Freshwater Mussels & Endangered Fish Glossary

Unless otherwise noted, these definitions represent a compilation from those previously published by Burch (1975),Clarke (1981), Cummings and Mayer (1992), Howells et al. (1996), and Webster’s Ninth New Collegiate Dictionary (1990).

anadromous—ascending rivers from the sea for breeding

anterior end—the shorter end of the shell as measured from the umbo to the margin. In a live animal, the foot extends from this end, and this end is usually embedded in the substrate

beak cavity—a cavity that extends into the beak, usually under the pseudocardinal teeth (Oesch 1995)

biramous—having two branches

bradytictic—species with females gravid for long periods, usually becoming gravid in late summer or early fall and remaining gravid through the spring or early summer

branchial septa—a dividing membrane within the gills or associated structures

demibranchs—one-half of one of the paired gills of a freshwater bivalve (Parmalee and Bogan 1998)

dendritic—resembling a tree in structure

dorsal margin—the margin of the shell with the umbo, hinge, and ligament

endemic—restricted or peculiar to a locality or region

elliptical—oval

elongate—stretched out

excurrent aperture—an opening of the mantle at the posterior end of the mussel which expels water, wastes, glochidia or sperm; may be tube-like; above incurrent aperture

extant—in reference to a population, present as live individuals in a particular area

extirpated—in reference to a population, no longer present as live individuals in a particular area

fish host—a species of fish parasitized by the glochidia of a mussel species

foot—a muscular part of the body used for movement on or within the substrate; extends from antero-ventral shell area

glochidia—the parasitic life stage of a freshwater mussel

gravid—condition when glochidia or young mussels are present in the gills

growth rests—a ridge formed during an intermediate stage of growth when this area was the edge of the shell (Clarke 1981)

hermaphroditic—an individual that has both male and female sexual organs (Clarke 1981).

hinge line—the dorsal edge where the two valves of a shell meet; connected by the hinge ligament

incurrent aperture—an opening of the mantle at the posterior end of the mussel which brings water and other resources into the body of the mussel; may be tube-like; below excurrent aperture

interdental projection—a "tooth" that juts out from the interdentum

interdentum—the area between the pseudocardinal and lateral teeth

iridescent—a play of colors producing rainbow effects (as in a soap bubble)

lateral teeth—the long blade-like structures along the hinge line of each valve and closest to the posterior end of the shell

left valve—the shell half on the left side when the hinge is facing up and the anterior end is directed forward (away from you)

mantle—the folds or lobes of the body wall of a mollusk that lines the shell and bears shell-secreting glands; the number of lobes or folds varies by species.

marsupium—structures for enclosing or carrying eggs or young

nacre—the inner surface of the shell

obovate—ovate with the narrower end basal

pallial line—a linear impression on the inner surface of the shell, parallel to the ventral margin, the line of attachment of the mantle muscles.

papillae—small finger-like sensory projections associated with the outer margins of the apertures, and in some species, associated with the outer margins of the mantle.

periostracum—the external surface of the shell

plications—parallel ridges

posterior end—the longer end of the shell as measured from the umbo to the margin. In a live animal, the siphons extend from this end.

posterior ridge—a ridge along the external surface of the shell extending from the umbo to the posterio-ventral margin.

pseudocardinal teeth—the compact, often triangular, tooth like structures along the hinge line of each valve and closest to the anterior end of the shell.

rays—streaks or lines of color on the external surface of the shell.

right valve—the right half of the shell when the hinge is facing up and the anterior end is directed forward (away from you)

sexually dimorphic—a condition in which males and females of the same species are morphologically different, usually indicated in unionoids by an expanded posterior marsupial area in the female in contrast to a more pointed or bluntly rounded area in the male. (Parmalee and Bogan 1998)

shell—the hard outer covering of the mussel.

subovate—less than ovate

suprabranchial chamber—the dorsal portion of the gills

tachytictic—species with females gravid for short periods, usually late spring or early summer

teeth—the structures along the hinge line on the inner surface of the shell that serve to lock the two valves together when the animal closes its shell.

terminus—an extreme point or element

trapezoidal—a four sided form having only two sides parallel

umbo—the raised part at the dorsal margin of each valve, an external structure

valves—one of the two halves of the shell

vestigial—a structure that is less fully developed compared with closely related forms

ventral margin—the bottom edge of the shell, opposite the hinge line