Bivalves

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  BIVALVES (Acephala, Lamellibranchia, Pelecypoda) by J.M. Poutiers click for previous page  General Remarks  GENERAL REMARKS B ivalves are aquatic molluscs that show a fundamental bilateral symmetry. Their characteristic shell iscomposed of  2 calcified valves  lying on the right and left sides of the body. Both valves are typicallyequally convex ( equivalve shell ), but they may differ in size and shape ( inequivalve shell ) as a result ofan alteration of bilateral symmetry.Valves are articulated along amarginal process of the dorsal side calledthe  hinge , and are connected by an elastic and poorly calcified structure, the  ligament . Under the actionof the ligament, the 2 valves tend to open along their anterior, posterior and mainly ventral margins.Theyare closed by the pulling action of 1 or 2 (sometimes 3)  adductor muscles . These are fixed to the innerside of valves by areas leaving well-defined imprints, the  adductor muscle scars . posteriormargin ligamenthinge plateposterioradductor scarpallial sinuspallial lineanterioradductorscar anteriormargin lateral toothcardinal toothumbo dorsal margin  midline of valveheightlengthinflation ventral margin escutcheonligamentleft valvelunuleumbo dorsal of entire shellinterior of left valve right valve main features of a bivalve shell posterioradductormuscledigestiveglandheartintestineanusexhalantsiphoninhalantsiphongillmantlegonadfootmouthanterioradductormusclestomachheartligamenthingegutshellmantlelobebranchiallamellaepallialcavityinnerdemibranchouterdemibranchanastomosesctenidialaxisperiostracum internal lateral view after removalof mantle and left valve general anatomy of bivalves  diagrammatic transverse section foot124 Bivalves  The soft, unsegmented body of bivalves is laterally compressed, but has neither a head ( Acephala ) nor amasticatory apparatus. It is covered by the  mantle , an overgrowing sheet of tissue composed of 2 lobes,each one lining and secreting the respective valve. Pallial lobes  are fused dorsally with the visceral mass,and enclose ventrally a rather wide pallial cavity communicating with the outside.They are tightly attachedto the interior of valves along a well-defined  pallial line , close to the ventral margin of the shell. Themantle-lobe margins may be somewhat fused, forming 2 siphons posteriorly through which water is takenin (ventral, inhalant siphon) or expelled (dorsal, exhalant siphon). The  foot , a muscular ventral structure,is sometimes hatchet-shaped (Pelecypoda) and enables a burrowing locomotion or a fixation to hardsubstrates by means of elastic filaments ( byssus ). Many bivalves exhibit a pair of respiratory lamellousgills (Lamellibranchia), whose activity produces a complex system of currents largely concerned with thecollection of food. Most forms of bivalves are microphagous and feed either on plankton or organic mattersuspended in water (suspension-feeders), or on food collected on the substrate floor (deposit-feeders).However, a few species have evolved specialized feeding strategies (carnivores, xylophages).In the majority of bivalves, sexes are separate and reproductive cells are released into the water wherefertilizationoccurs;larvaehavearelativelylongfree-swimmingplanktoniclife,followedbyametamorphosisleading to the definitive benthonic mode of life. However, some species may exhibit various forms ofhermaphrodism,and fertilization may occur in thepallialcavity,sometimes withprotection of eggs or larvaein a “brooding chamber”. The planktonic larval stage may be reduced or totally absent, and then younghatch directly as benthic organisms.The biodiversity of the malacological fauna in the Western Central Pacific is probably the greatest in the world,butnoreliableestimateofthebivalvediversityispresentlyavailable.However,arecentevaluationofthenearbyJapanese fauna may give an idea of the rich biodiversity in the area.Japanese bivalves comprise about 1 600species of marine and brackish-water species allocated to 92 families, compared to a total of 10 000 speciesin the world.For the present contribution, 189 species belonging to 35 families have been selected, mainly onthe basis of size, abundance, distribution, and commercial interest. Only those species that are known to beused as food are included in this guide but, in view of the paucity of detailed informations on fisheries in manyplaces,manyotherspeciesareprobablycollectedlocallyinthearea.Inordertoselectthesespecies,theauthorhas largely used his “Annotated list of marine and brackish-water species of interest to fisheries” (1992, FAOunpublishedreport)thathasbeencirculatedforimprovementamongspecialistsasabasisfortheFAOSpeciesCatalogue,“BivalvesoftheWorld”(inpreparation).Hehadtheopportunitytostudyspecimenscollectedinlocalmarkets of the Philippine Archipelago by Prof. V. Storch, Ruprecht-Karls-Universität (Heidelberg, Germany),thanks to courtesy of Drs R.von Cosel and R.Janssen,Senckenberg Museum (Frankfurt am Main,Germany).HehasalsogatheredaconsiderableamountofinformationonthespeciesexploitedinthecentralandnorthernPhilippines during a workshop in support of the present field guide which was held in October 1995 in thePhilippines, organized by FAO, MSI (Marine Science Institute, University of the Philippines), and ICLARM(International Centre for Living Aquatic Resources Management). Useful remarks have been made on thePectinidaeby DrH.H.Dijkstra,ZoölogischMuseum(Amsterdam,theNetherlands),andontheGlycymerididaeand Veneridae by Dr A.Matsukuma, Kyushu University (Fukuoka, Japan).In the Western Central Pacific, a large variety of species is traditionally collected by coastal populations forpersonal consumption. In the past, fishing effort and aquaculture have concentrated on a limited number ofbivalve species, but now an increasing number of species tends to be exploited and aquaculture experimentsare attempted in various places in order to counteract the depletion of natural beds by overexploitation orpollution. General Remarks/Glossary of Technical Terms GLOSSARY OF TECHNICAL TERMS Accessory plate  - calcareous and periostracal structure covering the soft parts in the Pholadidae, inaddition to the shell valves. Adductor muscle  - muscle connecting the 2 valves of a shell, tending to draw them together. Apophysis  - finger-like shelly structure to which the foot muscles are attached in the Pholadidae andTeredinidae. Branchial  - pertaining to the gills. Branchial lamella  - (see gill). Byssus  - clump of horny threads spun by the foot, by which a bivalve can anchor to a hard substrate. Cardinal area  - surface of the shell extending between umbo and hinge margin. Cardinal tooth  - (see tooth). Chomata  - marginal crenulations in Ostreidae and Gryphaeidae, occuring all around the inner side ofvalvesoronlynearthehinge,composedofsmalltuberclesorridgeletsontherightvalve,andcorrespondingpits on the left valve. General Remarks/Glossary of Technical Terms 125  Commissure  - line of junction of the valves. Concentric  - parallel to lines of growth. Cruciform muscles  - crossed muscles connecting valves and serving to retract the siphons, leaving 2small scars near the posteroventral end of pallial line in some bivalves (e.g., Tellinidae). Ctenidial axis  - (see gill). Ctenolium  - a row of small teeth on lower side of byssal notch in some Pectinidae. Demibranch  - (see gill). Denticle  - a small tooth. Ear  - lateral expansion of the dorsal part of a shell. Equilateral  - the condition of a valve when growth on either side of umbo is symmetrical. Equivalve  - the condition of a shell when valves are of the same shape and size. Escutcheon  - differenciated area extending along dorsal margin of valves, behind the umbones. Eulamellibranchiate type  - gill demibranchs composed of 2 lamellae. Branchial filaments and lamellaealways connected by tissular junctions (e.g., Veneridae). Filibranchiate type  - gill demibranchs composed of 2 lamellae.In addition to the ciliary junctions betweenbranchialfilaments,anastomosed tissularjunctionsmayunitelamellaeofeachdemibranch(e.g., Mytilidae,Pectinidae). Foot  - mobile and extensible muscular organ, used for locomotion or for attachment to substrate by meansof byssal threads. Gape  - opening or gap remaining between margins of valves, when shell is closed. Gill  - respiratory organ generally composed of 2 thin leaf-like structures (demibranches) suspended to adorsal axis (ctenidial axis);each demibranch may be either simple or bent back upon itself and then formedof 2 sheets (branchial lamellae). A lamella is constituted of many ciliated filaments parallel to each otherand interconnected by more or less complex junctions. Four main types of gill structures are currentlyrecognized among bivalves: the protobranchiate, filibranchiate, eulamellibranchiate, and septibranchiatetypes (see these terms). Growth marks  - (see sculpture). Hinge  - structures in the dorsal region of the shell, along which the valves meet, and that function in theopening and closing of the shell. Hinge line  - shell margin adjacent to the hinge. Hinge plate  - infolding of dorsal shell margin bearing hinge teeth and sockets, and lying in each valve ina plane parallel to that of junction of valves. Imbricate  - overlapping like tiles or shingles on a roof. Inequilateral  - the condition of a valve when growth on either side of umbo is assymmetrical. Inequivalve  - the condition of a shell when valves are not alike in shape or size. Keel  - a prominent, angular ridge. Lamellate  - with thin, flattened plates. Lateral tooth  - (see tooth). Lenticular  - shaped like a biconvex lens. Ligament  - horny, elastic structure joining the 2 valves dorsally. Ligamental area  - part of cardinal area occupied by the ligament. Lunule  - differentiated area extending along dorsal margin of valves, just in front of umbones. Mantle  - fleshy sheet surrounding vital organs and composed of 2 lobes, one lining and secreting eachvalve. Muscle scar  - impression marking the place of attachment of a muscle inside the shell. Nacreous  - pearly, often with multi-coloured hues, as in mother-of-pearl. Nymph  - narrow plateform extending behind umbo along dorsal margin, to which the external ligament isattached. Opisthogyrate  - the condition of a shell when umbones are directed posteriorly. Orbicular  - disk-shaped, nearly circular. 126 Bivalves
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