autobranch gills are usually used for suspension feeding in addition to respiration. The most accurate but most time-consuming method is the microscopic examination of sections through the outer prismatic layer of the shell. This forms the familiar coloured layer on the shell. In most autobranch species, it contains a crystalline style – a proteinaceous rod-shaped structure that rotates, stirring and macerating the stomach contents and releasing digestive enzymes. The mantle also usually forms the siphons, usually through fusion of the edges of the mantle, although some species make siphons from mucus tubes. The bivalve shell is usually composed of several layers of crystals of calcium carbonate (CaCO 3). Right image source: "shellnut" (Wikimedia Commons; Creative Commons Attribution-Share Alike 3.0 Unported license), Creative Commons Attribution-Share Alike 3.0 Unported license, Creative Commons Attribution 4.0 International license, Creative Commons Attribution 2.0 Generic license. The pallial line represents the attachment of the soft tissues to the interior of the shell; an embayment (pallial sinus) is present posteriorly in siphon-bearing bivalves, into which the siphons contract when the shell is closed. Remarkably, the mantles of some freshwater mussels are modified to resemble small fish. Growth lines parallel to the shell margins mark increments of growth of the shell, similar to tree rings. Evolutionary History of Bivalves– 4. In: The Digital Encyclopedia of Ancient Life. An elastic ligament connects the two valves, allowing them to spring open when the adductor muscles relax. The shell microstructure of freshwater bivalve Anodonta cygnea was observed from the ventral margin toward the intrapallial regions by scanning electronic microscopy during the spring/summer period. The bay scallop Argopecten irradians. The side of the shell with the hinge is dorsal and the opposite side (i.e., margin of shell growth) is ventral. Using more than one of these methods should increase the accuracy of the result. 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.
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