PelecypodsMorphological Features

Shell Morphology• Mantle - a thin membrane surrounding the body, secretes the shell valves,

ligament, and hinge teeth.• Shell consists of three distinct layers:

• Nacre- inner most layer, thin sheets of CaCO3 laid down horizontally (parallel to plane of shell) gives iridescent appearance; how it is laid down determines color; ranges from white to peach to pink to deep purple; Sphaeriids usually lack this layer and in corbiculids it is very thin

• Prismatic- 90% of shell; also CaCO3 laid down in vertical layers (90o to the horizontal plane)

• Periostracum- outer proteinaceous, horny layer; thickness varies with species; in unionids it is usually comprised of three layers

Structures on the Shell• Umbone (=beak)- area where first shell was laid down in development• Umbonal cavity (beak cavity)- inside, area under umbone• Interdentum- space between teeth types (ie. between lateral and

pseudocardinal teeth)• Pallial line - muscle scars• Lateral teeth - used as a locking mechanism to keep valves from sliding• Orientation: Right, Left valves; Dorsal and Ventral• Annual growth rings- much like tree rings; laid down as individual grows;

have been used to age species

• Alae- wing; condition is said to be "alate"• Flutings- broad grooves and channels in side of shells• Pustules- warts or knobs or tubercles; any bumps on the surface of the shell• Rays- green colored bands on side of shell; sometimes narrow sometimes

broad or may be restricted to certain parts of the shell

Internal Morphology• Mantle

• covers the body and lines the internal layer of the shell;• responsible for secreting the shell; • folds in the mantle produce siphons: inhalant (located vantrally) and exhalant (dorsal); located on

posterior end of body

• Gills • are very large and paired on both sides of body formed into large lamellae or leaf like structures that

almost completely surround body• are highly vascularized• associated with dorsal heart that sends blood from the gills to the visceral mass, mantle, foot and

back• have multiple functions

• Respiration- take up oxygen and give off carbon dioxide• Feeding- aid in filter feeding; water brought across the gills which contains food; food is trapped by mucous on gills

and is then moved by ciliary system to mouth; this process may produce pseudofeces• Reproduction- the gills act as a marsupium chamber for developing larvae or young

• Visceral Mass• contains most of the vital organs

• stomach and intestines• digestive gland (i.e., liver)• gonads • Heart

• dorsal three chambered; covered by a pericardium; pumps blood (hemolymph) around body• Kidney

• Foot • largest muscle of body used for locomotion and digging

• Muscles• Anterior and Posterior Adductor Muscles- close shell; no mechanism except relaxation

of these muscles and flexibility of hinge ligament to open shell• Anterior and Posterior Retractor muscles- used to retract foot into shell; and in doing so

pull the shell forward during locomotion• Protractor muscle- opposes action of retractor muscles

Other structures• Morphology of a bivalve differ from one specie to another and this is

reflected in their differing: tooth patterns and gill structure

TOOTH TYPE DESCRIPTION DESMODONT Teeth are reduced or absent all togetherDYSODONT Small simple teeth located near the edge of the valve

TAXODONT Numerous teeth arranged in a radial pattern fanning out upwards

ACTINODONT Numerous teeth arranged in a radial pattern fanning out downwards

ISODONT Large teeth found either side of the internal ligament pitSCHIZODONT Large teeth, sometimes groovedPACHYODONT Very large blunt teethHETERODONT Cardinal and lateral teeth

GILL TYPE DESCRIPTION

PROTOBRANCH This gill structure tends to occur in primitive groups, and appears as a small leaf-like structure

FILIBRANCH This gill structure consists of individual filaments forming 'W'-shaped structures that come together to form lamellar sheets.

EULAMELLIBRANCH These form 'W'-shaped gills with cross-partitions joining the filaments to create water-filled cavities in between them.

SEPTIBRANCHThese gills are only found in Poromyacea a super-family of the rock borers. They run transversely across the mantle cavity, enclosing the inner chamber, with only a small connection to the outer cavity

PelecypodsTaxonomic Classification

TaxonomyPhylum: Mollusca

Class: BivalviaSubclass: ProtobranchiaOrder: NuculoidaOrder: SolemyoidaSubclass: PteromorphiaOrder: MytiloidaOrder: PterioidaSubclass: PalaeoheterodontaOrder: TrigonioidaOrder: UnionoidaSubclass: HeterodontaOrder VeneroidaOrder MyoidaSubclass AnomalodesmataOrder Pholadomyoida

Subclass: Protobranchia• Early Cambrian to Recent• the simplest and most primitive of the bivalves• They have a protobranch gill structure, an equivalved aragonite shell and taxodont

dentition.• Order Nuculoida

• shell is aragonic with an interior that is nacreous or porcelaneous • periostracum is smooth. • valves are equal and have a row of sharp teeth along its hinge or border. • Large palps used for food collection. • Gills are small and used only for gas exchange. • Foot is longitudinally grooved and has a plantar sole. • Common Name: Nut Clams

Nuculanoidea

Malletiidae

Nuculoidea

Neilonellidae

Subclass: Protobranchia• Order Solemyoida (Devonian to Recent)

• Shell valves are thin, equal in size, elongate and lacking hinge teeth (dysodont). • have a large gills used for both feeding and gas exchange. • Their palps are small. • Common Name: awning clams

SolemyidaeNucinellidae

Subclass Pteromorphia• A very diverse group of normally byssate forms with variable musculature and

dentition. • Shells may be calcite, aragonite or both. • Many inequivalved, some equivalved. • Variable dentition. • Foot commonly reduced, absent in some. • Mantle mostly unfused and lacking siphons. • Gills fillibranch or eulamellibranch

• Order Mytiloida • Shell valves are quite thin, elongated and equal in size. • The valves are uncalcified along the outer edges and hinge teeth are absent.• They have one large ctenidia which is used both for feeding and gas exchange.• Their palps are small.

Mytilidae: these are the sea mussels

• Order Pterioida • Ordovician to Recent• Include pectinaceans (scallops and their kin), ostreaceans, and the fossil

inoceramids

Isognomonidae

Inoceramid Pteriidae

Pinnidae

Subclass Palaeoheterodonta• They are distinguished by having the two halves of the shell be of equal size and

shape, but by having the hinge teeth be in a single row, rather than separated into two groups

• Order Trigonioida • Bivalves with large trigonal and well developed schizodont teeth• Devonian to Recent

Trigonioidea

Yaadia valentina

• Order Unioinoida• freshwater clams• the only non-marine bivalves• Devonian to Recent

MuteloidaeUnionidae

Hyriidae

Subclass Heterodonta• (Triassic to Recent) • Heterodont hinge structure • eulamellibranch gill structure • almost always aragonitic shells • Adapted to varied modes of life, especially infaunal siphon feeding• The hinge structure may degenerate into a desmodont condition.

• Order Veneroida • mostly active equivalve heterodonts with true heterodont teeth • Usually thick-valved, and isomyarian.

Veneroida Cardiidae Veneroida Glossoidea Veneroida Tellinoidea

• Order Myoida • Thin-shelled burrowers and borers • very inequivalve • hinges are degenerate, with one cardinal tooth on each valve; • siphons well developed • Carboniferous to Recent

Hiatellidae

Gastrochaenidae

Pholadidae

Subclass Anomalodesmata• Middle Ordovician to Recent • Burrowing or boring forms • very modified, with aragonitic shells and desmodont dentition. • A small, specialized group, in which gills are not present. • The inhalent and suprabranchial (exhalent) cavity are separated by a pumping septum• Order Pholadomyoida

• The shells of species in this order are of equal size, as are the muscles that hold them closed • the margins at the hinges are thickened. • The margins of the mantle are also fused, and there is only a single hinge tooth, if any

Pholadomyidae

Cuspidariidae