Bivalvia
Overview
Bivalvia is a taxonomic class
Class (biology)
In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...

 of marine and freshwater molluscs
Mollusca
The Mollusca , common name molluscs or mollusksSpelled mollusks in the USA, see reasons given in Rosenberg's ; for the spelling mollusc see the reasons given by , is a large phylum of invertebrate animals. There are around 85,000 recognized extant species of molluscs. Mollusca is the largest...

. This class includes clam
Clam
The word "clam" can be applied to freshwater mussels, and other freshwater bivalves, as well as marine bivalves.In the United States, "clam" can be used in several different ways: one, as a general term covering all bivalve molluscs...

s, oyster
Oyster
The word oyster is used as a common name for a number of distinct groups of bivalve molluscs which live in marine or brackish habitats. The valves are highly calcified....

s, mussel
Mussel
The common name mussel is used for members of several families of clams or bivalvia mollusca, from saltwater and freshwater habitats. These groups have in common a shell whose outline is elongated and asymmetrical compared with other edible clams, which are often more or less rounded or oval.The...

s, scallop
Scallop
A scallop is a marine bivalve mollusk of the family Pectinidae. Scallops are a cosmopolitan family, found in all of the world's oceans. Many scallops are highly prized as a food source...

s, and many other families of molluscs that have two hinged shells. The class was known for some time as Pelecypoda, which is a reference to the soft parts of the animal, whereas the name Bivalvia simply describes the shell, which has two valves. Other names which have been used for this class include Lamellibranchia (referring to the plate-like gill elements, see ctenidium
Ctenidium (mollusc)
A ctenidium is a respiratory organ or gill which is found in many mollusks. This structure exists in bivalves and in many aquatic gastropods, i.e. in some freshwater snails and sea snails and also in some sea slugs...

), Acephala (they have no head), and Bivalva (two valves).

The total number of bivalve species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 is currently approximately 9,200.
Encyclopedia
Bivalvia is a taxonomic class
Class (biology)
In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...

 of marine and freshwater molluscs
Mollusca
The Mollusca , common name molluscs or mollusksSpelled mollusks in the USA, see reasons given in Rosenberg's ; for the spelling mollusc see the reasons given by , is a large phylum of invertebrate animals. There are around 85,000 recognized extant species of molluscs. Mollusca is the largest...

. This class includes clam
Clam
The word "clam" can be applied to freshwater mussels, and other freshwater bivalves, as well as marine bivalves.In the United States, "clam" can be used in several different ways: one, as a general term covering all bivalve molluscs...

s, oyster
Oyster
The word oyster is used as a common name for a number of distinct groups of bivalve molluscs which live in marine or brackish habitats. The valves are highly calcified....

s, mussel
Mussel
The common name mussel is used for members of several families of clams or bivalvia mollusca, from saltwater and freshwater habitats. These groups have in common a shell whose outline is elongated and asymmetrical compared with other edible clams, which are often more or less rounded or oval.The...

s, scallop
Scallop
A scallop is a marine bivalve mollusk of the family Pectinidae. Scallops are a cosmopolitan family, found in all of the world's oceans. Many scallops are highly prized as a food source...

s, and many other families of molluscs that have two hinged shells. The class was known for some time as Pelecypoda, which is a reference to the soft parts of the animal, whereas the name Bivalvia simply describes the shell, which has two valves. Other names which have been used for this class include Lamellibranchia (referring to the plate-like gill elements, see ctenidium
Ctenidium (mollusc)
A ctenidium is a respiratory organ or gill which is found in many mollusks. This structure exists in bivalves and in many aquatic gastropods, i.e. in some freshwater snails and sea snails and also in some sea slugs...

), Acephala (they have no head), and Bivalva (two valves).

The total number of bivalve species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 is currently approximately 9,200. These are placed within 1,260 genera and 106 families. Marine bivalves (including brackish water
Brackish water
Brackish water is water that has more salinity than fresh water, but not as much as seawater. It may result from mixing of seawater with fresh water, as in estuaries, or it may occur in brackish fossil aquifers. The word comes from the Middle Dutch root "brak," meaning "salty"...

 and estuarine species) represent about 8,000 species, combined in 4 subclasses and 99 families with 1,100 genera. The largest recent families are Veneridae
Veneridae
The Veneridae or venerids, also known as the Venus clams, are a very large family of minute to large, saltwater clams, marine bivalve molluscs. There are over 500 living species of venerid bivalves, most of which are edible, and many of which are exploited as a food source.Many of the most...

 with more than 680 species, and Tellinidae
Tellinidae
Tellinidae is a family of marine bivalve molluscs of the order Veneroida. They live under soft sediments in shallow seas.-Characteristics:Tellinids have rounded or oval, elongated shells, much flattened. The two valves are connected by a large external ligament. The two separate siphons are...

 and Lucinidae
Lucinidae
Lucinidae is a family of saltwater clams, marine bivalve molluscs.These bivalves are remarkable for their endosymbiosis with sulphide-oxidizing bacteria.-Characteristics:...

 each with over 500 species. The freshwater bivalves have 7 additional families, of which the Unionidae
Unionidae
Unionidae is a family of freshwater mussels, the largest in the order Unionoida, the bivalve mollusks sometimes known as river mussels, naiads, or simply as unionids.The range of distribution for this family is world-wide...

 contain about 700 species.

Bivalves have a shell consisting of two asymmetrically rounded halves called valves that are mirror images of each other, joined at one edge by a flexible ligament
Ligament
In anatomy, the term ligament is used to denote any of three types of structures. Most commonly, it refers to fibrous tissue that connects bones to other bones and is also known as articular ligament, articular larua, fibrous ligament, or true ligament.Ligament can also refer to:* Peritoneal...

 called the hinge. The shell is typically bilaterally symmetrical, with the hinge lying in the sagittal plane
Sagittal plane
Sagittal plane is a vertical plane which passes from front to rear dividing the body into right and left sections.-Variations:Examples include:...

. The adult maximum shell size of Recent species of bivalves ranges from 0.52 mm in Condylonucula maya (a nut clam) to a length of 1,532 mm in Kuphus polythalamia, a kind of shipworm
Shipworm
Shipworms are not worms at all, but rather a group of unusual saltwater clams with very small shells, notorious for boring into wooden structures that are immersed in sea water, such as piers, docks and wooden ships...

. However, the species generally regarded as the largest living bivalve is the giant clam
Giant clam
The giant clam, Tridacna gigas , is the largest living bivalve mollusc. T. gigas is one of the most endangered clam species. It was mentioned as early as 1825 in scientific reports...

 Tridacna gigas which can weigh more than 200 kilograms (441 lbs).

Bivalves are unique among the Mollusca in that they have lost their odontophore and radula
Radula
The radula is an anatomical structure that is used by molluscs for feeding, sometimes compared rather inaccurately to a tongue. It is a minutely toothed, chitinous ribbon, which is typically used for scraping or cutting food before the food enters the esophagus...

 in their transition to filter feeding.

Some bivalves are epifaunal; they attach to surfaces. Others are infaunal; they bury themselves in sediment. These forms typically have a strong digging foot. Some bivalves such as scallop
Scallop
A scallop is a marine bivalve mollusk of the family Pectinidae. Scallops are a cosmopolitan family, found in all of the world's oceans. Many scallops are highly prized as a food source...

s can swim
Nekton
Nekton refers to the aggregate of actively swimming aquatic organisms in a body of water able to move independently of water currents....

.

The term bivalve is derived from the Latin
Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...

 bis, meaning 'two', and valvae, meaning leaves of a door Not all animals that have two hinged shells are Bivalvia; other kinds include the bivalved gastropods (small sea snail
Sea snail
Sea snail is a common name for those snails that normally live in saltwater, marine gastropod molluscs....

s in the family Juliidae
Juliidae
Juliidae, common name the bivalved gastropods, is a family of minute sea snails, marine gastropod mollusks or micromollusks in the superfamily Oxynooidea, an opisthobranch group....

), the phylum Brachiopoda, and the minute crustaceans known as ostracodes and conchostrachans
Conchostraca
Clam shrimp are a taxon of bivalved branchiopod crustaceans that resemble the unrelated bivalved molluscs. They are extant, and known from the fossil record, from at least the Devonian period and perhaps before...

.

Taxonomy

No professional consensus exists on bivalve phylogeny. Many conflicts exist due to taxonomies
Taxonomy
Taxonomy is the science of identifying and naming species, and arranging them into a classification. The field of taxonomy, sometimes referred to as "biological taxonomy", revolves around the description and use of taxonomic units, known as taxa...

 based on single organ systems and conflicting naming schemes. More recent taxonomies use multiple organ systems, the fossil record, and molecular phylogenetics to draw more robust phylogenies. Due to the numerous fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...

 lineages, DNA sequence
DNA sequence
The sequence or primary structure of a nucleic acid is the composition of atoms that make up the nucleic acid and the chemical bonds that bond those atoms. Because nucleic acids, such as DNA and RNA, are unbranched polymers, this specification is equivalent to specifying the sequence of...

 data is of limited use should the subclass
Class (biology)
In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...

es turn out to be paraphyletic.

In his 1935 work Handbuch der systematischen Weichtierkunde (Handbook of Systematic Malacology), Johannes Thiele
Johannes Thiele
Johannes Thiele, full name Karl Hermann Johannes Thiele was a German zoologist specialized in malacology. His Handbuch der systematischen Weichtierkunde is a standard work...

 introduced a mollusc taxonomy based upon the 1909 work by Cossmann and Peyrot. Thiele's system divided the bivalves into three orders:
  • Taxodonta – taxodont dentition
  • Anisomyaria – either a single adductor muscle
    Adductor muscle
    - Humans :* Adductor muscles of the hip, the most common reference in humans, but may also refer to** Adductor brevis muscle, a muscle in the thigh situated immediately behind the pectineus and adductor longus...

     or one adductor muscle much larger than the other
  • Eulamellibranchiata – all forms with lamellibranch ctenidia

The last was divided into four sub-orders: Schizodonta, Heterodonta, Adapedonta and Anomalodesmata.

The systematic layout presented here follows Norman D. Newell
Norman D. Newell
Norman Dennis Newell was professor of geology at Columbia University, and chairman and curator of invertebrate paleontology at the American Museum of Natural History in New York.-Personal life:...

's 1965 classification based on hinge tooth morphology
Morphology (biology)
In biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....

:
Subclass
Subclass
Subclass may refer to:* Subclass , a taxonomic rank intermediate between class and superorder* Subclass , a class that is derived from another class or classes...

Order
Order (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...

Palaeotaxodonta *Nuculoida
Nuculoida
Nuculanoida is an order of bivalves. It belongs to the subclass Protobranchia. They are distinguished from other bivalves by the presence of relatively primitive, "protobranchiate" gills, with a row of short teeth along the hinge of the shell. The shells are often nacreous.-Families:*...

Cryptodonta
Cryptodonta
Cryptodonta is a subclass of the bivalves. It contains a single extant order, Solemyoida, while the Praecardioida are known only from fossils....

†Praecardioida
Solemyoida
Solemyoida
Solemyoida is an order of bivalve molluscs.-Families in the order Solemyoida:* Manzanellidae Chronic, 1952 * Solemyidae J. E. Gray, 1840-References:...

Pteriomorphia Arcoida
Arcoida
The Arcoida is an extant order of bivalve molluscs. This order dates back to the lower Ordovician period. They are distinguished from related groups, such as the mussels, by having a straight hinge to the shells, and the adductor muscles being of equal size....

 (ark shells)
†Cyrtodontoida

Limoida (file shells)

Mytiloida (true mussels)

Ostreoida
Ostreoida
The order Ostreoida includes the true oysters and a number of other related families of bivalves.About eleven families are recognised within it, but suborders, superfamilies and subfamilies are used relatively heavily in this order. The following classification represents a synthesis of different...

 (oysters, formerly included in Pterioida)

†Praecardioida

Pterioida
Pterioida
Pterioida is an order of large and medium-sized saltwater clams, marine bivalve mollusks. It includes five families, among them the Pteriidae -Families within the order Pterioida:* Isognomonidae* Malleidae* Pinnidae...

 (pearl oysters, pen shells)
Paleoheterodonta
Paleoheterodonta
Paleoheterodonta is a subclass of bivalve molluscs. It contains the extant order Unionoida and the prehistoric Trigonioida. They are distinguished by having the two halves of the shell be of equal size and shape, but by having the hinge teeth be in a single row, rather than separated into two...

†Trigonioida

Unionoida
Unionoida
Unionoida is an order of freshwater mussels, aquatic bivalve mollusks. The order includes most of the larger freshwater mussels, including the freshwater pearl mussels...

 (freshwater mussels)

†Modiomorpha
Heterodonta
Heterodonta
Heterodonta is a subclass of bivalve molluscs, including the clams and cockles. They are distinguished by having the two halves of the shell be equally sized, and having a few cardinal teeth separated from a number of long lateral teeth. Their shells lack a nacreous layer, and the gills are...

†Cycloconchidae

†Hippuritoida

†Lyrodesmatidae

Myoida
Myoida
Myoida is an order of bivalve molluscs. They are burrowing molluscs, with well-developed siphons. The shell is relatively soft, and lacks a nacreous layer...

 (most "soft shell calms" razor clams)

†Redoniidae

Veneroida
Veneroida
The Veneroida or veneroids are an order of bivalve molluscs. They include some familiar forms such as saltwater clams and cockles, and a number of freshwater bivalves including zebra mussels....

 (most "hard shell calms", cockle
Cockle
Cockle may refer to:* Cockle , a group of edible saltwater clams * Lolium temulentum, a tufted grass plant* Berwick cockles, a confectionery from ScotlandCockleshell* The Mark II canoes used in Operation Frankton in 1942...

s, etc.)
Anomalodesmata Pholadomyoida
Pholadomyoida
Pholadomyoida is an order of bivalve molluscs. It is the sole member of the subclass Anomalosdesmata. Their shells are of equal size, as are the muscles that hold them closed, and the margins at the hinges are thickened...



The monophyly
Monophyly
In common cladistic usage, a monophyletic group is a taxon which forms a clade, meaning that it contains all the descendants of the possibly hypothetical closest common ancestor of the members of the group. The term is synonymous with the uncommon term holophyly...

 of the Anomalodesmata is disputed, but this is of less consequence as that group does not include higher-level prehistoric taxa. The standard view now is that Anomalodesmata resides within the subclass Heterodonta
Heterodonta
Heterodonta is a subclass of bivalve molluscs, including the clams and cockles. They are distinguished by having the two halves of the shell be equally sized, and having a few cardinal teeth separated from a number of long lateral teeth. Their shells lack a nacreous layer, and the gills are...

.

An alternative systematic scheme exists according to gill
Gill
A gill is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen from water, afterward excreting carbon dioxide. The gills of some species such as hermit crabs have adapted to allow respiration on land provided they are kept moist...

 morphology. This distinguishes between Protobranchia, Filibranchia, and Eulamellibranchia. The first corresponds to Newells Palaeotaxodonta and Cryptodonta, the second to his Pteriomorphia, with the last corresponding to all other groups. In addition, Franc separated the Septibranchia from his eulamellibranchs, but this would seem to make the latter paraphyletic.

In May 2010 a new taxonomy of the Bivalvia was published in the journal Malacologia
Malacologia
Malacologia is a peer-reviewed scientific journal in the field of malacology, the study of mollusks. The journal publishes articles in the fields of molluscan systematics, ecology, population ecology, genetics, molecular genetics, evolution, and phylogenetics.The journal specializes in publishing...

. In this classification 324 families were recognized as valid, 214 of which are known exclusively as fossils and 110 families occur in the Recent with or without a fossil record. This publication consisted of two parts :
  • Nomenclator of Bivalve Names of the Family-Group and above
  • Classification of Bivalve Families (under the redaction of Rüdiger Bieler, Joseph G. Carter and Eugene V. Coan)

Biodiversity of extant bivalves

Huber gives a total number of about 9,200 living bivalves combined in 106 families. He states that the number of 20,000 living species, often encountered in literature, could not be verified and shows the following table.
Number of Families Genera Species
PROTOBRANCHIA 10 49 700
Nuculoidea
Nuculoidea
Nuculoidea is an superfamily of bivalves. It belongs to the order Nuculoidea. It comprises only one family: Nuculidae...

 
1 8 170
Sapretoidea  1 ca. 5 10
Solemyoidea  1 2 30
Manzanelloidea  1 2 20
Nuculanoidea  6 32 460
PTERIOMORPHA
Pteriomorpha
Pteriomorphia is a mollusc subclass of the Bivalvia. Apart from the orders Arcoida, Mytiloida, Ostreoida, and Pterioida, it also contains some extinct and probably basal families, such as the Evyanidae, Colpomyidae, Bakevelliidae, Cassianellidae and Lithiotidae.Pteriomorpha molluscs have...

25 240 (incl. 2 freshwater) 2000 (incl. 11 freshwater)
Mytiloidea  1 50 (1 freshwater) 400 (5 freshwater)
Arcoidea  7 60 (1 freshwater) 570 (6 freshwater)
Pinnoidea  1 3 (+) 50
Pterioidea  5 9 80
Ostreoidea  2 23 80
Dimyoidea  1 3 15
Anomioidea  2 9 30
Plicatuloidea  1 1 20
Pectinoidea
Pectinoidea
Pectinoidea is a superfamily of marine bivalve mollusks.The Pectinoidea include the following families:-Further reading:*...

 
4 68 500
Limoidea  1 8 250
PALAEOHETERODONTA 7 (incl. 6 freshwater) 171 (incl. 170 freshwater) 908 (incl. 900 freshwater)
Trigonioidea  1 1 8
Unionoidea  (6 freshwater) (170 freshwater) (900 freshwater)
HETERODONTA
Heterodonta
Heterodonta is a subclass of bivalve molluscs, including the clams and cockles. They are distinguished by having the two halves of the shell be equally sized, and having a few cardinal teeth separated from a number of long lateral teeth. Their shells lack a nacreous layer, and the gills are...

64 (incl. 1 freshwater) 800 (incl. 16 freshwater) 5600 (incl. 270 freshwater)
Crassatelloidea  5 65 420
Thyasiroidea  1 ca. 12 ca. 100
Lucinoidea  2 ca. 85 ca. 500
Galeommatoidea  ca. 4 ca. 100 ca. 500
Cyamioidea  3 22 140
Solenoidea  2 17 (2 freshwater) 130 (4 freshwater)
Hiatelloidea  1 5 25
Gastrochaenoidea  1 7 30
Chamoidea  1 6 70
Cardioidea  2 38 260
Tellinoidea  5 110 (2 freshwater) 900 (15 freshwater)
Glossoidea  2 20 110
Arcticoidea  2 6 13
Cyrenoidea  1 6 (3 freshwater) 60 (30 freshwater)
Sphaerioidea  (1 freshwater) (5 freshwater) (200 freshwater)
Veneroidea  4 104 750
Hemidonacoidea  1 1 6
Cyrenoidoidea  1 1 6
Ungulinoidea  1 16 100
Mactroidea  4 46 220
Dreissenoidea  1 3 (2 freshwater) 20 (12 freshwater)
Myoidea  3 15 (1 freshwater) 130 (1 freshwater)
Pholadoidea  2 34 (1 freshwater) 200 (3 freshwater)
Limoidea  1 8 250
(ANOMALODESMATA) (14) (71) (770)
Pholadomyoidea  2 3 20
Clavagelloidea  1 2 20
Pandoroidea  7 30 250
Verticordioidea  2 16 160
Cuspidarioidea  2 20 320

Anatomy

Bivalve shells vary greatly in shape; some are globular, others flattened, while others are elongated to aid burrowing. The shipworm
Shipworm
Shipworms are not worms at all, but rather a group of unusual saltwater clams with very small shells, notorious for boring into wooden structures that are immersed in sea water, such as piers, docks and wooden ships...

s 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 that permit the animal to dig tunnels through wood.

Nervous system

The sedentary habit of the bivalves has led to the development of a simpler nervous system
Nervous system
The nervous system is an organ system containing a network of specialized cells called neurons that coordinate the actions of an animal and transmit signals between different parts of its body. In most animals the nervous system consists of two parts, central and peripheral. The central nervous...

 than in other molluscs; they have no brain
Brain
The brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...

. In all but the simplest forms the neural ganglia are united into two cerebropleural ganglia on either side of the oesophagus. The pedal ganglia, controlling the foot, are at its base, and the visceral ganglia (which can be quite large in swimming bivalves) under the posterior adductor muscle. These ganglia are both connected to the cerebropleural ganglia by nerve fibres. There may also be siphonal ganglia in bivalves with a long siphon.

Senses

The sensory organs of bivalves are not well developed and are largely a function of the posterior mantle margins. The organs are usually tentacle
Tentacle
A tentacle or bothrium is one of usually two or more elongated flexible organs present in animals, especially invertebrates. The term may also refer to the hairs of the leaves of some insectivorous plants. Usually, tentacles are used for feeding, feeling and grasping. Anatomically, they work like...

 mechanoreceptor
Mechanoreceptor
A mechanoreceptor is a sensory receptor that responds to mechanical pressure or distortion. There are four main types in the glabrous skin of humans: Pacinian corpuscles, Meissner's corpuscles, Merkel's discs, and Ruffini corpuscles...

s or chemoreceptors.

Scallop
Scallop
A scallop is a marine bivalve mollusk of the family Pectinidae. Scallops are a cosmopolitan family, found in all of the world's oceans. Many scallops are highly prized as a food source...

s have complex eyes with a lens
Lens (anatomy)
The crystalline lens is a transparent, biconvex structure in the eye that, along with the cornea, helps to refract light to be focused on the retina. The lens, by changing shape, functions to change the focal distance of the eye so that it can focus on objects at various distances, thus allowing a...

 and retina
Retina
The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...

, but most other bivalves have much simpler eyes, if any. There are also light-sensitive cells in all bivalves that can detect a shadow falling over the animal.

Many bivalves possess a number of tentacles, which have chemoreceptor cells to taste the water, as well as being sensitive to touch. These are typically found near the siphons, but in some species may fringe the entire mantle cavity.

Another notable sensory organ found in bivalves is the osphradium
Osphradium
The osphradium is the olfactory organ in certain molluscs, linked with the respiration organ.The main function of this is to test incoming water for silt and other possible food particles.It is used by all members of the Genus Conus....

, a patch of sensory cells located below the posterior adductor muscle. It may serve to taste the water, or measure its turbidity
Turbidity
Turbidity is the cloudiness or haziness of a fluid caused by individual particles that are generally invisible to the naked eye, similar to smoke in air. The measurement of turbidity is a key test of water quality....

, but it is probably not homologous with the structure of the same name found in snails and slugs.

In the septibranchs the inhalant siphon is surrounded by vibration-sensitive tentacles for detecting prey.

Statocyst
Statocyst
The statocyst is a balance sensory receptor present in some aquatic invertebrates, including bivalves, cnidarians, echinoderms, cephalopods, and crustaceans. A similar structure is also found in Xenoturbella. The statocyst consists of a sac-like structure containing a mineralised mass and numerous...

s within the organism help the bivalve to sense and correct its orientation.

Muscles

The muscular system is composed of the posterior and anterior adductor muscles, although the anterior muscles may be reduced or even lost in some species.

The paired anterior and posterior pedal retractor muscles operate the animal's foot. In some bivalves, such as oyster
Oyster
The word oyster is used as a common name for a number of distinct groups of bivalve molluscs which live in marine or brackish habitats. The valves are highly calcified....

s and scallop
Scallop
A scallop is a marine bivalve mollusk of the family Pectinidae. Scallops are a cosmopolitan family, found in all of the world's oceans. Many scallops are highly prized as a food source...

s, these retractors are absent.

Circulation and respiration

Bivalves have an open circulatory system
Circulatory system
The circulatory system is an organ system that passes nutrients , gases, hormones, blood cells, etc...

 that bathes the organs in hemolymph
Hemolymph
Hemolymph, or haemolymph, is a fluid in the circulatory system of some arthropods and is analogous to the fluids and cells making up both blood and interstitial fluid in vertebrates such as birds and mammals...

. The heart
Heart
The heart is a myogenic muscular organ found in all animals with a circulatory system , that is responsible for pumping blood throughout the blood vessels by repeated, rhythmic contractions...

 has three chambers; two auricles receiving blood from the gill
Gill
A gill is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen from water, afterward excreting carbon dioxide. The gills of some species such as hermit crabs have adapted to allow respiration on land provided they are kept moist...

s, and a single ventricle
Ventricle (heart)
In the heart, a ventricle is one of two large chambers that collect and expel blood received from an atrium towards the peripheral beds within the body and lungs. The Atria primes the Pump...

. The ventricle is muscular and pumps hemolymph into the aorta
Aorta
The aorta is the largest artery in the body, originating from the left ventricle of the heart and extending down to the abdomen, where it branches off into two smaller arteries...

, and through this to the rest of the body. Many bivalves have only a single aorta, but most also have a second, usually smaller, aorta serving the hind parts of the animal.

Oxygen is absorbed into the hemolymph in the gills, which hang down into the mantle cavity, and also assist in filtering food particles from the water. The wall of the mantle cavity is a secondary respiratory surface, and is well supplied with capillaries
Capillary
Capillaries are the smallest of a body's blood vessels and are parts of the microcirculation. They are only 1 cell thick. These microvessels, measuring 5-10 μm in diameter, connect arterioles and venules, and enable the exchange of water, oxygen, carbon dioxide, and many other nutrient and waste...

. Some species, however, have no gills, with the mantle cavity being the only location of gas exchange. Bivalves adapted to tidal environments can survive for several hours out of water by closing their shells and keeping the mantle cavity filled with water.

The hemolymph usually lacks any respiratory pigment
Biological pigment
Biological pigments, also known simply as pigments or biochromes are substances produced by living organisms that have a color resulting from selective color absorption. Biological pigments include plant pigments and flower pigments...

, although some species are known to possess haemoglobin dissolved directly into the serum
Blood serum
In blood, the serum is the component that is neither a blood cell nor a clotting factor; it is the blood plasma with the fibrinogens removed...

.

Mantle and shell

In bivalves the mantle
Mantle (mollusc)
The mantle is a significant part of the anatomy of molluscs: it is the dorsal body wall which covers the visceral mass and usually protrudes in the form of flaps well beyond the visceral mass itself.In many, but by no means all, species of molluscs, the epidermis of the mantle secretes...

 forms a thin membrane
Biological membrane
A biological membrane or biomembrane is an enclosing or separatingmembrane that acts as a selective barrier, within or around a cell. It consists of a lipid bilayer with embedded proteins that may constitute close to 50% of membrane content...

 surrounding the body which secretes the valves, ligament
Ligament
In anatomy, the term ligament is used to denote any of three types of structures. Most commonly, it refers to fibrous tissue that connects bones to other bones and is also known as articular ligament, articular larua, fibrous ligament, or true ligament.Ligament can also refer to:* Peritoneal...

 and hinge teeth. The mantle lobes secrete the valves and the mantle crest secretes the ligament and hinge teeth. The mantle is attached to the shell by the mantle retractor muscles at the pallial line. In some bivalves the mantle edges fuse to form siphons, which take in and expel water for suspension feeding.

The shell is composed of two calcareous
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:...

 valves, which are made of either calcite
Calcite
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...

 (as with oysters) or both calcite and aragonite
Aragonite
Aragonite is a carbonate mineral, one of the two common, naturally occurring, crystal forms of calcium carbonate, CaCO3...

, usually with the aragonite forming an inner layer (as with the Pterioida
Pterioida
Pterioida is an order of large and medium-sized saltwater clams, marine bivalve mollusks. It includes five families, among them the Pteriidae -Families within the order Pterioida:* Isognomonidae* Malleidae* Pinnidae...

). The outermost layer is the periostracum
Periostracum
The periostracum is a thin organic coating or "skin" which is the outermost layer of the shell of many shelled animals, including mollusks and brachiopods. Among mollusks it is primarily seen in snails and clams, i.e. in bivalves and gastropods, but it is also found in cephalopods such as the...

, composed of a horny organic substance
Corneous
Corneous is a biological and medical term meaning horny, in other words made out of a substance similar to that of horns and hooves in some mammals....

. This forms the familiar coloured layer on the shell.

The shell is added to in two ways; at the open edge and by a gradual thickening throughout the animal's life.

The shell halves are held together at the animal's dorsum
Dorsum (biology)
In anatomy, the dorsum is the upper side of animals that typically run, fly, or swim in a horizontal position, and the back side of animals that walk upright. In vertebrates the dorsum contains the backbone. The term dorsal refers to anatomical structures that are either situated toward or grow...

 by the ligament
Ligament
In anatomy, the term ligament is used to denote any of three types of structures. Most commonly, it refers to fibrous tissue that connects bones to other bones and is also known as articular ligament, articular larua, fibrous ligament, or true ligament.Ligament can also refer to:* Peritoneal...

, which is composed of the tensilium and resilium. The ligament opens the shell using muscles.

Modes of feeding

The majority of bivalves are filter feeder
Filter feeder
Filter feeders are 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 feeding are clams, krill, sponges, baleen whales, and many fish and some sharks. Some birds,...

s, using their gill
Gill
A gill is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen from water, afterward excreting carbon dioxide. The gills of some species such as hermit crabs have adapted to allow respiration on land provided they are kept moist...

s to capture particulate food from the water. In almost all species, the water current enters the shell from the posterior ventral surface of the animal, and then passes upwards through the gills in a U-shape, so that it exits just above the intake. In burrowing species, there may be elongated siphons stretching from the body to the surface, one each for the inhalant and exhalant streams of water.

The gills of filter-feeding bivalves have become highly modified to increase their ability to capture food. For example, the cilia on the gills, which originally served to remove unwanted sediment, are adapted to capture food particles, and transport them in a steady stream of mucus
Mucus
In vertebrates, mucus is a slippery secretion produced by, and covering, mucous membranes. Mucous fluid is typically produced from mucous cells found in mucous glands. Mucous cells secrete products that are rich in glycoproteins and water. Mucous fluid may also originate from mixed glands, which...

 to the mouth. The filaments of the gills are also much longer than those in more primitive bivalves, and are folded over to create a groove through which food can be transported. The structure of the gills varies considerably, and can serve as a useful means for classifying bivalves into groups.

Some bivalves feed by scraping detritus from the bottom, and this may be the primitive mode of feeding for the group, before the gills became adapted for filter feeding. These primitive bivalves hold onto the substratum with a pair of tentacles at the edge of the mouth, each of which has a single palp, or flap. The tentacles are covered in mucus, which traps the food particles, and transports them back to the palps using cilia. The palps then serve to sort the particles, ejecting those that are too large to be digestible.

A few bivalves, such as Poromya, are carnivorous, eating much larger prey than the tiny phytoplankton
Phytoplankton
Phytoplankton are the autotrophic component of the plankton community. The name comes from the Greek words φυτόν , meaning "plant", and πλαγκτός , meaning "wanderer" or "drifter". Most phytoplankton are too small to be individually seen with the unaided eye...

 consumed by the filter feeders. In these animals, the gills are relatively small, and form a perforated barrier separating the main mantle cavity from a smaller chamber through which the water is exhaled. Muscles pump water through the cavity, sucking in small crustaceans and worms. The prey are then seized in the palps and consumed.

The unusual genus Entovalva is parasitic, and lives only in the gut of sea cucumbers.

Digestive tract

The digestive tract of typical bivalves consists of an oesophagus
Esophagus
The esophagus is an organ in vertebrates which consists of a muscular tube through which food passes from the pharynx to the stomach. During swallowing, food passes from the mouth through the pharynx into the esophagus and travels via peristalsis to the stomach...

, stomach
Stomach
The stomach is a muscular, hollow, dilated part of the alimentary canal which functions as an important organ of the digestive tract in some animals, including vertebrates, echinoderms, insects , and molluscs. It is involved in the second phase of digestion, following mastication .The stomach is...

, and intestine
Intestine
In human anatomy, the intestine is the segment of the alimentary canal extending from the pyloric sphincter of the stomach to the anus and, in humans and other mammals, consists of two segments, the small intestine and the large intestine...

. A number of digestive glands open into the stomach, often via a pair of diverticula; these secrete enzymes to digest food in the stomach, but also include cells that phagocytose food particles, and digest them intracellularly.

In the filter feeding bivalves, an elongated rod of solidified mucus referred to as the crystalline style projects into the stomach from an associated sac. Cilia in the sac cause the style to rotate, winding in a stream of food-containing mucus from the mouth, and churning the stomach contents. This constant motion propels food particles into a sorting region at the rear of the stomach, which distributes smaller particles into the digestive glands, and heavier particles into the intestine.

Carnivorous bivalves have a greatly reduced style, and a chitin
Chitin
Chitin n is a long-chain polymer of a N-acetylglucosamine, a derivative of glucose, and is found in many places throughout the natural world...

ous gizzard
Gizzard
The gizzard, also referred to as the ventriculus, gastric mill, and gigerium, is an organ found in the digestive tract of some animals, including birds, reptiles, earthworms and some fish. This specialized stomach constructed of thick, muscular walls is used for grinding up food; often rocks are...

 that helps grind up the food before digestion.

Excretory system

Like most other molluscs, the excretory organs of bivalves are nephridia
Nephridium
A Nephridium is an invertebrate organ which occurs in pairs and function similar to kidneys. Nephridia remove metabolic wastes from an animal's body. They are present in many different invertebrate lines. There are two basic types, metanephridia and protonephridia, but there are other...

. There are two nephridia, each consisting of a long, glandular tube, which opens into the body cavity just beneath the heart, and a bladder
Bladder
Bladder usually refers to an anatomical hollow organBladder may also refer to:-Biology:* Urinary bladder in humans** Urinary bladder ** Bladder control; see Urinary incontinence** Artificial urinary bladder, in humans...

. Waste is voided from the bladders through a pair of openings near the front of the upper part mantle cavity, where it can easily be washed away in the stream of exhalant water.

Reproduction

The sexes are usually separate, but some hermaphroditism is known. Bivalves practice external fertilization
External fertilization
External fertilization is a form of fertilization in which a sperm cell is united with an egg cell external to the bodies of the reproducing individuals. In contrast, internal fertilization takes place inside the female after insemination through copulation....

. The gonad
Gonad
The gonad is the organ that makes gametes. The gonads in males are the testes and the gonads in females are the ovaries. The product, gametes, are haploid germ cells. For example, spermatozoon and egg cells are gametes...

s are located close to the intestines, and either open into the nephridia, or through a separate pore into the mantle cavity.

Typically bivalves start life as a trochophore
Trochophore
A trochophore is a type of free-swimming planktonic marine larva with several bands of cilia.By moving their cilia rapidly, a water eddy is created. In this way they control the direction of their movement...

, later becoming a veliger
Veliger
A veliger is the planktonic larva of many kinds of marine and freshwater gastropod molluscs, as well as most bivalve mollusks.- Description :...

. Freshwater bivalves of the Unionoida
Unionoida
Unionoida is an order of freshwater mussels, aquatic bivalve mollusks. The order includes most of the larger freshwater mussels, including the freshwater pearl mussels...

 have a different life cycle: they become a glochidium
Glochidium
The glochidium is a special microscopic larval stage of larger freshwater mussels, aquatic bivalve mollusks in the families Unionidae and Margaritiferidae, the river mussels and European freshwater pearl mussels....

, which attaches to any firm surface to avoid the danger of being swept downsteam. Glochidia can be serious pests of fish
Fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...

 if they lodge in the fish gills.

Some of the species in the freshwater mussel family, Unionidae
Unionidae
Unionidae is a family of freshwater mussels, the largest in the order Unionoida, the bivalve mollusks sometimes known as river mussels, naiads, or simply as unionids.The range of distribution for this family is world-wide...

, commonly known as pocketbook mussels have evolved a remarkable reproductive strategy. The edge of the female's body that protrudes from the valves of the shell develops into an imitation of a small fish complete with markings and false eyes. This decoy moves in the current and attracts the attention of real fish. Some fish see the decoy as prey, while others see a conspecific. Whatever they see, they approach for a closer look and the mussel releases huge numbers of larvae from her gills, dousing the inquisitive fish with her tiny, parasitic young. These glochidia larvae are drawn into the fish's gills where they attach and trigger a tissue response that forms a small cyst
Cyst
A cyst is a closed sac, having a distinct membrane and division on the nearby tissue. It may contain air, fluids, or semi-solid material. A collection of pus is called an abscess, not a cyst. Once formed, a cyst could go away on its own or may have to be removed through surgery.- Locations :* Acne...

 in which the young mussel resides. It feeds by breaking down and digesting the tissue of the fish within the cyst.

Behaviour

The radical structure of the bivalves reflects their behaviour in several ways. The most significant is the use of the closely fitting valves as a defence against predation and, in intertidal
Intertidal zone
The intertidal zone is the area that is above water at low tide and under water at high tide . This area can include many different types of habitats, with many types of animals like starfish, sea urchins, and some species of coral...

 species, against desiccation. The entire animal can be contained within the shell, which is held shut by the powerful adductor muscles. This defence is difficult to overcome except by specialist predators such as sea star
Sea star
Starfish or sea stars are echinoderms belonging to the class Asteroidea. The names "starfish" and "sea star" essentially refer to members of the class Asteroidea...

s and oystercatcher
Oystercatcher
The oystercatchers are a group of waders; they form the family Haematopodidae, which has a single genus, Haematopus. They are found on coasts worldwide apart from the polar regions and some tropical regions of Africa and South East Asia...

s.

Feeding

Most bivalves are filter feeder
Filter feeder
Filter feeders are 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 feeding are clams, krill, sponges, baleen whales, and many fish and some sharks. Some birds,...

s although some have taken up scavenging and predation. Nephridia remove the waste material. Buried bivalves feed by extending a siphon to the surface (indicated by the presence of a pallial sinus, the size of which is proportional to the burrowing depth, and represented by their hinge teeth).

Feeding types

There are four feeding types, defined by their gill structure:
  • Protobranchs use their ctenidia solely for respiration, and the labial palps to feed
  • Septibranchs possess a septum
    Septum
    In anatomy, a septum is a wall, dividing a cavity or structure into smaller ones.-In human anatomy:...

     across the mantle cavity which pumps in food.
  • Filibranchs and lamellibranchs trap food with a mucous
    Mucus
    In vertebrates, mucus is a slippery secretion produced by, and covering, mucous membranes. Mucous fluid is typically produced from mucous cells found in mucous glands. Mucous cells secrete products that are rich in glycoproteins and water. Mucous fluid may also originate from mixed glands, which...

     coating on the ctenidia; the filibranchs and lamellibranchs are differentiated by the way the ctenidia are joined

Movement

Razor shell
Razor shell
The razor shell, Ensis arcuatus, also called razor clam or razor fish, is a bivalve of the family Pharidae. It is found on sandy beaches in Northern Europe and Eastern Canada, such as Prince Edward Island, where it is most populous in the world. It prefers coarser sand than its relatives E. ensis...

s can dig themselves into the sand with great speed to escape predation. Scallops, and file clam
Lima (genus)
Lima is a genus of file shells or file clams, marine bivalve molluscs in the family Limidae, the file shells, within the subclass Pteriomorphia.The shells are obliquely trigonal, and strongly radially ribbed, the ribs scabrous to spinose....

s can swim to escape a predator, clapping their valves together to create a jet of water. Cockle
Cockle (bivalve)
Cockle is the common name for a group of small, edible, saltwater clams, marine bivalve molluscs in the family Cardiidae.Various species of cockles live in sandy sheltered beaches throughout the world....

s can use their foot to leap from danger. However these methods can quickly exhaust the animal. In the razor shells the siphons can break off only to grow back later.

Defensive secretions

The file shells
Limidae
Limidae is the only family of bivalve molluscs in the order Limoida.-Genera:* Acesta H. and A. Adams, 1858* Divarilima Powell, 1958* Escalima Iredale, 1929 * Lima Bruguière, 1789 * Limaria Link, 1807 * Limatula S. V. Wood, 1839...

 can produce a noxious secretion when threatened, and the fan shells
Limidae
Limidae is the only family of bivalve molluscs in the order Limoida.-Genera:* Acesta H. and A. Adams, 1858* Divarilima Powell, 1958* Escalima Iredale, 1929 * Lima Bruguière, 1789 * Limaria Link, 1807 * Limatula S. V. Wood, 1839...

 of the same family have a unique, acid-producing organ.

Comparison with brachiopods

Bivalves are superficially similar to brachiopod
Brachiopod
Brachiopods are a phylum of marine animals that have hard "valves" on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, while the front can be opened for feeding or closed for protection...

s, but the construction of the shell is completely different in the two groups. In brachiopods, the two valves are on the dorsal and ventral surfaces of the body, while in bivalves, they are on the left and right sides.

Bivalves appeared late in the Cambrian explosion
Cambrian explosion
The Cambrian explosion or Cambrian radiation was the relatively rapid appearance, around , of most major phyla, as demonstrated in the fossil record, accompanied by major diversification of other organisms, including animals, phytoplankton, and calcimicrobes...

 and came to dominate over brachiopod
Brachiopod
Brachiopods are a phylum of marine animals that have hard "valves" on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, while the front can be opened for feeding or closed for protection...

s during the Palaeozoic. By the Permian-Triassic extinction event
Permian-Triassic extinction event
The Permian–Triassic extinction event, informally known as the Great Dying, was an extinction event that occurred 252.28 Ma ago, forming the boundary between the Permian and Triassic geologic periods, as well as the Paleozoic and Mesozoic eras...

 bivalves were undergoing a huge radiation
Radiation
In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...

 while brachiopods were devastated, losing 95% of their diversity.

It had long been considered that bivalves are better adapted to aquatic life than the brachiopods were, causing brachiopods to be out-competed
Competition (biology)
Competition is an interaction between organisms or species, in which the fitness of one is lowered by the presence of another. Limited supply of at least one resource used by both is required. Competition both within and between species is an important topic in ecology, especially community ecology...

 and relegated to minor niche
Ecological niche
In ecology, a niche is a term describing the relational position of a species or population in its ecosystem to each other; e.g. a dolphin could potentially be in another ecological niche from one that travels in a different pod if the members of these pods utilize significantly different food...

s in later strata. These taxa appeared in textbooks as an example of replacement by competition. Evidence included the use of an energetically efficient ligament-muscle system for opening valves, requiring less food to subsist. However the prominence of bivalves over brachiopods might instead be due to chance disparities in their response to extinction events.

External links

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