| Bivalves Fossil range: Cambrian - Recent |
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 "Acephala" from Ernst Haeckel's Kunstformen der Natur, 1904
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Anomalosdesmata |
Bivalves are molluscs belonging to the class Bivalvia. Molluscs are animals belonging to the phylum Mollusca. There are around 250000 extant Species within the phylum with an estimated 70000 They have two-part shells, and typically both valves are symmetrical along the hinge line. Symmetry generally conveys two primary meanings The first is an imprecise sense of harmonious or aesthetically-pleasing proportionality and balance such that it reflects beauty or The class has 30,000 species, including scallops, clams, oysters and mussels. In Biology, a species is one of the basic units of Biological classification and a Taxonomic rank. A scallop (ˈskɒləp or /ˈskæləp/ is a marine Bivalve Mollusk of the family Pectinidae. Clam is a word which can be used for all some or only a few Species of Bivalve Mollusks the word is a Common name which has The common name oyster is used for a number of different groups of Bivalve Mollusks most of which live in marine habitats or Brackish water. The common name mussel is used for members of several different families of Clams or Bivalve Molluscs, from both saltwater and freshwater habitats Other names for the class include Bivalva, Pelecypoda, and Lamellibranchia.
Bivalves are exclusively aquatic; they include both marine and freshwater forms. However some, for instance the mussels, can survive out of water for short periods by closing their valves.
Bivalves are unique among the molluscs for lacking a radula; they feed by siphoning and filtering large particles from water. The radula is a toothed Chitinous ribbon typically used for scraping cutting and chewing food before it enters the Esophagus. Some bivalves are epifaunal: that is, they attach themselves to surfaces in the water, by means of a byssus or organic cementation. Fauna is all of the Animal life of any particular region or time The word Byssus has two related meanings and one rather different one according to the context IN BIOLOGY The filaments created by numerous different kinds of marine Others are infaunal: they bury themselves in sand or other sediments; these forms typically have a strong digging foot. Fauna is all of the Animal life of any particular region or time Some bivalves, such as scallops, can swim. A scallop (ˈskɒləp or /ˈskæləp/ is a marine Bivalve Mollusk of the family Pectinidae.
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Systematics
The systematic layout presented here is according to Newell's 1965 classification based on hinge teeth morphology. There exists no robust phylogeny, and due to the plethora of fossil lineages, DNA sequence data is only of limited use should the subclasses turn out to be paraphyletic. FOSSIL is a standard protocol for allowing serial communication for Telecommunications programs under the DOS Operating system. A DNA sequence or genetic sequence is a succession of letters representing the Primary structure of a real or hypothetical DNA Molecule In Phylogenetics, a group of organisms is said to be paraphyletic if the group contains its most recent common ancestor but does not contain all The monophyly of the Anomalosdesmata is especially disputed, but this is of less consequence as that group does not include higher-level prehistoric taxa. A clade is a taxonomic group comprising a single Common ancestor and all the descendants of that ancestor A taxon (plural taxa) or taxonomic unit, is a name designating an organism or a group of Organisms In Biological nomenclature according to
Subclass Palaeotaxodonta
Subclass Cryptodonta
- †Praecardioida
- Solemyoida
Subclass Pteriomorphia (oysters, mussels, etc)
- Arcoida
- †Cyrtodontoida
- Mytiloida
- Ostreoida - formerly included in Pterioida
- †Praecardioida
- Pterioida
Subclass Paleoheterodonta
- †Trigonioida
- Unionoida (typical freshwater mussels)
- †Modiomorpha
Subclass Heterodonta (typical clams, cockles, rudists, etc)
- †Cycloconchidae
- †Hippuritoida
- †Lyrodesmatidae
- Myoida
- †Redoniidae
- Veneroida
Subclass Anomalosdesmata
There also exists an alternative systematic scheme according to gill morphology (Franc 1960). Nuculoida is an order of Bivalves It is the sole member of the Subclass Palaeotaxodonta. This article is about the taxonomic rank for the sequence of species in a taxonomic list see Taxonomic order In scientific classification used Nuculoida is an order of Bivalves It is the sole member of the Subclass Palaeotaxodonta. Cryptodonta is a Subclass of the Bivalves It contains a single extant order Solemyoida, while the Praecardioida are known only from Solemyoida is an order of Bivalve Molluscs Families in the order Solemyoida Manzanellidae Chronic 1952 Pteriomorphia is a Mollusc Subclass of the Bivalvia. Apart from the orders Arcoida, Mytiloida, Ostreoida, and The Arcoida is an Extant order of clams or Bivalve Molluscs This order dates back to the lower Ordovician period Mytiloida is an order of Molluscs in the subclass Pteriomorpha containing the single family Mytilidae. The order Ostreoida includes the True oysters and a number of other related families of Bivalves About eleven families are recognised Pterioida is an order of large and medium-sized saltwater clams marine Bivalve Mollusks It includes five families among them the Paleoheterodonta is a Mollusc Subclass of the Bivalvia. It contains the extant order Unionoida (freshwater mussels and the prehistoric The Superfamily Trigoniacea are medium-sized saltwater Clams marine Bivalve Mollusks This superfamily is the sole representative Unionoida is an order within the class Bivalvia. Members of the order Unionoida which includes the pearly freshwater Mussels are found on six continents Heterodonta is a Mollusc subclass in the class Bivalvia and contains the orders † Cycloconchidae, † Hippuritoida (rudists † Lyrodesmatidae Clam is a word which can be used for all some or only a few Species of Bivalve Mollusks the word is a Common name which has Rudists are a group of bizarrely shaped marine Heterodont Bivalves that arose during the Jurassic, and became so diverse during the Myoida is an order of Bivalve Molluscs Classification Superfamily Anomalodesmacea Superfamily The Veneroida or veneroids are an order of Bivalve Molluscs They include some familiar forms such as saltwater Clams and Pholadomyoida is an order of Bivalve Molluscs It is the sole member of the Subclass Anomalosdesmata. Pholadomyoida is an order of Bivalve Molluscs It is the sole member of the Subclass Anomalosdesmata. A gill is an anatomical structure found in many aquatic organisms This distinguishes between Protobranchia, Filibranchia, and Eulamellibranchia. The first corresponds to Newell's Palaeotaxodonta + Cryptodonta, the second to his Pteriomorphia, and the last contains all other groups. In addition, Franc separated the Septibranchia from his eulamellibranchs, but this would seem to make the latter paraphyletic.
Anatomy
The shapes of bivalve shells vary greatly - some are rounded and globular, others are flattened and plate-like, while still others, have become greatly elongated in order to aid burrowing. The shipworms of the family Teredinidae have greatly elongated bodies, but the shell valves are much reduced and restricted to the anterior end of the body, where they function as burrowing organs, allowing the animal to dig tunnels through wood. Shipworms are not worms at all but rather a group of unusual saltwater clams with very reduced shells notorious for boring into (and eventually destroying wooden structures which [1]
Nervous system
The sedentary habit of the bivalves has led to the development of a simpler nervous system than in other molluscs - so simple, in fact, that there is no brain. The nervous system is a Network of specialized cells that communicate information about an animal's surroundings and itself The brain is the center of the Nervous system in animals All Vertebrates and the majority of Invertebrates have a brain In all but the simplest forms the neural ganglia are united into two cerebropleural ganglia either side of the oesophagus. In Anatomy, a ganglion (pl ganglia) is a tissue mass. Neurology In neurological contexts ganglia are composed mainly of The esophagus or oesophagus (see American and British English spelling differences) sometimes known as the gullet, is an organ in 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. [2] These ganglia are both connected to the cerebropleural ganglia by nerve fibres. There may also be siphonal ganglia in bivalves with a long siphon. A siphon (also spelled syphon) is a continuous tube that allows liquid to drain from a reservoir through an intermediate point that is higher than the reservoir the flow being
Senses
The sensory organs of bivalves are not well developed, and are largely a function of the posterior mantle margins. The organs are usually tentacles and most are typically mechanoreceptors and chemoreceptors. A mechanoreceptor is a Sensory receptor that responds to mechanical pressure or distortion A chemosensor, also known as chemoreceptor, is a Sensory receptor that transduces a chemical signal into an Action potential.
Scallops have complex eyes with a lens and retina, but most other bivalves have much simpler eyes, if any. A scallop (ˈskɒləp or /ˈskæləp/ is a marine Bivalve Mollusk of the family Pectinidae. The lens is a transparent biconvex structure in the Eye that along with the Cornea, helps to Refract Light to be focused The vertebrate retina is a light sensitive part inside the inner layer of the Eye. There are also light-sensitive cells in all bivalves, that can detect shadows falling on the animal. [2]
In the septibranchs the inhalant siphon is surrounded by vibration-sensitive tentacles for detecting prey. [3]
Statocysts within the organism help the bivalve to sense its orientation, which then be corrected if need be. The statocyst is a balance organ present in some aquatic Invertebrates ( Cnidarians Ctenophores Bilaterians) [4]
Muscles
The muscular system is comprised of the posterior and anterior adductor muscles, although the anterior may be reduced or even lost in some species. In fields of Anatomy, anatomical terms of location are descriptive terms to help identify relative positions or directions within a species In Human anatomy, the adductor muscles of the hip is a group of Muscles of the hip.
The paired anterior and posterior pedal retractor muscles operate the animal's foot. The foot is an Anatomical structure found in many Animals It is the terminal portion of a limb which bears weight and allows Locomotion. In some bivalves, such as oysters and scallops, these retractors are absent. The common name oyster is used for a number of different groups of Bivalve Mollusks most of which live in marine habitats or Brackish water. A scallop (ˈskɒləp or /ˈskæləp/ is a marine Bivalve Mollusk of the family Pectinidae.
Circulation
Bivalves have an open circulatory system that bathes the organs in hemolymph. This is an article about the rock music band "Circulatory System" Hemolymph or haemolymph is the Blood analogue used by all Arthropods and most Mollusks that have an Open circulatory system.
Mantle and shell
In bivalves the mantle, a thin membrane surrounding the body, secretes the main shell valves, ligament and hinge teeth, the mantle lobes secreting the valves and the mantle crest the other parts. The bivalve shell is one kind of Seashell, and in life it is composed of two parts two valves In Anatomy, the term ligament is used to denote three different types of structures Fibrous tissue that connects Bones to other bones 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 purposes. A siphon (also spelled syphon) is a continuous tube that allows liquid to drain from a reservoir through an intermediate point that is higher than the reservoir the flow being Filter feeders (also known as suspension feeders) are Animals that feed by straining suspended matter and food particles from water typically by passing the water
The shell is composed of two calcareous valves, which are made of either calcite (as with, e. Calcareous refers to a Sediment, Sedimentary rock, or Soil type which is formed from or contains a high proportion of Calcium carbonate in the For other uses see Valve (disambiguation. For the electronic component see Thermionic valve. Calcite is a carbonate mineral and the most stable polymorph of Calcium carbonate ( Ca[[carbon C]] O 3 g. oysters) or both calcite and aragonite, usually with the aragonite forming an inner layer, as with the pterioida. Aragonite is a Carbonate mineral, one of the two common naturally occurring polymorphs of Calcium carbonate, Ca[[carbon C]] O 3 Pterioida is an order of large and medium-sized saltwater clams marine Bivalve Mollusks It includes five families among them the The outermost layer is the periostracum, composed of a horny organic substance. The periostracum is a thin organic coating or "skin" which is the outermost layer of the shell of many Mollusks for example the shells of many bivalves This forms the familiar coloured layer on the shell. [5] 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. 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 (like humans that walk upright In Anatomy, the term ligament is used to denote three different types of structures Fibrous tissue that connects Bones to other bones The ligament opens the shells.
Reproduction
The sexes are usually separate, but some hermaphroditism is known. A hermaphrodite is an organism having both male and female reproductive organs Bivalves practice external fertilisation. External fertilization is a form of Fertilization in which a Zilo cell is united with an Egg cell external to the body of the female
Typically the marine bivalve will start life as a trochophore, later becoming a veliger. A trochophore (ˈtrɒkəfɔər also spelled trocophore) is a type of marine Larva with several bands of Cilia. A veliger is the free-swimming Planktonic Larva of many kinds of marine and fresh-water Gastropod Molluscs as well as a number of Freshwater bivalves have a different life cycle: they become a glochidium, which attaches to any firm surface to avoid the danger of being swept downsteam. Glochidium is Larva of big freshwater Bivalves See also Veliger Trochophore Glochidia can become serious pests of fish. Fish are aquatic Vertebrate animals that are typically ectothermic (previously Cold-blooded) covered with scales, and equipped with two
Behaviour
The radical structure of the bivalves affects 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 such as mussels, drying out. 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 the Starfish and Oystercatcher. Starfish (also called sea stars) are any Echinoderms belonging to the class Asteroidea. "Oystercatchers" is also a novel by Susan Fletcher. The oystercatchers are a group of Waders they form the family
Feeding
Most bivalves are filter feeders (although some have taken up scavenging and predation), extracting organic matter from the sea in which they live. Filter feeders (also known as suspension feeders) are Animals that feed by straining suspended matter and food particles from water typically by passing the water Nephridia remove the waste material. Nephridia are invertebrate organs which function similarly to Kidneys They remove Metabolic wastes from an animal's body 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, defind by their gill structure. The Protobranchs use their ctenida solely for respiration, and the labial palps catch their food. The filibranchs and lamellibranchs trap the food with a mucous coating on the ctenida; the filibranchs and lamellibranchs are differentiated by the way the ctenida are joined. Finally, the septibranchs possess a septum across the mantle cavity, which pumps in food. A septum ( Latin: something that encloses; plural Septa) is a partition separating two cavities or spaces
Movement
Razor shells (Ensis spp. The razor shell, Ensis arcuatus, also called razor clam or Razor fish, is a Bivalve of the family Solenidae. ---- Ensis is a Genus of medium-sized edible saltwater Clams littoral Bivalve Mollusks in the family ) can dig themselves into the sand with great speed to escape predation. Scallops can swim to escape an enemy, 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 (Limidae) can produce a noxious secretion when threatened, and the fan shells of the same family have a unique, acid-producing organ. Limidae is a family of Bivalve Molluscs in the order Limoida.
Comparison to Brachiopods
Bivalves are laterally combined and have a shell composed of two valves. The valved shell makes them superficially similar to brachiopods, but the construction of the shell is completely different in the two groups: in brachiopods, the two valves are on the upper and lower surfaces of the body, while in bivalves, they are on the left and right sides. Brachiopods (from Latin brachium, arm + New Latin -poda, foot are a small phylum of Benthic Invertebrates Also
Bivalves appeared late in the Cambrian explosion and came to dominate over brachiopods during the Palaeozoic; indeed, by the end-Permian extinction, bivalves were undergoing a huge radiation in numbers while brachiopods (along with around 95% of all species) were devastated. The Cambrian explosion or Cambrian radiation was the seemingly rapid appearance of most major groups of complex Animals around, as evidenced by the Brachiopods (from Latin brachium, arm + New Latin -poda, foot are a small phylum of Benthic Invertebrates Also The Paleozoic or Palaeozoic Era (from the Greek palaio (παλαιο "old" and zoe (ζωη "life" meaning "ancient life" The Permian–Triassic (P–Tr extinction event, informally known as the Great Dying, was an Extinction event that occurred, and 70 percent of terrestrial
Bivalves appear to be better adapted to aquatic life than the Brachiopods were. Far more sophisticated than the brachiopods, bivalves use an energetically-efficient ligament-muscle system for opening valves, and thus require less food to subsist.
References
- ^ "Description" in [1]
- ^ a b Nervous System and Sense Organs
- ^ In "an analysis of the evolution of the septibranch condition"
- ^ "a statocyst. . . " in [2]
- ^ "The shell of bivalve molluscs" in [3]
- Franc, A. (1960): Classe de Bivalves. In: Grassé, Pierre-Paul: Traite de Zoologie 5/II.
- Newell, N. D. (1969): [Bivalvia systematics]. In: Moore, R. C. : Treatise on Invertebrate Paleontology Part N.
- Jay A. Schneider (Nov 2001). "Bivalve Systematics During the 20th Century" 75 (6): 1119–1127. doi:. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document.
External links
- Museum of Paleontology - Palaeontology from the University of California, Berkeley
- Bioerosion website at The College of Wooster
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