Bivalve Glossary

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Glossary

A
Accessory plate-
An extra, small, horny plate over the hinge area or siphons.
Adductor muscle-
A major muscle of a bivalve that, when contracted, closes the valves.
Adductor scar-
A small, circular impression on the inside of the valve marking the attachment point of an adductor muscle.
Anterior-
The front end of the body, the end of the shell from which the foot and byssal threads exit.
Apophysis-
Long, slender extension of the interior of the valve, projecting from just under the umbones of pholad bivalves (shipworms) where the foot retractor muscles attach.
Auricle-
On scallops, the wings to either side of the hinge (see also wings)

B
Benthos-
Refers to those organisms that live on the ocean floor and beaches. Also refers to the sea bottom itself.
Bifid-
Referring to the hinge teeth: Divided into two parts by a groove.
Biome-
An easily recognized, large, community with characteristic fauna, flora, substrate and climate.
Bivalve-
A class of phylum-Mollusca having a hinged, two part shell.
Byssal gap-
The openings between the margins of the valves through which the byssus passes
Byssal thread-
In mussels and some scallops, one of the organic threads secreted by the byssal gland and used for temporary attachment to a substrate.
Byssus-
In rock oysters, a tough knot of organic threads secreted from the byssal gland in the foot used for permanent attachment to the substrate.

C
Cancelate sculpture-
The pattern formed on the shell exterior by radial ribs crossing concentric sculpture.
Cardinal teeth-
Hinge teeth immediately below the umbo. (see hinge teeth)
Cementation-
Permanent fixation to the substrate by a sessile bivalve.
Chondrophore-
An internal shelf near the hinge to which the internal ligament attaches in certain bivalves.
Community-
A group of different species living within the same environment.
Concentric sculpture-
Centering around the umbo and parallel to the shell margin.
Conchiolin-
Proteinaceous material that makes up the periostracum and forms the organic matrix for calcareous parts of the shell giving the shell a horny texture instead of a chalky one.
Crystalline style-
The enzyme-rich rod that projects into the stomach and releases digestive enzymes as it is slowly dissolved.
Ctenidium (ctenidia pl.)-
The gills, characteristic of all molluscan genera, divided into four major orders in the class Bivalvia: Septibranchia; Eulamellibranchia; Filibranchia; Protobranchia (see also lamellar gills).

D
Diameter-
A measure of the shell around the valve margins.
Dimyarian-
Where the two adductor muscles are of approximately equal size.
Dorsal-
The surface closest to the hinge.

E
Epifauna-
Living on or attached to, but not burrowed into, the substrate.
Equilateral-
Anterior and posterior ends of the the valve are the same size and shape and the umbones are centered.
Equivalve-
Each valve of the shell being of the same shape and size.
Escutcheon-
An oval shaped area on the shell exterior near the hinge and extending posteriorly form the umbones.
Exhalent siphon-
The dorsal most of the tubular extension of the mantle edge through which water exits the mantle cavity removing wastes and indigestible material.
External ligament-
The part of the hinge ligament visible when the valves are closed.

F
Filter feeder-
Any animal that obtains nourishment by filtering suspended particulate food from water (suspension feeder) or by ingesting sediment to digest the organic matter that accumulates on the particles of sand (deposit feeder).
Foot-
The muscular organ used for locomotion.
Fossette-
A pit shaped chondrophore.

G
Gaping-
Incapable of completely closing the valves.
Gill-
The respiratory organ that extracts dissolved oxygen, salts and nutrients from the water.
Gonad-
The reproductive gland.
Growth lines/rings-
Concentric sculpture on each valve which mark the successive positions of the shell margin during earlier growth.

H
Height-
The measure of the shell through the umbones and perpendicular to the adductor muscles.
Heterodont-
Having differentiated cardinal and lateral teeth.
Heteromyarian-
Where the two adductor muscles are unequal in size.
Hinge-
A horny ligament, located internally, externally or partially internally, near the umbones, that holds the valves slightly opened when the animal is feeding or relaxed
Hinge plate-
The in-folded dorsal margin of a valve below the umbo.
Hinge teeth-
Small projections of the hinge plate that align the valves so that they close correctly. (see cardinal teeth and lateral teeth)

I
Incremental lines-
Faint, concentric growth lines.
Indigenous-
Refering to organisms that are native to a particular region, not having been introduced.
Inequlateral-
The posterior and anterior ends of the shell being different in size and shape.
Inequivalve-
One valve of the shell being larger, flatter or otherwise different from the other.
Infauna-
Those animals that live burrowed in the substrate.
Inhalant siphon-
The ventral most tubular extension of the mantle edge that takes oxygen rich water and nutrients into the mantle cavity and over the gills.
Internal ligament-
The portion of the ligament not visible when the valves are closed.
Intertidal-
The area between the high water mark and the low water mark( see Littoral zone).
Invasive/Introduced species-
Organisms that have been intentionally or accidentally placed into a region where they did not naturally develop as a species and where their natural predators and/or prey do not exist.

K
Kingdom-
The highest taxonomic grouping in the hierarchy.

L
Labial palps-
Paired ciliated flaps on the mouth.
Lamellar gills/Lamellae-
The large flattened plate-like gills that act as the feeding organs (see ctenidium).
Lateral teeth-
The hinge teeth posterior and anterior to the cardinal teeth, usually longer and running parallel to the mantle
margin. (see hinge teeth)
Length-
the greatest horizontal distance parallel to the adductor muscles from the posterior to the anterior end of the shell.
Littoral zone-
The marginal shore area covered by the tide (see intertidal zone) and includes the splash zone where only spray form waves reaches.
Lower littoral zone- Range of the littoral zone that remains submerged except during low spring tides.
Middle littoral zone- Range of the littoral zone submerged by most high-tides and exposed during most low-tides.
Upper littoral zone- Range of the littoral zone submerged only during the high spring tides.
Lunules-
The heart shaped impression on the dorsal midline of the shell just anterior of the umbones. Usually found on the Venus (Family-Veneridae) clams.

M
Mantle-
The outer most portion of a bivalve's soft body parts (visceral mass) that secretes the shell and periostracum and which folds on itself to form the siphons.
Mantle cavity-
The space enclosed by the mantle which holds the visceral mass.
Mantle scar-
A broad area of attachment caused by the mantle edge too wide to be called a pallial line.
Monomyarian-
Having only one, large adductor muscle to close the valves.
Muscle scar-
A small, circular impression on the inside of the valve made by the adductor muscles (see also adductor scar).
Myophore-
An extension of the shell's interior in pholad bivalves (shipworms) to which the foot retractor muscles attach (see also apophysis).

N
Nacre-
The inner most, mother-of-pearl layer of the shell.
Nearshore-
The part of a beach made up of the submerged area from the low-tide line seaward. Also called the Subtidal zone.
Neck-
Another name for the fused siphons of a bivalve.
Nephridium-
An excretory tube usually opening to the exterior of the body through the anus.
Niche-
The role or function of a species within an ecosystem in the case of most bivalves: filter feeding, herbivorous infauna.
Nymph-
A thickened projection along the hinge margin that supports an external ligament or reinforces the normal hinge structure.

O
Opisthoqyrate-
Having the umbones pointed posteriorly.
Orthogyrate-
Having the umbones pointed toward each other.
Osphradium-
The chemosensory organ in the siphon that tests the quality of incoming water.

P
Pallets-
Found only in the wood boring, pholadid bivalves (shipworms), these are the calcareous, feather-like extensions secreted by the distal end of the siphon and used to close the burrow.
Pallial muscle-
A muscle that controls the mantle and who's attachment forms the pallial line.
Pallial sinus/Sinus-
A "C" shaped bend in the attachment scar of the mantle signifying where the siphons are retracted into the shell.
Pallium-
The mantle.
Palp proboscis-
Found in protobanchial bivalves, the long ciliated tentacles extending from the labial palps and used for feeding.
Pedal gape-
The space between the valves through which the foot exits.
Pelecypoda-
"Hatchet-Footed", another name for class bivalvia.
Pericardial sac-
The organ that holds the circulatory heart.
Periostracum-
The paper-like outer most layer of most molluscan shells, produced by the mantle edge, that provides protection from the
corrosive elements of sea water.
Perismatic layer-
The thick, calcium carbonate, middle layer of the valve.
Posterior-
Towards the rear of the body, the end in which the siphons exit the shell.
Prodissochonch-
The shell secreted by the veliger, preserved at the umbones of some adult shells.
Prosogyrate-
Having the umbones pointed anteriorly.

R
Radial sculpture-
Rib-like, external shell sculpting originating at the umbones and extending to the ventral shell margin.
Resilifer-
A projecting spoon-like chondrophore.
Resilium-
The internal part of the hinge ligament.
Ribs-
The sculpture that makes up radial sculpture, also called radial ribbing, running roughly parallel to the anterior and posterior
shell margins.

S
Scar-
A marking on the interior of a valve that indicates the attachment point of a muscle or the mantle.
Sessile-
Fixed to the substrate and usually incapable of movement.
Shell-
The ridged, external part of a bivalve consisting of two valves held together by a tough hinge.
Shelly-
Composed of calcium carbonate rather than concholin (see periostracum) giving the shell a chalky texture.
Sinus/Pallial sinus-
A "C" shaped bend in the attachment scar of the mantle signifying where the siphons are retracted into the shell.
Siphon-
A tube like extension of the posterior mantle used to funnel water through the mantle cavity for respiration, feeding and the removal of wastes.
Socket-
A recess for the reception of a tooth, whether lateral or cardinal, or chondrophore from the opposite valve
Species-
A group of potentially interbreeding organisms that is reproductively isolated from all others.
Substrate-
Any base for attachment or burrowing.
Suprabranchial chamber-
The space within the mantle cavity that contain the ctenidium.
Surf zone-
The area affected by wave action, extending form the high-water mark to the point where waves break.

T
Taxodont Dentition-
A long row of many, small, uniform teeth along the hinge line, anterior and posterior of the umbones. Usually found in Nuculid, Nuculanid and Glycymeridid clams.
Tentacles-
The long, thin projections from the mantle of scallops that are sensitive to stimuli (also called a cirrus [pl. cirri]).
Trochophore-
The first larval form of a mollusc, roughly ovoid in shape with a "crown" of cilia on the anterior end, bands of cilia around the body, precedes the veliger form.

U
Umbo (umbones pl.)-
The oldest, first formed part of the valve, also called a "beak" due to its shape, found above the hinge.
Umbonal ridge-
An angled or rounded ridge beginning at the umbones and extending to the posterior end of a shell.

V
Valve-
One half of the bivalve shell.
Veliger-
The free swimming larval form of a mollusc that metamorphoses into the adult form.
Ventral-
The edge of the valve farthest from the umbo.
Visceral mass-
The internal organs and soft body parts of the bivalve.

W
Wing-
On scallops, the flattened projection located at one or both ends of the hinge line (see also auricle).

Last updated Monday, August 28, 2006, by Lisa Ferrier