Bivalvia 202240 223839207 2008-07-06T02:19:10Z 67.78.196.2 /* Anatomy */ Minor grammar edit {{Taxobox | name = Bivalves | fossil_range = [[Cambrian]] - Recent | image = Haeckel Acephala.jpg | image_width = 250px | image_caption = "Acephala" from [[Ernst Haeckel]]'s ''[[Kunstformen der Natur]]'', 1904 | regnum = [[Animal]]ia | phylum = [[Mollusca]] | classis = '''Bivalvia''' | classis_authority = [[Carolus Linnaeus|Linnaeus]], [[1758]] | subdivision_ranks = [[Subclass]]es | subdivision = [[Anomalosdesmata]]<br/> [[Cryptodonta]]<br/> [[Heterodonta]]<br/> [[Paleoheterodonta]]<br/> [[Palaeotaxodonta]]<br/> [[Pteriomorphia]]<br/> and see text }} [[Image:CornishMussels.JPG|thumb|250px|right|Mussels in the intertidal zone in [[Cornwall]], England.]] [[Image:JurassicMarineIsrael.JPG|thumb|250px|Fossil [[gastropod]] and attached mytilid bivalves on a Jurassic limestone bedding plane in southern [[Israel]].]] [[Image:Aviculopecten_subcardiformis01.JPG|thumb|right|250px|''Aviculopecten subcardiformis''; an extinct pectenoid bivalve from the Logan Formation (Lower [[Carboniferous]]) of [[Wooster, Ohio]] (external mold).]] '''Bivalves''' are [[mollusc]]s belonging to the class '''Bivalvia'''. They have two-part [[animal shell|shell]]s, and typically both valves are [[symmetry|symmetrical]] along the hinge line. The class has 30,000 [[species]], including [[scallop]]s, [[clam]]s, [[oyster]]s and [[mussel]]s. 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. Some bivalves are [[Fauna (animals)#Epifauna|epifaunal]]: that is, they attach themselves to surfaces in the water, by means of a [[byssus]] or organic cementation. Others are [[Fauna (animals)#Infauna|infaunal]]: they bury themselves in sand or other sediments; these forms typically have a strong digging foot. Some bivalves, such as [[scallop]]s, can swim. ==Systematics== The systematic layout presented here is according to Newell's 1965 classification based on hinge teeth morphology. There exists no robust phylogeny, and due to the plethora of [[fossil]] lineages, [[DNA sequence]] data is only of limited use should the [[subclass]]es turn out to be [[paraphyletic]]. The [[monophyly]] of the Anomalosdesmata is especially disputed, but this is of less consequence as that group does not include higher-level prehistoric [[taxa]]. Subclass [[Palaeotaxodonta]] *[[Order (biology)|Order]] [[Nuculoida]] Subclass [[Cryptodonta]] *†[[Praecardioida]] *[[Solemyoida]] Subclass [[Pteriomorphia]] (oysters, mussels, etc) *[[Arcoida]] *†[[Cyrtodontoida]] *[[Mytiloida]] *[[Ostreoida]] - formerly included in Pterioida *†[[Praecardioida]] *[[Pterioida]] Subclass [[Paleoheterodonta]] *†[[Trigonioida]] *[[Unionoida]] (typical freshwater mussels) *†[[Modiomorpha]] Subclass [[Heterodonta]] (typical [[clam]]s, [[cockle]]s, rudists, etc) *†[[Cycloconchidae]] *†[[Hippuritoida]] *†[[Lyrodesmatidae]] *[[Myoida]] *†[[Redoniidae]] *[[Veneroida]] Subclass [[Anomalosdesmata]] *[[Pholadomyoida]] There also exists an alternative systematic scheme according to [[gill]] morphology (Franc 1960). This distinguishes between [[Protobranchia]], [[Filibranchia]], and [[Eulamellibranchia]]. The first corresponds to Newell's Palaeotaxodonta + Cryptodonta, the second to his Pteriomorphia, and the last contains all other groups. In addition, Franc separated the [[Septibranchia]] from his eulamellibranchs, but this would seem to make the latter paraphyletic. ==Anatomy== [[Image:Oyster anatomy.jpg|left|thumb|Drawing of [[oyster]] anatomy]] [[Image:Margaritifiera-margaritifiera-Anatomy.png|thumb|Drawing of anatomy of [[Freshwater pearl mussel]] ''Margaritifera margaritifera'']] [[Image:Valve-InternalView.png|thumb|A diagram of the internal anatomy of the right hand valve of a bivalve such as a venerid]] [[Image:Giant_clam_or_Tridacna_gigas.jpg|thumb|right|200px|[[Giant clam]], ''Tridacna gigas''.]] 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 [[shipworm]]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, allowing the animal to dig tunnels through wood.<ref>"Description" in [http://64.233.183.104/search?q=cache:OyzesLnucFUJ:www.marine.csiro.au/crimp/nimpis/spSummaryPDF.asp%3Ftxa%3D10036+teredo+burrowing&hl=en&ct=clnk&cd=3]</ref> ===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]]. In all but the simplest forms the neural [[ganglia]] are united into two cerebropleural ganglia 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.<ref name="EL">[http://el.erdc.usace.army.mil/zebra/zmis/zmishelp4/nervous_system_and_sense_organs.htm Nervous System and Sense Organs<!-- Bot generated title -->]</ref> 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 tentacles and most are typically [[mechanoreceptor]]s and [[chemoreceptor]]s. [[Scallop]]s have complex eyes with a [[lens (anatomy)|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 shadows falling on the animal.<ref name="EL">[http://el.erdc.usace.army.mil/zebra/zmis/zmishelp4/nervous_system_and_sense_organs.htm Nervous System and Sense Organs<!-- Bot generated title -->]</ref> In the [[septibranch]]s the inhalant siphon is surrounded by vibration-sensitive tentacles for detecting prey.<ref>In [http://adsabs.harvard.edu/abs/1981RSPTB.294..413A "an analysis of the evolution of the septibranch condition"]</ref> [[Statocyst]]s within the organism help the bivalve to sense its orientation, which can then be corrected if need be.<ref>"a statocyst..." in [http://www.manandmollusc.net/advanced_introduction/moll101pelecypoda.html]</ref> ===Muscles=== The muscular system is comprised of the [[Anatomical terms of location#Invertebrate directional terms|posterior and anterior]] [[adductor muscles]], although the anterior 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]]s and [[scallop]]s, these retractors are absent. ===Circulation=== Bivalves have an open [[circulatory system]] that bathes the organs in [[hemolymph]]. ===Mantle and shell=== {{main|Bivalve shell}} In bivalves the [[mantle (mollusk)|mantle]], a thin [[membrane]] surrounding the body, secretes the main shell valves, [[ligament]] and [[Hinge tooth|hinge teeth]], the mantle lobes secreting the valves and the mantle crest the other parts. 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 [[siphon]]s, which take in and expel water for [[suspension feeding]] purposes. The shell is composed of two [[calcareous]] [[valve]]s, which are made of either [[calcite]] (as with, e.g. 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.<ref>"The shell of bivalve molluscs" in [http://paleo.cortland.edu/tutorial/Bivalves/bivalvia.htm]</ref> 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 (biology)|dorsum]] by the [[ligament]], which is composed of the tensilium and resilium. The ligament opens the shells. ===Reproduction=== The sexes are usually separate, but some [[hermaphroditism]] is known. Bivalves practice [[external fertilisation]]. Typically the marine bivalve will start life as a [[trochophore]], later becoming a [[veliger]]. 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. Glochidia can become serious [[pest]]s of [[fish]]. ==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 zone|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]]. ===Feeding=== Most bivalves are [[filter feeder]]s (although some have taken up scavenging and predation), extracting organic matter from the sea in which they live. [[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, 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. ===Movement=== [[Razor shell]]s (''[[Ensis]]'' spp.) 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 shell]]s ([[Limidae]]) can produce a noxious secretion when threatened, and the [[fan shell]]s of the same family have a unique, acid-producing organ. ==Comparison to Brachiopods== Bivalves are laterally combined and have a shell composed of two valves. The valved shell makes them superficially similar to [[brachiopod]]s, 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. Bivalves appeared late in the [[Cambrian explosion]] and came to dominate over [[brachiopod]]s during the [[Palaeozoic]]; indeed, by the [[Permian-Triassic extinction event|end-Permian extinction]], bivalves were undergoing a huge radiation in numbers while brachiopods (along with around 95% of all species) were devastated. It had long been considered that bivalves are better adapted to aquatic life than the brachiopods were, causing brachiopods to be [[Competition (biology)|out-competed]] and relegated to minor niches in later fossil strata. In fact, these taxa even appeared in textbooks as an example of replacement by competition. Evidence adduced for this included use of an energetically-efficient ligament-muscle system for opening valves, requiring less food to subsist. Lately, however, this interpretation of the interaction between brachiopods and bivalves has been largely exploded. It seems instead that the reason for the prominence of bivalves over brachiopods has to do with chance disparities in their response to extinction events. <ref>{{cite journal |journal=Paleobiology |pages=383–396 |title=Clams and Brachiopods-Ships that Pass in the Night |last=Gould |first=Stephen |authorlink=Stephen Jay Gould |coauthors=C. Bradford Calloway |volume=6 |issue=4 |date=Autumn, 1980 |url=http://www.jstor.org/pss/2400538 }}</ref> ==References== <references/> * Franc, A. (1960): Classe de Bivalves. ''In:'' Grassé, Pierre-Paul: ''Traite de Zoologie'' 5/II. * Newell, N.D. (1969): [Bivalvia systematics]. ''In:'' Moore, R.C.: ''Treatise on Invertebrate Paleontology'' Part N. * {{cite journal |pages=1119–1127 |title=Bivalve Systematics During the 20th Century |author=Jay A. Schneider |volume=75 |issue=6 |month=Nov |year=2001 |doi=10.1666/0022-3360(2001)075<1119:BSDTC>2.0.CO;2 |journal=of Paleontology }} ==External links== {{commonscat|Bivalvia}} *[http://www.ucmp.berkeley.edu/ Museum of Paleontology] - [[Palaeontology]] from the [[University of California, Berkeley]] *[http://www.wooster.edu/geology/Bioerosion/Bioerosion.html Bioerosion website at The College of Wooster] [[Category:Bivalves| ]] [[Category:Fossils]] [[zh-min-nan:Siang-khak-kong]] [[bg:Миди]] [[ca:Bivalve]] [[cs:Mlži]] [[da:Muslinger]] [[de:Muscheln]] [[et:Karbid]] [[es:Bivalvia]] [[fa:دوکفه‌ای‌ها]] [[fr:Bivalvia]] [[ko:이매패류]] [[hr:Školjkaši]] [[io:Bivalvo]] [[is:Samlokur]] [[it:Bivalvia]] [[he:צדפות]] [[la:Bivalvia]] [[lv:Gliemenes]] [[lb:Muschelen]] [[lt:Dvigeldžiai]] [[hu:Kagylók]] [[mk:Школки]] [[nl:Tweekleppigen]] [[ja:二枚貝]] [[no:Muslinger]] [[nn:Musling]] [[nds:Musseln]] [[pl:Małże]] [[pt:Bivalvia]] [[qu:Lakachu]] [[ru:Двустворчатые]] [[simple:Bivalve]] [[sk:Lastúrniky]] [[sr:Шкољке]] [[fi:Simpukat]] [[sv:Musslor]] [[tr:Midyeler]] [[uk:Двостулкові]] [[zh:双壳纲]]