Dictionary:

bivalve

  (bī'vălv')

A mollusk, such as an oyster or a clam, that has a shell consisting of two hinged valves.

1. Having a shell consisting of two hinged valves.
2. Consisting of two similar separable parts.

(Bivalves)

Phylum: Mollusca

Class: Bivalvia

Number of families: 105

Thumbnail description
Bilaterally symmetrical mollusks, with a reduced head and typically two external shell valves, many of which are commercially important for human consumption and pearl production; some have major impacts on the world economy and environment

Evolution and systematics

The evolutionary history of bivalves is represented by an extensive fossil record. It begins in the Cambrian period (544–505 million years ago [mya]) with a laterally compressed stenothecid monoplacophoran (primitive single-shelled marine mollusk) as the most likely immediate ancestor. By the Middle Ordovician period (about 460 mya), recognizable members of all modern subclasses had appeared. Bivalves formed important components of marine communities since they first diversified, especially in shallow-water marine sediments, but also in the intertidal zone, the deep sea, and freshwater habitats. In the Cretaceous period (145–65 mya), epibenthic rudist bivalves formed tropical reef-like structures similar to modern coral reefs. Rudists were massive extinct cemented bivalves that had two different-sized shell valves.

Most authors think that scaphopods and bivalves are closely related taxa, based on the configuration of the nervous system, lateral expansion of the mantle, and elongation of the foot for burrowing. Within the class of bivalves itself, the widely variable shell characteristics (shape, sculpture, color, hinge teeth) have been historically and consistently used in identification and classification at all levels. Various other evolutionary schemes have been based primarily on single organ systems, especially the ligament, stomach, digestive tract, and gills. Elements of each of these continue to be important in modern comprehensive phylogenetic analyses.

Current classification schemes recognize five subclasses of bivalves. The Protobranchia (e.g., families Nuculidae, Sole-myidae) are presumably the most primitive, using simple gills solely for respiration and palp proboscides (enlarged labial palps, normally used for sorting food particles) for collecting food from the sediment surface. The Pteriomorphia (e.g., Mytilidae, Pteriidae) include many of the most familiar bivalves, all of which share an epibenthic habitat (byssate or cemented), an unfused mantle edge, and a reduced foot. Byssate bivalves are attached to their substrate by a byssus (bundle of elastic collagen-rich threads) secreted by the foot. The Pale-oheterodonta (e.g., Unionidae, Margaritiferidae) include the freshwater mussels, with their specialized glochidia larvae. Heterodonta (e.g., Veneridae, Donacidae) is the most species-rich and most widely distributed subclass, containing the classic burrowing clams with well-developed hinges, siphons, and active feet. The Anomalodesmata (e.g., Pandoridae, Clavagellidae) include the most unusual and most specialized bivalves, featuring modified ctenidia (gills), an edentulous hinge, fused mantle margins, and hermaphroditism. The evolutionary relationships, and thus the accepted classification, among and within bivalve subgroups continue to be revised through the application of phylogenetic analysis. These analyses makes use of a wide range of morphological (especially anatomical) and molecular characters.

Physical characteristics

Most bivalved mollusks have laterally compressed bodies and a shell consisting of two calcareous valves hinged dorsally by interlocking teeth and an elastic ligament. The shell valves are usually similar in size, sculpture, and color, and retain a permanent record of shell growth in their concentric layers. Growth can be traced from the first stage (prodissoconch) at the umbo (rounded or pointed extremity) to the latest stage at the ventral margin. The shells of many bivalve species are rather bland, although some groups (e.g., Pectinidae, Spondylidae) show characteristic colors or color patterns. Lining each valve is mantle tissue that secretes the aragonitic or calcitic shell, including its organic outer layer, or periostracum, and the interior aragonitic nacre (mother-of-pearl) present in many species. Between the two shell-mantle layers, an internal mantle (or pallial) cavity contains the ctenidia and visceral mass (containing digestive and reproductive organs), the latter ending in a muscular extensible foot. The foot is usually equipped with a gland that secretes the byssus for attachment to hard substrates.

The posterior mantle edge of many bivalves is fused into incurrent and excurrent siphons that direct water in and out of the mantle cavity for respiration, feeding, and discharge of waste and reproductive products. In most bivalves, water flows in and out of the posterior end; some, however, are secondarily modified for an anterior-posterior flow. Paired adductor muscles connect the inner valve surfaces, enabling the bivalve to close; relaxation of these muscles allows the ligament to open the valves. The head of bivalves is reduced, so that the cephalic eyes, tentacles, and radular teeth typical of most mollusks are absent. The stomach is one of the most complex organs of bivalves, comprising various ciliated sorting areas and ridges as well as the crystalline style, which is an enzymatic rod that rotates against a hardened gastric shield lining the dorsal stomach surface to facilitate extracellular digestion. The bivalve nervous system consists simply of three pairs of ganglia connected by nerve connectives. The circulatory system is equipped with a three-chambered heart, simple vessels, and open hemocoels. The hemostatic pressure of the fluid in these blood cavities is responsible for the expansion and retraction of many bivalve structures, particularly the foot, siphons, and tentacles.

Distribution

Bivalves are found worldwide, including aquatic habitats at high and low latitudes. All are aquatic, requiring water for reproductive processes, respiration, and typically for feeding. One supratidal species (Enigmonia) lives in the tidal spray on mangrove leaves or seawalls in Australia, achieving the most nearly "terrestrial" life mode of any bivalve. Several independent lineages of bivalves have invaded freshwater habitats, where they have diversified to produce one of the most endemic faunas as well as some of the most important biofoulers among invertebrates. Many species have been introduced to non-native locations accidentally or intentionally, the latter often in commercial aquaculture for human use or consumption.

Habitat

All bivalves are aquatic, requiring water for reproductive processes, respiration, and typically for feeding. They range in depth from the intertidal zone to the deep sea; one supratidal species (Enigmonia) lives in the tidal spray on mangrove leaves or seawalls in Australia, achieving the most "terrestrial" life-mode of any bivalve. Several independent lineages of bivalves have invaded freshwater habitats, where they have diversified to produce one of the most endemic faunas as well as some of the most important biofoulers among invertebrates. Most bivalves are free-living, either epibenthic (e.g., Pectinidae) or infaunal, burrowing into sand or mud with the muscular foot (e.g., Veneridae, Donacidae). Others are cemented by one valve (e.g., Ostreidae) or permanently attached by byssal threads (e.g., Mytilidae, Dreissenidae). Specialized members of the class burrow into rock or wood (e.g., Pholadidae, Teredinidae), using one or a combination of chemical and mechanical methods. Commensal and parasitic forms (e.g., Galeommatoidea) live associated with, attached to, or within the bodies of other invertebrates.

The habitat of a bivalve is often reflected in the form of its shell. Nestlers and cementing bivalves frequently take the shape of their substrates. Individuals in calm waters often have more delicate or leaflike shell sculpture than their counterparts in fast-flowing currents.

Behavior

Most bivalves are relatively sedentary organisms; however, many are capable of considerable levels of activity. The former class name Pelecypoda means "hatchet-foot," referring to the laterally compressed foot typically used for burrowing in sand or mud. Bivalves move downward into the substrate by extending the foot into the sediment, anchoring the foot by expanding its tip, and pulling the shell downward toward the anchor by muscular action. Byssally attached bivalves (e.g., Mytilidae, Dreissenidae) can break their byssal threads to relocate, and use the foot to move across a hard substrate in a sequence similar to that used for burrowing. They then produce a new byssus for reattachment. Other bivalves can actively swim by waving the foot or tentacles (e.g., Solenidae, Limidae) or by jet-propelling themselves by rapidly clapping the shell valves together (e.g., Pectinidae).

Although the reduction of the bivalve head has eliminated cephalic eyes and other sense organs, many bivalves (e.g., Galeommatidae, Pectinidae) have tentacles and/or photoreceptors along the mantle margins or in the vicinity of the siphons. These structures allow bivalves to respond to changes in light intensity by retracting the siphons and closing the valves. More sophisticated eyes, equipped with retina and lens, are found in several families of epibenthic bivalves (e.g., Cardiidae, Pectinidae).

Feeding ecology and diet

Most bivalves are suspension feeders, filtering food particles from the water column. The expansive ctenidia, in addition to functioning in gas exchange, are the main feeding organs. Their cilia-covered surface collects and sorts particles from currents flowing through the mantle cavity, conveying them to marginal food grooves, then anteriorly toward the labial palps flanking the mouth.

The most primitive bivalves were probably deposit feeders, collecting detritus from the sediment surface. This method is still used by living Nuculoida, using specialized structures known as palp proboscides. Other specialists feed by direct absorption of dissolved organic matter, or DOM (e.g., Sole-myidae), or by active capture of small crustaceans and worms through use of a raptorial incurrent siphon (e.g., Cuspidariidae). Others possess symbiotic organisms, supplementing their energy reserves with by-products from their inhabitants. Examples of symbiotic relationships include chemoautotrophic bacteria in Solemyidae and Lucinidae that facilitate habitation of anoxic muds, and zooxanthellae in Cardiidae that provide photosynthetic products in shallow eutrophic waters. The wood-eating Teredinidae are enabled by symbiotic cellulolytic (cellulose-digesting) bacteria that are stored in pouches along the bivalve's esophagus.

Reproductive biology

Bivalves are usually dioecious, with eggs and sperm shed into the water column where external fertilization occurs. Some species are consecutive or simultaneous hermaphrodites, with protandry (male phase preceding female phase) most common. Internal fertilization has been recorded for a few groups (Galeommatoidea, Teredinidae), using tentacles or siphons as copulatory organs. External sexual dimorphism is evident in only a few bivalves (Carditidae, Unionoidea).

Larval development is plesiomorphically planktotrophic, with free-swimming veliger larvae that feed in the plankton for a few weeks. Some bivalves brood their larvae in the supra-branchial chamber or in specialized brood pouches, releasing late-stage veligers or direct-developed juveniles through the excurrent opening. Settlement of larvae is time-dependent but is often delayed in the absence of suitable habitat. Freshwater mussels (Unionoidea) are characterized by specialized glochidia larvae that require attachment to the gills or fins of fish to complete their life cycles. Many of these larvae have specialized hooks for attachment, and some bivalve-fish relationships are species-specific. Many unionoideans possess specialized flaps on the mantle edge that they wave in the water column to attract the attention of the required fish; some of these "lures" mimic small fish or the invertebrate prey of the fish host.

Conservation status

Freshwater pearl mussels (Unionoida) are among the world's most gravely threatened fauna. In eastern North America, the group's center of evolutionary diversification, 35% of the 297 native species are presumed extinct, with another 69% formally listed as endangered or threatened. Human-introduced pollution, especially from agriculture and industry, as well as other forms of habitat alteration (dredging, damming) have been blamed for much of the decline. Such factors can adversely impact not only the mussels themselves, but also the obligate fish hosts of their larvae, potentially resulting in population declines. Introduced species, especially the Asian clam (Corbicula) and two species of zebra mussels (Dreissena) have further impacted unionoid populations through competition for space and food resources. One-hundred ninety-five species of bivalves have been placed on the 2002 IUCN Red List; all but 10 of these are freshwater pearl mussels. Twenty-nine species of freshwater pearl mussels are protected under CITES.

Marine bivalves are much less affected by human activities. There are no known recent extinctions in this group, and none are currently listed as threatened or endangered. Many species are partially protected by local and national laws regulating the commercial and private harvesting of shellfish. The giant clams (Tridacna, Hippopus; Tridacnidae) are the single marine group regulated internationally, as a result of overcollecting; eight species are included on the 2002 IUCN Red List, and the entire family is protected under CITES.

Significance to humans

Many kinds of bivalves, especially clams, cockles, mussels, oysters, and scallops, have served as important food sources for fish, vertebrates, other invertebrates, and humans. Aboriginal populations of many cultures have left evidence of eating bivalves in their kitchen middens (mounds or deposits of refuse from meals). Recent practices rely both on harvesting wild populations and on aquaculture in either open or closed aquatic systems. Members of the marine Pteriidae and freshwater Unionoidea have been sources of natural pearls and mother-of-pearl shell for centuries. Since the 1950s, cultured pearls have increased the quantity and quality of this biological gem through aquaculture and husbandry. The Japanese perfected the process of culturing pearls using pearl oysters of the species Pinctada fucata (Gould, 1850).

Bivalves have also had negative impacts on human activities. Because most bivalves are filter feeders, they are frequent vectors of human disease related to the concentration of bacteria, viruses, pesticides, industrial wastes, toxic metals, and petroleum derivatives from the water column. Shipworms (Teredinidae) have a long historical record of bioerosion of such human-made wooden structures as ships and docks. Species introduced in freshwaters of the United States, such as the biofouling zebra mussel Dreissena, have required millions of dollars to repair clogged water treatment plants and irrigation systems. Damage caused by such species to the environment, in terms of altered habitat and impact on native species, is irreversible; their spread has been largely unstoppable.

Species accounts

Resources

Gosling, Elizabeth, ed. The Mussel Mytilus: Ecology, Physiology, Genetics and Culture. Amsterdam: Elsevier Science Publishers, 1992.

Harper, E. M., J. D. Taylor, and J. A. Crame, eds. The Evolutionary Biology of the Bivalvia. London: The Geological Society, 2000.

Kennedy, Victor S., Roger I. E. Newell, and Albert F. Ebel, eds. The Eastern Oyster: Crassostrea virginica. College Park: Maryland Sea Grant College, 1996.

Landman, Neil H., Paula M. Mikkelsen, Rüdiger Bieler, and Bennett Bronson. Pearls: A Natural History. New York: Harry N. Abrams, 2001.

Morton, Brian. "The Evolutionary History of the Bivalvia." In Origin and Evolutionary Radiation of the Mollusca, edited by John D. Taylor. Oxford, U.K.: Oxford University Press, 1996.

Morton, Brian, Robert S. Prezant, and Barry Wilson. "Class Bivalvia." In Mollusca: The Southern Synthesis, Fauna of Australia, Vol. 5, Part A. Melbourne, Australia: CSIRO Publishing, 1998.

Nalepa, Thomas F., and Donald W. Schloesser, eds. Zebra Mussels: Biology, Impacts, and Control. Boca Raton, FL: Lewis Publishers, 1993.

Turner, Ruth D. A Survey and Illustrated Catalogue of the Teredinidae. Cambridge, MA: Museum of Comparative Zoology, Harvard University, 1966.

Adamkewicz, S. Laura, and Miroslav G. Harasewych. "Systematics and Biogeography of the Genus Donax (Bivalvia: Donacidae) in Eastern North America." American Malacological Bulletin 13, no. 1–2 (1996): 97–103.

Boss, Kenneth J., and Ruth D. Turner. "The Giant White Clam from the Galapagos Rift, Calyptogena magnifica species novum." Malacologia 20, no. 1 (1980): 161–194.

Carlos, A. A., B. K. Baillie, and T. Maruyama. "Diversity of Dinoflagellate Symbionts (Zooxanthellae) in a Host Individual." Marine Ecology Progress Series 195 (2000): 93–100.

Carlton, James T. "Introduced Marine and Estuarine Mollusks of North America: An End-of-the-20th-Century Perspective." Journal of Shellfish Research 11, no. 2 (1992): 489–505.

Giacobbe, Salvatore. "Epibiontic Mollusc Communities on Pinna nobilis L. (Bivalvia, Mollusca)." Journal of Natural History 36 (2002): 1385–1396.

Giribet, Gonzalo, and Ward Wheeler. "On Bivalve Phylogeny: A High-Level Analysis of the Bivalvia (Mollusca) Based on Combined Morphology and DNA Sequence Data." Invertebrate Biology 121, no. 4 (2002): 271–324.

Johnson, Claudia C. "The Rise and Fall of Rudist Reefs." American Scientist 90, no. 2 (2002): 148–153.

Mikkelsen, Paula M., and Rüdiger Bieler. "Biology and Comparative Anatomy of Divariscintilla yoyo and D. troglodytes, Two New Species of Galeommatidae (Bivalvia) from Stomatopod Burrows in Eastern Florida." Malacologia 31, no. 1 (1989): 175–195.

Morton, Brian. "Biology and Functional Morphology of the Watering Pot Shell Brechites vaginiferus (Bivalvia: Anomalodesmata: Clavagelloidea)." Journal of Zoology (London) 257 (2002): 545–562.

Rosewater, Joseph. "The Family Tridacnidae in the Indo-Pacific." Indo-Pacific Mollusca 1, no. 6 (1965): 347–396.

Smith, Brian J. "Revision of the Recent Species of the Family Clavagellidae (Mollusca, Bivalvia)." Journal of the Malacological Society of Australia 3, no. 3–4 (1976): 187–209.

Wilson, James G. "Population Dynamics and Energy Budget for a Population of Donax variabilis (Say) on an Exposed South Carolina Beach." Journal of Experimental Marine Biology and Ecology 239, no. 1 (1999): 61–83.

American Malacological Society. Web site: <

Conchologists of America. Web site: <

Freshwater Mollusk Conservation Society. Web site: <

"NS&T Mussel Watch Project." National Status and Trends Program, National Ocean Service, National Oceanographic and Atmospheric Administration. [11 Aug. 2003]. . "Zebra Mussel Information." United States Geological Survey (USGS). 9 July 2003 [11 Aug. 2003]. .

[Article by: Paula M. Mikkelsen, PhD]

One of the five classes in the phylum Mollusca, sometimes known as Pelecypoda. All bivalves are aquatic, living at all depths of the sea and in brackish and fresh waters. With about 25,000 living species, Bivalvia is second to class Gastropoda (over 74,000) in molluscan species diversity. However, the total biomass of bivalves is much greater, and certain bivalve species are numerically dominant in many benthic ecosystems. The most primitive bivalves are infaunal, burrowing into soft sediments, but many families are epifaunal, attached to rocks or shells or residing on the sediment surface. Bivalves are well represented in the fossil record from the early Paleozoic because of their calcareous shells.

In general, bivalves are bilaterally symmetrical and laterally compressed. They have a fleshy mantle that secretes the shell enclosing the body (see illustration). The mouth is located anteriorly in bivalves; and in the Lamellibranchiata, the largest subclass, the mouth is flanked by paired labial palps that act to sort food prior to ingestion. Sensory organs are located on the outer mantle margin that has the closest contact with the environment. Frequently these sensory organs are borne on tentacles, and they are sensitive to tactile and chemical stimuli. Certain species of scallops have highly developed light-sensing organs or “eyes” on their mantle tentacles.

Bivalve anatomy.

The shell consists of two valves with a noncalcified connecting ligament holding the valves together at a hinge plate. The shell layers consist of an outer horny periostracum (protective layer) that can be either absent or eroded in some species, a middle prismatic layer consisting of crystalline calcium carbonate, and an inner lamellar or nacreous layer. In some families such as the Mytilidae (mussels) or the Pteriidae (winged or pearl oysters), the nacreous layer can exhibit a beautiful iridescent sheen, whereas in most bivalves the inner layer is smooth but with a chalky appearance. Hinge ligament tension holds the valves in a gaping position, with valve closure effected by adductor muscles.

The ciliated molluscan gills, properly called ctenidia, are enlarged in the subclass Lamellibranchiata and occupy a substantial portion of the mantle cavity. The ctenidia consist of layered filaments which function primarily to pump water into the mantle cavity and to filter particulate food from the incurrent water stream. The ctenidia of bivalves of the subclass Protobranchia also serve to pump water, but they are smaller and less developed than in the lamellibranchs and do not serve to filter food particles. Protobranch bivalves are deposit feeders that gather food by extending thin muscular palp proboscides to probe soft sediments and entrap organic detrital particles. Bivalves of the subclass Septibranchia (sometimes called Anomalodesmata) have highly modified ctenidia that lack filaments. A septum divides the mantle cavity into dorsal and ventral chambers, and water is pumped by muscular contraction of the septum wall.

Some bivalves have a foot for locomotion. If present, the foot can be extended from the shell by blood pressure and dilated to act as an external anchor while movement is effected by contraction of retractor muscles. Some bivalves of the family Pectinidae (scallops) lack a foot but are highly active swimmers through clapping their valves and jetting water through orifices (openings) near the hinge. Some bivalves, such as oysters and giant clams, are sedentary and lack a foot as adults.

Bivalves exhibit a wide range of reproductive strategies. Most bivalves are dioecious or have separate sexes, while others exhibit various forms of hermaphrodism. For example, as mature adults, scallops carry both eggs and sperm, while oysters exhibit protandric hermaphrodism in which the oysters first develop as males and in subsequent years change sex to develop ovaries. Most species of bivalves shed eggs and sperm directly into the water, where fertilization occurs; in others, eggs may be held in a brood chamber, where they are fertilized by sperm in incurrent water, and released as well-developed larvae into the water. Most bivalves go through several planktonic stages prior to settlement and metamorphosis to their benthic form.

Many species of bivalves are actively farmed either for human consumption of the meats or for shell products. Most of the gem-quality pearls sold in the world originate from farmed pearl oysters of the genus Pinctada in Japan, Australia, and islands of the tropical Pacific. Fresh-water pearls are produced from fresh-water mussels in the United States, China, and Japan. Other species of bivalves are of economic concern by virtue of being pest organisms or biological invaders.

The fossil record of the Bivalvia can be traced to the Lower Cambrian Fordilla. The Ordovician was a major period of bivalve speciation, but throughout the Paleozoic the Bivalvia remained second to bivalves of the phylum Brachiopoda in species diversity and abundance. During the Mesozoic Era, the brachiopods declined in importance. It is probable that diverse adaptations of the Bivalvia to avoid predatory gastropods, arthropods, and fish evolving during the Mesozoic were a major factor in the replacement of the more exposed brachiopods as the dominant bivalves. The evolutionary radiation occurring during the Mesozoic includes the emergence of many species of bivalves that bore into rocks, hard corals, and wood. The Mesozoic emergent family Ostreidae, which includes oysters, remains to the present. The transition from the Mesozoic to Cenozoic began with the extinction of many ancient families and the emergence of several modern families. See also Lamellibranchia; Mollusca; Protobranchia; Septibranchia.

Any soft-bodied mollusk, such as a clam, scallop, oyster or mussel, that has two shells hinged together by a strong muscle.

(click to enlarge)
Internal structure of a clam. A ligament hinges the shell's two halves (valves) open, and the … (credit: © Merriam-Webster Inc.)

Any member of the mollusk class Bivalvia, or Pelecypoda, characterized by having a two-halved (valved) shell. Clams, cockles, mussels, oysters, scallops, and shipworms are bivalves. Most are completely enclosed by the shell, the two valves of which are joined by an elastic ligament, and by two sheets of tissue called the mantle. Bivalves have no head. They feed on phytoplankton by pumping water across the gills and trapping food particles that are then moved to the mouth. Bivalves are found in most parts of the ocean from the intertidal zone to abyssal depths.

For more information on bivalve, visit Britannica.com.

Shellfish members of the Class Bivalvia. Molluscs enclosed between two shells which are hinged together. Includes oysters, clams, arkshells, mussels. Called also lamellibranch.

Bivalves Fossil range: Cambrian - Recent
"Acephala" from Ernst Haeckel's Kunstformen der Natur, 1904
Scientific classification
Kingdom: Animalia Phylum: Mollusca Class: Bivalvia Linnaeus, 1758
Subclasses
Anomalosdesmata Cryptodonta Heterodonta Paleoheterodonta Palaeotaxodonta Pteriomorphia and see text

Mussels in the intertidal zone in Cornwall, England.

Fossil gastropod and attached mytilid bivalves on a Jurassic limestone bedding plane in southern Israel.

Bivalves are mollusks belonging to the class Bivalvia. They typically have two-part shells, with both valves being symmetrical along the hinge line. The class has 30,000 species, including scallops, clams, oysters and mussels. Other names for the class include Bivalva, Pelecypoda, and Lamellibranchia.

Bivalves are exclusively aquatic; they include both marine and freshwater forms.

Bivalves lack a radula and feed by siphoning and filtering large particles from water. Some bivalves are epifaunal: that is, they attach themselves to surfaces in the water, by means of a byssus or organic cementation. Others are infaunal: they bury themselves in sand or other sediments. These forms typically have a strong digging foot. Some bivalves, such as scallops, can swim.

Systematics

The systematic layout presented here is according to Newell's 1965 classification based on hinge teeth morphology. There exists no robust phylogeny, and due to the plethora of fossil lineages, DNA sequence data is only of limited use should the subclasses turn out to be paraphyletic. The monophyly of the Anomalosdesmata is especially disputed, but this is of less consequence as that group does not include higher-level prehistoric taxa.

Subclass Palaeotaxodonta

• Nuculoida

Subclass Cryptodonta

• †Praecardioida
• Solemyoida

Subclass Pteriomorphia (oysters, mussels, etc)

• Arcoida
• †Cyrtodontoida
• Mytiloida
• Ostreoida - formerly included in Pterioida
• †Praecardioida
• Pterioida

Subclass Paleoheterodonta

• †Trigonioida
• Unionoida (typical freshwater mussels)
• †Modiomorpha

Subclass Heterodonta (typical clams, cockles, rudists, etc)

• †Cycloconchidae
• †Hippuritoida
• †Lyrodesmatidae
• Myoida
• †Redoniidae
• Veneroida

Subclass Anomalosdesmata

• Pholadomyoida

There also exists an alternative systematic scheme according to gill morphology (Franc 1960). This distinguishes between Protobranchia, Filibranchia, and Eulamellibranchia. The first corresponds to Newell's Palaeotaxodonta + Cryptodonta, the second to his Pteriomorphia, and the last contains all other groups. In addition, Franc separated the Septibranchia from his eulamellibranchs, but this would seem to make the latter paraphyletic.

Anatomy

Bivalves are filter-feeders which extract organic matter from the water in which they live. They have an open circulatory system that bathes the organs in hemolymph. Nephridia remove the waste material. Bivalves are laterally combined and have a shell composed of two valves. The valved shell makes them superficially similar to brachiopods, but the construction of the shell is completely different in the two groups: in brachiopods, the two valves are on the upper and lower surfaces of the body, while in bivalves, they are on the left and right sides.

Drawing of oyster anatomy

Photo of anatomy of Crassostrea gigas

Drawing of anatomy of Freshwater pearl mussel Margaritifera margaritifera

Internal anatomy of the valve

Pre-history

Giant clam, Tridacna gigas.

Bivalves appeared late in the Cambrian explosion and came to dominate over brachiopods during the Palaeozoic; indeed, by the end-Permian extinction, bivalves were undergoing a huge radiation in numbers while brachiopods (along with ~95% of all species) were devastated.

This raises two questions: how did the bivalves come to challenge the brachiopoda niche before the extinction event, and how did the bivalves escape the fate of extinction? Although inevitable biases exist in the fossil record and our documentation thereof, bivalves essentially appear to be better adapted to aquatic life. Far more sophisticated than the brachiopods, bivalves use an energetically-efficient ligament-muscle system for opening valves, and thus require less food to subsist. Furthermore, their ability to burrow allows for evasion of predators: buried bivalves feed by extending a siphon to the surface (indicated by the presence of a palial sinus, the size of which is proportional to the burrowing depth, and represented by their hinge teeth). Some bivalves can bore directly into rock and wood, a process known as bioerosion. Additionally, some bivalves became much more mobile: many of the scallops eject water vigorously by suddenly closing the valves, and this created rapid propulsion and permits the scallop to evade slower predators such as starfish.

With such a wide range of adaptations it is unsurprising that the shapes of bivalve shells vary greatly - some are rounded and globular, others are flattened and plate-like, while still others, such as the razor shell Ensis, have become greatly elongated in order to aid burrowing. The shipworms of the family Teredinidae have greatly elongated bodies, but the shell valves are much reduced and restricted to the anterior end of the body, where they function as burrowing organs, allowing the animal to dig tunnels through wood.

References

• Franc, A. (1960): Classe de Bivalves. In: Grassé, Pierre-Paul: Traite de Zoologie 5/II.
• Newell, N.D. (1969): [Bivalvia systematics]. In: Moore, R.C.: Treatise on Invertebrate Paleontology Part N.
• Jay A. Schneider (Nov 2001). "Bivalve Systematics During the 20th Century" 75 (6): 1119–1127. DOI:10.1666/0022-3360(2001)075%3C1119:BSDTC%3E2.0.CO;2. 

External links

Wikimedia Commons has media related to:

Bivalvia

• Museum of Paleontology - Palaeontology from the University of California, Berkeley
• Bioerosion website at The College of Wooster

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Dansk (Danish)
n. - toskallet skaldyr
adj. - toskallet

Nederlands (Dutch)
tweekleppig (schaaldier), oester

Français (French)
n. - bivalve
adj. - bivalve

Deutsch (German)
n. - Muschel
adj. - zweischalig, zweiklappig

Ελληνική (Greek)
adj. - (ζωολ.) δίθυρος, (φυτολ.) δίλοβος
n. - (ζωολ.) δίθυρο, (φυτολ.) δίλοβο

Italiano (Italian)
bivalve, valvola doppia

Português (Portuguese)
adj. - bivalve
n. - molusco (m) bivalve (Zool.)

Русский (Russian)
двустворчатый моллюск

Español (Spanish)
n. - molusco de dos valvas
adj. - bivalvo

Svenska (Swedish)
adj. - tvåskalig
n. - tvåskaligt skaldjur

中文（简体） (Chinese (Simplified))
双壳贝, 两瓣的, 双壳的

中文（繁體） (Chinese (Traditional))
n. - 雙殼貝
adj. - 兩瓣的, 雙殼的

한국어 (Korean)
n. - 쌍각류의 조개
adj. - 쌍각류의, 양 판의

日本語 (Japanese)
n. - 二枚貝
adj. - 二枚貝の

العربيه (Arabic)
‏(صفه) ذو صمامين (الاسم) حيوان ذو صمامين‏

עברית (Hebrew)
n. - ‮רכיכה דו-צדפתית‬
adj. - ‮עם צדפה כפולה על צירים‬

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