| image = Haeckel Acephala.jpg | image_width = 250px | image_caption = "Acephala", from Ernst Haeckel's Kunstformen der Natur (1904) | regnum = Animalia | phylum = Mollusca | classis = Bivalvia | classis_authority = Linnaeus, 1758 | subdivision_ranks = Subclasses | subdivision = Anomalosdesmata Cryptodonta Heterodonta Paleoheterodonta Palaeotaxodonta Pteriomorphia see text }} , England]] and attached mytilid bivalves in a Jurassic limestone (Matmor Formation) in southern Israel]] ; a fossil of an extinct scallop from the Logan Formation of Wooster, Ohio (external mold)]]

Bivalvia (common name bivalves) is a taxonomic class of marine and freshwater molluscs. This class includes clams, oysters, mussels, scallops, 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), Acephala (they have no head), and Bivalva (two valves).

The total number of bivalve species is currently approximately 9,200. These are placed within 1,260 genera and 106 families. Marine bivalves (including brackish water 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 with more than 680 species, and Tellinidae and Lucinidae each with over 500 species. The freshwater bivalves have 7 additional families, of which the Unionidae 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 called the hinge. The shell is typically bilaterally symmetrical, with the hinge lying in the sagittal plane. 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. However, the species generally regarded as the largest living bivalve is the giant clam 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 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 scallops can swim.

The term bivalve is derived from the Latin 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 snails in the family Juliidae), the phylum Brachiopoda, and the minute crustaceans known as ostracodes and conchostrachans.

Taxonomy

No professional consensus exists on bivalve phylogeny. Many conflicts exist due to taxonomies 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 lineages, DNA sequence data is of limited use should the subclasses turn out to be paraphyletic.

In his 1935 work Handbuch der systematischen Weichtierkunde (Handbook of Systematic Malacology), Johannes Thiele 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 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's 1965 classification based on hinge tooth morphology:

! Subclass

Order (biology)>Order

Palaeotaxodonta

*Nuculoida

Cryptodonta

†Praecardioida

Solemyoida

Pteriomorphia

Arcoida (ark shells)

†Cyrtodontoida

Limoida (file shells)

Mytiloida (true mussels)

Ostreoida (oysters, formerly included in Pterioida)

†Praecardioida

Pterioida (pearl oysters, pen shells)

Paleoheterodonta

†Trigonioida

Unionoida (freshwater mussels)

†Modiomorpha

Heterodonta

†Cycloconchidae

†Hippuritoida

†Lyrodesmatidae

Myoida (most "soft shell calms" razor clams)

†Redoniidae

Veneroida (most "hard shell calms", cockles, etc.)

Anomalodesmata

Pholadomyoida

The monophyly 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.

An alternative systematic scheme exists according to gill 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. 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.

10 !Nuculoidea !Sapretoidea !Solemyoidea !Manzanelloidea !Nuculanoidea !PTERIOMORPHA !Mytiloidea !Arcoidea !Pinnoidea !Pterioidea !Ostreoidea !Dimyoidea !Anomioidea !Plicatuloidea !Pectinoidea !Limoidea !PALAEOHETERODONTA !Trigonioidea !Unionoidea !HETERODONTA !Crassatelloidea !Thyasiroidea !Lucinoidea !Galeommatoidea !Cyamioidea !Solenoidea !Hiatelloidea !Gastrochaenoidea !Chamoidea !Cardioidea !Tellinoidea !Glossoidea !Arcticoidea !Cyrenoidea !Sphaerioidea !Veneroidea !Hemidonacoidea !Cyrenoidoidea !Ungulinoidea !Mactroidea !Dreissenoidea !Myoidea !Pholadoidea !Limoidea !(ANOMALODESMATA) !Pholadomyoidea !Clavagelloidea !Pandoroidea !Verticordioidea !Cuspidarioidea

Number of !! Families !! Genera !! Species
! PROTOBRANCHIA	|	49	700
|	1	8	170
|	1	ca. 5	10
|	1	2	30
|	1	2	20
|	6	32	460
	|
|	25	240 (incl. 2 freshwater)	2000 (incl. 11 freshwater)
|	1	50 (1 freshwater)	400 (5 freshwater)
|	7	60 (1 freshwater)	570 (6 freshwater)
|	1	3 (+)	50
|	5	9	80
|	2	23	80
|	1	3	15
|	2	9	30
|	1	1	20
|	4	68	500
|	1	8	250
	|
|	7 (incl. 6 freshwater)	171 (incl. 170 freshwater)	908 (incl. 900 freshwater)
|	1	1	8
|	(6 freshwater)	(170 freshwater)	(900 freshwater)
	|
|	64 (incl. 1 freshwater)	800 (incl. 16 freshwater)	5600 (incl. 270 freshwater)
|	5	65	420
|	1	ca. 12	ca. 100
|	2	ca. 85	ca. 500
|	ca. 4	ca. 100	ca. 500
|	3	22	140
|	2	17 (2 freshwater)	130 (4 freshwater)
|	1	5	25
|	1	7	30
|	1	6	70
|	2	38	260
|	5	110 (2 freshwater)	900 (15 freshwater)
|	2	20	110
|	2	6	13
|	1	6 (3 freshwater)	60 (30 freshwater)
|	(1 freshwater)	(5 freshwater)	(200 freshwater)
|	4	104	750
|	1	1	6
|	1	1	6
|	1	16	100
|	4	46	220
|	1	3 (2 freshwater)	20 (12 freshwater)
|	3	15 (1 freshwater)	130 (1 freshwater)
|	2	34 (1 freshwater)	200 (3 freshwater)
|	1	8	250
|	(14)	(71)	(770)
|	2	3	20
|	1	2	20
|	7	30	250
|	2	16	160
|	2	20	320

Anatomy

anatomy]] Margaritifera margaritifera]] , 2: growth lines, 3: ligament, 4: umbo]]

, Tridacna gigas]] Bivalve shells vary greatly in shape; some are globular, others flattened, while others are elongated 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 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 than in other molluscs; they have no brain. 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 mechanoreceptors or chemoreceptors.

Scallops have complex eyes with a lens and retina, 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, a patch of sensory cells located below the posterior adductor muscle. It may serve to taste the water, or measure its turbidity, 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.

Statocysts 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 oysters and scallops, these retractors are absent.

Circulation and respiration

Bivalves have an open circulatory system that bathes the organs in hemolymph. The heart has three chambers; two auricles receiving blood from the gills, and a single ventricle. The ventricle is muscular and pumps hemolymph into the aorta, 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. 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, although some species are known to possess haemoglobin dissolved directly into the serum.

Mantle and shell

In bivalves the mantle forms a thin membrane surrounding the body which secretes the valves, ligament 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 valves, which are made of either calcite (as with oysters) or both calcite and aragonite, usually with the aragonite forming an inner layer (as with the Pterioida). The outermost layer is the periostracum, composed of a horny organic substance. 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 by the ligament, which is composed of the tensilium and resilium. The ligament opens the shell.

Digestive system

Modes of feeding

The majority of bivalves are filter feeders, using their gills 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 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 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, stomach, and 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 chitinous gizzard that helps grind up the food before digestion.

Excretory system

Like most other molluscs, the excretory organs of bivalves are nephridia. There are two nephridia, each consisting of a long, glandular tube, which opens into the body cavity just beneath the heart, and a bladder. 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. The gonads 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, later becoming a veliger. Freshwater bivalves of the Unionoida have a different life cycle: they become a glochidium, which attaches to any firm surface to avoid the danger of being swept downsteam. Glochidia can be serious pests of fish if they lodge in the fish gills.

Some of the species in the freshwater mussel family, Unionidae, 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 in which the young mussel resides. It feeds by breaking down and digesting the tissue of the fish within the cyst.

Behaviour

bivalves with their siphons visible]] 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 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 stars and oystercatchers.

Feeding

Most bivalves are filter feeders 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 across the mantle cavity which pumps in food.

Filibranchs and lamellibranchs trap food with a mucous coating on the ctenidia; the filibranchs and lamellibranchs are differentiated by the way the ctenidia are joined

Movement

Razor shells can dig themselves into the sand with great speed to escape predation. Scallops, and file clams can swim to escape a predator, clapping their valves together to create a jet of water. Cockles 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 can produce a noxious secretion when threatened, and the fan shells of the same family have a unique, acid-producing organ.

Comparison with brachiopods

, a bivalve with taxodont dentition from the Pliocene of Cyprus]] Bivalves are 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 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 and came to dominate over brachiopods during the Palaeozoic. By the Permian-Triassic extinction event bivalves were undergoing a huge radiation 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 and relegated to minor niches 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.

References

External links

Marine Bivalve Shells of the British Isles – Online Identification Guide

Museum of Paleontology - Palaeontology from the University of California, Berkeley

Bioerosion website at The College of Wooster

A Database of Cambodian Marine Bivalves

Category:Fossils

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