Bivalvia, commonly called the bivalves, is a taxonomic class of marine and freshwater molluscs that have a laterally compressed body enclosed by a shell in two hinged parts. This class includes the clams, oysters, mussels, scallops and numerous other families. The majority are filter feeders and have no head or rasping tongue. The gills have evolved into ctenidia, specialised organs for feeding and breathing. Most bivalves bury themselves in sediment on the seabed where they are safe from predation. Others lie on the sea floor or attach themselves to rocks or other hard surfaces. A few bore into wood, clay or stone and live inside these substances. Some bivalves, such as the scallops, can swim.
The shell of a bivalve is composed of calcium carbonate and consists of two, usually similar, parts called valves. These are joined together at one edge by a flexible ligament that, in conjunction with nearby interlocking "teeth" on the two valves, are known collectively as the hinge. This arrangement allows the shell to be opened and closed without the two halves becoming disarticulated. The shell is typically bilaterally symmetrical, with the hinge lying in the sagittal plane. Adult shell sizes vary from fractions of a millimetre to over a metre in length but the majority of species do not exceed ten centimetres (four inches).
Bivalves have long been a part of the diet of coastal communities. Oysters were cultured in ponds by the Romans and mariculture has more recently become an important source of bivalves for food. Modern knowledge of molluscan reproductive cycles has led to the development of hatcheries and new culture techniques. A better understanding of the hazards of eating raw and undercooked shellfish has led to improved storage and processing. Besides their use as food, oysters are the most common source of natural pearls. The shells of bivalves are used in craftwork and in the manufacture of jewellery and buttons. Bivalves have also been used in the biocontrol of pollution.
Bivalves first appeared in the fossil record in the early Cambrian more than 500 million years ago. The total number of living species is approximately 9,200. These species are placed within 1,260 genera and 106 families. Marine bivalves (including brackish water and estuarine species) represent about 8,000 species, combined in four subclasses and 99 families with 1,100 genera. The largest recent marine families are Veneridae with more than 680 species and the Tellinidae and Lucinidae, each with over 500 species. The freshwater bivalves include seven families, the largest of which is theUnionidae with about 700 species.
| Bivalvia Temporal range: early Cambrian–Recent[1][2] | |
|---|---|
 | |
| "Acephala", from Ernst Haeckel'sKunstformen der Natur (1904) | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Mollusca |
| Class: | Bivalvia Linnaeus, 1758 |
| Subclasses | |
Bivalvia, commonly called the bivalves, is a taxonomic class of marine and freshwater molluscs that have a laterally compressed body enclosed by a shell in two hinged parts. This class includes the clams, oysters, mussels, scallops and numerous other families. The majority are filter feeders and have no head or rasping tongue. The gills have evolved into ctenidia, specialised organs for feeding and breathing. Most bivalves bury themselves in sediment on the seabed where they are safe from predation. Others lie on the sea floor or attach themselves to rocks or other hard surfaces. A few bore into wood, clay or stone and live inside these substances. Some bivalves, such as the scallops, can swim.
The shell of a bivalve is composed of calcium carbonate and consists of two, usually similar, parts called valves. These are joined together at one edge by a flexible ligament that, in conjunction with nearby interlocking "teeth" on the two valves, are known collectively as the hinge. This arrangement allows the shell to be opened and closed without the two halves becoming disarticulated. The shell is typically bilaterally symmetrical, with the hinge lying in the sagittal plane. Adult shell sizes vary from fractions of a millimetre to over a metre in length but the majority of species do not exceed ten centimetres (four inches).
Bivalves have long been a part of the diet of coastal communities. Oysters were cultured in ponds by the Romans and mariculture has more recently become an important source of bivalves for food. Modern knowledge of molluscan reproductive cycles has led to the development of hatcheries and new culture techniques. A better understanding of the hazards of eating raw and undercooked shellfish has led to improved storage and processing. Besides their use as food, oysters are the most common source of natural pearls. The shells of bivalves are used in craftwork and in the manufacture of jewellery and buttons. Bivalves have also been used in the biocontrol of pollution.
Bivalves first appeared in the fossil record in the early Cambrian more than 500 million years ago. The total number of living species is approximately 9,200. These species are placed within 1,260 genera and 106 families. Marine bivalves (including brackish water and estuarine species) represent about 8,000 species, combined in four subclasses and 99 families with 1,100 genera. The largest recent marine families are Veneridae with more than 680 species and the Tellinidae and Lucinidae, each with over 500 species. The freshwater bivalves include seven families, the largest of which is theUnionidae with about 700 species.
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[edit]Etymology
The taxonomic term Bivalvia was first used by Linnaeus in the 10th edition of his Systema Naturae in 1758 to refer to molluscs that had shells composed of two valves.[3] More recently the class was known as Pelecypoda, meaning "axe-foot" (based on the shape of the soft parts of the animal). Other names that have been used for this class include Lamellibranchia (based on the plate-like gill elements, the ctenidium) and Acephala (based on the fact that, unlike other molluscs, these animals have no head).[4]
The name bivalve is derived from the Latin bis, meaning "two", and valvae, meaning "leaves of a door".[4] Not all animals that have shells with two hinged parts are classified under Bivalvia; other animals with paired valves include certain gastropods (small sea snails in the family Juliidae),[5] members of the phylum Brachiopoda [6] and the minute crustaceans known as ostracods[7] and conchostrachans.[8]
[edit]Anatomy
Digestive system
[edit]Modes of feeding
Most bivalves are filter feeders, using their gills to capture particulate food such as phytoplankton from the water. The Protobranchs feed in a different way, scraping detritus from the seabed, and this may be the original mode of feeding used by all bivalves 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, and cilia, which transport the particles back to the palps. These then sort the particles, rejecting those that are unsuitable or too large to digest, and conveying others to the mouth.[20]
In the Filibranchia and Eulamellibranchia, water is drawn into the shell from the posterior ventral surface of the animal, passes upwards through the gills and doubles back to be expelled just above the intake. In burrowing species, there may be two elongated, retractable siphons reaching up to the seabed, one each for the inhalant and exhalant streams of water. The gills of filter-feeding bivalves are known as ctenidia and 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, have become 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.[28]
A few bivalves, such as the granular poromya (Poromya granulata), are carnivorous, eating much larger prey than the tiny microalgae consumed by other bivalves. 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 draw water in through the inhalant siphon which is modified into a cowl-shaped organ, sucking in small crustaceans and worms at the same time. The siphon can be retracted quickly and inverted, bringing the prey within reach of the mouth. The gut is modified so that large food particles can be digested.[27]
The unusual genus, Entovalva, is endosymbiontic, being found only in the oesophagus of sea cucumbers. It has mantle folds that completely surround its small valves. When the sea cucumber sucks in sediment, the bivalve allows the water to pass over its gills and extracts fine organic particles. To prevent itself from being swept away, it attaches itself with byssal threads to the host'sthroat. The sea cucumber is unharmed.[29]
[edit]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 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.[30] Waste material is consolidated in the rectum and voided as pellets into the exhalent water stream through an anal pore. Feeding and digestion are synchronized with diurnal and tidal cycles.[31]
Carnivorous bivalves have a greatly reduced style, and a chitinous gizzard that helps grind up the food before digestion. In other ways their gut is similar to that of filter-feeding bivalves.[20]
[edit]Excretory system
Like most other molluscs, the excretory organs of bivalves are a pair of nephridia. Each of these consists of a long, looped, glandular tube, which opens into the body cavity just beneath the heart, and a bladder to store urine. There are also pericardial glands, either lining the auricles of the heart or attached to the pericardium, which serve as extra filtration organs. Metabolic waste is voided from the bladders through a pair of openings near the front of the upper part of the mantle cavity, from where it joins the stream of exhalant water.[32]
[edit]Reproduction and development
The sexes are usually separate in bivalves but some hermaphroditism is known. The gonads are located close to the intestines, and either open into the nephridia, or through a separate pore into the mantle cavity.[33] The ripe gonads of male and females release sperm and eggs into the water column. Spawning may take place continually or be triggered by environmental factors such as day length, water temperature or the presence of sperm in the water. Some species are "dribble spawners" but others release their gametes in batches or all at once. Mass spawning events sometimes take place when all the bivalves in an area synchronise their release of spawn.[34]
Fertilization is usually external. Typically, there is a short stage lasting a few hours or days before the eggs hatch into trochophore larvae. These later develop into veliger larvae which settle on the seabed and undergo metamorphosis into juveniles known as spat.[35] In some species, such as those in the genus Lasaea, females draw water containing sperm in through their inhalant siphons and fertilisation is inside the female. These species then brood the young inside their mantle cavity, eventually releasing them into the water column as veliger larvae or as crawl-away juveniles.[36]
Most of the bivalve larvae that hatch from eggs in the water column feed on diatoms or other phytoplankton. In temperate regions, about 25% of species are lecithotrophic, depending on nutrients stored in the yolk of the egg where the main energy source is lipids. The longer the period is before the larva first feeds, the larger the egg and yolk need to be. The reproductive cost of producing these energy-rich eggs is high and they are usually smaller in number. For example, the Baltic tellin (Macoma balthica) produces few, high-energy eggs. The larvae hatching out of these rely on the energy reserves and do not feed. After about four days they become D-stage larvae, the stage at which they first develop hinged, D-shaped valves. These larvae have a relatively small dispersal potential before settling out. The common mussel (Mytilus edulis) produces ten times as many eggs that hatch into larvae and soon need to feed in order to survive and grow. They can disperse more widely as they remain planktonic for a much longer time.[37]
Freshwater bivalves in the order Unionoida have a different life cycle. Sperm is drawn into a female's gills with the inhalant water and internal fertilization takes place. The eggs hatch into glochidialarvae that develop within the female's shell. Later they are released and attach themselves parasitically to the gills or fins of a fish host. After several weeks they drop off their host, undergo metamorphosis and develop into juveniles on the substrate. An advantage of this to the molluscs is that they may be able to disperse upstream with their temporary hosts rather than being constantly swept downstream by the water flow.[38]
Some of the species in the freshwater mussel family, Unionidae, commonly known as pocketbook mussels, have evolved an unusual reproductive strategy. The female's mantle protrudes from the shell and develops into an imitation small fish, complete with fish-like 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 its gills, dousing the inquisitive fish with its 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 around each larva. The larvae then feed by breaking down and digesting the tissue of the fish within the cysts. After a few weeks they release themselves from the cysts and fall to the stream bed as juvenile molluscs. The fish are relatively unharmed.[39]
[edit]Comparison with brachiopods
Brachiopods are shelled marine organisms that superficially resembled bivalves in that they were of similar size and had a hinged shell in two parts. However, brachiopods evolved from a very different ancestral line, and the resemblance to bivalves only arose because of a similar lifestyle. The differences between the two groups are due to their separate ancestral origins. Different initial structures have been adapted to solve the same problems, a case of convergent evolution. In modern times, brachiopods are not as common as bivalves.[40]
Both groups have a shell consisting of two valves, but the organization of the shell is quite different in the two groups. In brachiopods, the two valves are positioned on the dorsal and ventral surfaces of the body, while in bivalves, the valves are on the left and right sides of the body, and are normally mirror images of one other. Brachiopods have a lophophore, a coiled, rigid cartilaginous internal apparatus adapted for filter feeding, a feature shared with two other major groups of marine invertebrates, the bryozoans and the phoronids. Brachiopod shells are often made of calcium phosphate as well as calcium carbonate, whereas bivalve shells are composed entirely of calcium carbonate.[41]
[edit]Evolutionary history
The Cambrian explosion took place 543 to 525 million years ago. In this geologically brief period all the major animal phyla appeared and these included the first creatures with mineralized skeletons. Brachiopods and bivalves made their appearance at this time and left their fossilized remains behind in the rocks.[42] The fossils of both were formed when the sediment in which they were buried hardened into rock. Often it is the impression made by the valves that remains as a fossil rather than the valves themselves. During the Early Ordovician there was a great increase in the diversity of bivalve species and the dysodont, heterodont and taxodont dentitions evolved. By the early Silurian, the gills were becoming adapted for filter feeding and during theDevonian and Carboniferous periods siphons first appeared which, with the newly developed muscular foot, allowed the animals to bury themselves deep in the sediment.[43]
By the middle of the Paleozoic, around 400 million years ago, the brachiopods were among the most abundant filter feeders in the ocean and over 12,000 fossil species are recognized.[44] By the Permian-Triassic extinction event 250 million years ago, bivalves were undergoing a huge radiation of diversity. The bivalves were hard hit by this event but re-established themselves and thrived during the Triassic period that followed. In contrast, the brachiopods lost 95% of their species diversity.[41] Scientists have speculated that the ability of some bivalves to burrow and thus avoid predators was a major factor in their success. Other new adaptations within various families allowed species to occupy previously unused evolutionary niches. These included increasing relative buoyancy in soft sediments by developing spines on the shell, gaining the ability to swim, and in a few cases, adopting predatory habits.[43]
It was for a long time thought that bivalves were better adapted to aquatic life than brachiopods were, out-competing and relegating them to minor niches in later ages. These two taxa appeared in textbooks as an example of replacement by competition. Evidence given for this included the fact that bivalves needed less food to subsist because of their energetically efficient ligament-muscle system for opening and closing valves. An alternative view is that the prominence of modern bivalves over brachiopods might merely be due to chance disparities in their response to extinction events.[45]
[edit]Diversity of extant bivalves
The adult maximum size of living species of bivalve ranges from 0.52 mm (0.02 in) in Condylonucula maya,[46] a nut clam, to a length of 1,532 millimetres (60.3 in) in Kuphus polythalamia, an elongated, burrowing shipworm.[47] However, the species generally regarded as the largest living bivalve is the giant clam Tridacna gigas, which can grow to a length of 1,200 millimetres (47 in) and a weight of more than 200 kilograms (441 lbs).[48] The largest known extinct bivalve is a species of Platyceramus whose fossils measure up to 3,000 mm (118 in) in length.[49]
In his 2010 treatise, Compendium of Bivalves, Markus Huber gives the total number of living bivalve species as about 9,200 combined in 106 families.
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