A list of key terms used in this handbook to describe mussel structure, classification, biology, and life history

Adductor muscle - internal tissue structure that acts to hold valves closed.

Adhesive membrane - bivalve secretion that acts to hold glochidia together in the water tubes.

Anterior (see also: Posterior) - "front" edge of a bivalve; edge of shell closest to umbo; usually shortened and associated with the foot.

Attachment thread (see also: Byssal thread) - either a fibrous thread used to attach the bivalve to a substrate or, part of a glochidium attached to the rudimentary foot.

Beak (see also: Umbo) - dorsal protuberance on bivalve shell that generally rises above the hinge; the oldest part of the shell.

Beak Sculpture - (see also: Concentric scuplture) - shape and arrangement of contours inherent to umbo; formed by early growth geometry.

Bivalve - a laterally compressed mollusk with 2 valves (2 "half-shells").

Brood pouches (see also: Gill pouches) - vertically partitioned interlamellar space used by some bivalves as a marsupium; rather than broadcasting gametes directly into the environment,  eggs and developing glochidia stay within the protective confines of the bivalve shell.

Byssal thread - a tough fibrous structure secreted by certain bivalves; used to attach the bivalve to substrate.

Cilia - a hair-like array in bivalves; moved in a coordinated fashion to transfer and sort food and water through feeding palps and gills.

Cladistics - a style of mathematical analysis that uses shared parametric similarities to define taxa; uses inferred evolutionary relationships when characteristics are not found in ancestral groups.

Color pattern - characteristic of some bivalve shells, often useful for identification; color pattern may vary within the same taxon due to age and local environmental conditions.

Color ray - an external shell characteristic; dark green bands radiating from the umbo, and alternating with the lighter-colored portions of the periostracum.

Compressed - side to side flattening; the opposite of Inflated.

Concentric sculpture - a trait of beak sculpture, in which the annual growth rings appear as concentric shapes . Often worn to indistinctness in older specimens.

Cyst (see also: Encystment and Glochidium) - a protective tissue formation found in host species; formed by glochidia during transformation to adult characteristics.

Digestive tract - a system in lamellibranch bivalves that breaks down food particles into usable chemical components for growth and maintenance; comprised of crystalline style, gastric shield, esophagus, digestive diverticula, sorting region, intestine, stomach, typhlostyle, rectum, and anus.

Dorsal  - "top" or "back" edge of a shell; the edge containing the umbo.

Double-looped sculpture - a trait of beak sculpture wherein the growth rings appear as two loops.

Ducts (tube-like) - bivalve anatomical structure that connects ovaries to suprabranchial chamber; used to transport ova to chamber for fertilized egg development into glochidia.

Ectoparasites - parasitic lifestyle of freshwater mussel glochidia, either by hook attachment to fins or body, or for hookless types, to fish gills; draws food resources from host body directly through glochidial mantle surface.

Embryo - early life stage of growth and differentiation characterized by formation of fundamental tissues and organs; in bivalves, contained within egg membrane.

Encystment - process used by glochidium to stimulate tissue growth in host species; provides protection from host's attempts at rejection.

Eulamellibranch gills - specialized gill formation found in freshwater mussels and other bivalves with fused filaments forming a solid sheet-like structure, the folding of which forms water tubes.

Exhalant siphon (see: Siphon)

Extralimital - refers to taxa occuring outside their natural range.

Family - a taxon of the rank below superfamily and above subfamily, tribe, and genus; Unionidae and Margaritiferidae are North American freshwater mussel families.
  subfamily - a taxon of the rank below family and above tribe and genus.
  superfamily - a taxon of the rank above family, comprised of a group of closely  related families; super-family Unionoidea is comprised of Unionidae and Margaritiferidae in North America.

Foot - bivalve organ located anteriorly; used for locomotion.
  muscular foot - an anteriorly directed, compressed, blade-like molluscan organ; usually adapted for locomotion.
  rudimentary foot - a small organ found in salt water mussels (Mytilidae), with a gland that secretes horny threads that harden on contact with sea water forming byssal threads; or, an anatomical component of a glochidium that gives rise to the muscular foot in successive life stages.

Gill pouches - anatomical structure of bivalves used to protect developing embryos; located in the gills.

Gills - two pairs are present in bivalves; used primarily for feeding and secondarily  for respiration; food particles are sieved and sorted from inhalant water using cilia as a transport mechanism.
  outer gills - the outermost pair of gills in certain bivalves; the gill structures closest to the mantle.

Gill filaments - structure of bivalves that forms the basis of gill structure; fused into sheets in freshwater mussels and lined with cilia.

Glochidia (see also: Larvae) - analogous to free living veliger stage in other bivalves, an immature freshwater mussel life stage modified for a parasitic existence; released through siphon or temporary aperture (e.g. Lampsilis) larvae may attach to host species; facilitates movement away from parent; drops from host after adult features are formed.

Growth lines

Height  - maximum projected straight-line distance on a valve between the dorsal and ventral edges (including umbo, if raised above shell).

Hinge line - the juncture along which 2 valves connect

Hinge teeth - opposing sockets and grooves that lock into teeth and ridges on bivalve shells to prevent lateral slippage.

Hooks - structures on glochidial shells in Anodonta; after being expelled through the siphon, glochidia sink to the bottom; the shells clamp shut onto the flesh of a passing fish host and are attached firmly by hooks.

Host - fish species selected by the glochidium for attachment; provides protection, food, and serves to disperse the glochidium.

Host tissue (see: Cyst) - tissue in a host species that the glochidium attaches to; such tissue is stimulated to grow around the glochidium, forming a cyst.

Inhalant (see: Siphon)

Incurrent (see: Siphon)

Inflated - refers to bivalve shell form, a rounding of the valves when viewed anteriorly or posteriorly; the opposite of compressed.

Interdental tooth - supplementary tooth found between the lateral teeth and pseudocardinal teeth on a bivalve.

Intestine (see: Digestive tract)

Invasive - refers to a species introduced into an area outside its normal range.

Juvenile - a life stage of an organism that, except for size and reproductive organs, is identical to adult stage.

Larva - an immature life stage; in bivalves either a veliger (free swimming form), or a glochidium (parasitic form).

Lateral tooth (see: Teeth) - bivalve shell structure of interlocking ridges and grooves; located parallel and adjacent to shell hinge.

Length - projected straight-line distance from anterior-most point of a bivalve shell to the most posterior point.

Lure - a dispersal strategy used by some species of bivalve, wherein the glochidia are suspended in a mucus matrix and floated in the water to attract a host fish species; or, a moveable adapted siphon tissue or mantle structure found in some freshwater mussels that is used to attract host fish species. A lure may aid glochidial attachment by  resembling food that attracts a potential host fish to the vicinity of the exhalant siphon.

Mantle - large sheet of tissue that lies within bivalve shells; secretes shell layers.

Marsupia (see: Gill pouches)

Marsupial gill pouches (see: Gill pouches)

Metamorphosis - process of marked body change; in freshwater mussels characterized by the formation of free-living adult body features from the parasitic glochidial stage.

Molecular data - data derived from DNA analysis, used to describe taxa and define taxonomic relationships.

Mouth - structure of bivalves that takes in food particles from the gills to begin the digestive process.

Mucilaginous line - secretion used by bivalves to entangle waterborne food particles; these are then sorted by cilia in the gills, and then moved along food grooves into the mouth and esophagus of the digestive tract.

Nacre - innermost layer of bivalve shell; comprised of aragonite and/or calcite deposited within an organic matrix (conchiolin); usually lustrous, resembling "mother of pearl", and sometimes with distinctive areas of bluish or pinkish color.

Nodulous - refers to sharp or rounded knobs occurring on a bivalve shell.

Nomenclature - in zoological nomenclature, a system of scientific names for taxa devised by Linnaeus and others, and the provisions for their formation, treatment and use.

Ovaries - egg production organs in bivalves, in which they surround the gut; in freshwater mussels, unfertilized eggs remain within the female, and male spermatozoa are introduced through the inhalant siphon for fertilization.

Oviduct - anatomical structure in bivalves that acts as a corridor from ovaries to the site of fertilization; eggs are generally held within the shell of freshwater mussels until glochidia are developed.

Packet - an accumulation of glochidia (larvae) within a bivalve, enabling a taxon to release high numbers of larvae at one time.

Pallial line - an interior shell scar, located where the bivalve shell is attached to the mantle.

Parasite (see: Ectoparasite)

Periostracum - outermost layer of a bivalve shell; usually of a horny organic material which protects the inner shell layers from acidic conditions.

Phylogeny - the evolutionary history of a group of organisms.

Posterior (see also: Anterior) - the "rear" of a bivalve; edge of a shell farthest from the umbo; area proximal to the siphon.

Posterior ridge - an arched or raised area of a valve, located on the posterior between the umbo and the valve margin.

Posterior slope - area on a valve located between valve margin and posterior ridge.

Pseudocardinal teeth - opposing sockets and teeth on bivalve shells that are located anteriorly of the hinge.

Sheath - a structure used to hold and later release glochidia in discrete bursts from an adult bivalve.

Shell - protective anatomical structure in bivalves secreted by the mantle and consisting of two or three layers: periostracum, prismatic layer, and nacre; comprised of two more or less convex valves.

Shell shape - the general shape, in silhouette, of a valve.
  subcircular - silhouette almost circular.
  subelliptical - silhouette elongate with rounded anterior and posterior margins.
  subovate - silhouette roughly oval with one side more flattened.
  subquadrate - silhouette with roughly four sides, and the length and height subequal.
  subrhomboidal- (see: Subquadrate)
  subtrapezoidal- silhouette with roughly four sides, the length being greater than the height.
  subtriangular - silhouette roughly three sided, or the anterior end of the valve is an equilateral triangle.

Silhouette - (see: Profile)

Siphon - tubular structure in bivalves formed by fusion of mantle tissue surrounding inhalant and exhalant organs; used to transfer food and water into and expel wastes from bivalve.
  exhalant - siphon structure used to expel water and waste from bivalve visceral mass.
  incurrent (see: Siphon, inhalant).
  inhalant - siphon structure used to transport water and food particles into bivalve shell.

Spawning - the production of young; initiated by union of spermatozoa and egg.

Spermatazoa - male gametes; generally released through the exhalant siphon for intake by nearby females.

Substrate - the structure of the bottom on, or in which, bivalve lives.

Suprabranchial chamber (see: Water tubes).

Synonomy - relationship between two or more synonyms; (see: Synonym).

Synonym - each of two or more scientific names of the same taxonomic rank used to denote the same taxon.

Umbo - dorsal prominence on bivalve shell located adjacent to the hinge; the oldest part of the shell.

Valve - one half of the complete shell of a bivalve.

Ventrolateral shoulder - an inflation or arching of the valve (shell) surface located along its ventral margin.

Visceral mass - in bivalves, the digestive, excretory, circulatory, and genital organs contained within the mantle.

Water tubes - an anatomical space in certain bivalves formed by folding of eulamellibranch gills; characteristically used by Sphaeridae for nurturing of larvae; young bivalves are shed directly from gills when development is complete (i.e., there is no glochidial stage).

Width - the distance measured between the outer edges of the left and right valves.