Activity: Types of Hinges

The hinge is an important feature of the bivalve shell. It joins the two valves, and — importantly — keeps them aligned when the valves close (via the adductor musclesAdductor Muscle:
One of usually two large muscles (one anterior, one posterior) that contract to close the shell of a bivalve and maintain it in that condition; the position of these muscles is usually clearly marked on the shell interior as an adductor muscle scar or impression
) or open (via the ligamentLigament:
Elastic uncalcified structure that connects the two bivalve shells at the hinge line and functions as a spring to open the valves when the adductor muscles relax.
). It is widely used in taxonomy, and is usually characteristic at the family level. Ask your students to find specimens with the following basic types of bivalve hinges.

TaxodontTaxodont:
Having many, small, similarly-shaped hinge teeth.
— made up of many, small, similarly-shaped teeth. This is the most primitive kind of bivalve hinge. It keeps the valves closely aligned, but does not allow the valves to open very wide, limiting the bivalve’s movement.

Example: Ark Shells

HeterodontHeterodont:
Having more than one kind of hinge teeth.
— made up of two kinds of teeth: more central cardinal teethCardinal Teeth:
Hinge teeth located directly below the umbo in a heterodont hinge.
(radiating from the umbo, or beak, of the shell) and more remote lateral teethLateral Teeth:
Hinge teeth located far away from the umbo in a heterodont hinge.
. Lateral teeth can be absent in some species. This is a more derived kind of bivalve hinge. It allows the valves to open more widely, and was essential in the evolution of siphons, swimming, and other traits that require a wider gap.

Examples: Hard-shelled Clam, Lucine Clam, Crassatella Clam, Kitten’s Paw, Freshwater Mussel

EdentateEdentate:
Lacking hinge teeth.
— lacking teeth. Bivalves that lack teeth usually have other means of aligning their shells, such as a ligament or muscles. The Soft-shelled Clam (at right) has a spoon-shaped process at the hinge line; this is a resiliferResilifer:
Hinge structure supporting an internal ligament.
, a support for an internal ligamentLigament:
Elastic uncalcified structure that connects the two bivalve shells at the hinge line and functions as a spring to open the valves when the adductor muscles relax.
.

Examples: Blue Mussel, Soft-shelled Clam, Zebra Mussel