BIVAVLIA
MORPHOLOGY

General Morphology

Beak: The region of initial growth of shell. The beak can be curved to either the anterior or, less commonly, the posterior. The general region of the beak is often called the umbone.

Auricle: A wing-like protrusion along the dorsal margin, this can be either anterior and/or posterior of the beak. See the wing-like projection in the examples.

Byssal notch: Anterior depression below the auricle from which the byssal threads emerge. Such a notch can be viewed directly beneath the anterior auricle in this example.

Escutcheon: A small curved area on the dorsal margin posterior to the beak. Both valves must be joined to view the escutcheon.

Lunule: A small curved area on the dorsal margin anterior to the beak.

Equivalved: Both valves being equal in size and shape.

Inequivalved: Two valves of unequal size and shape.

Equilateral: An individual valve that is symmetrical along its mid-line as in most brachiopods.

Inequilateral: Valves that are not symmetrical along their mid-line as is the case for most bivalve species.

Dentition and Ligaments

Cardinal teeth: The teeth immediately below the beak (see Figure 3 below).

Lateral teeth: The teeth extending laterally from the beak.

Edentulous space: Hinge region lacking teeth, usually present between the cardinal and lateral teeth.

Resilifer: A small depression along the hinge plate which holds an internal ligament; may be a single pit or consist of multiple pits.

Taxodont dentition: A series of small parallel to sub parallel teeth which are perpendicular to hinge line.

Heterodont dentition: having cardinal teeth and lateral teeth either in front and/or behind beak.

Desmodont dentition: having an internal ligament and a chondrophore, but usually lacking well defined teeth.

Schizodont dentition: having prominent bifurcating or diverging teeth.

Figure 3 - Bivalve Dentition

 

Musculature

Pallial line: Line of mantle attachment (see Figure 4 below).

Pallial sinus: An indentation in the posterior part of the pallial line where the siphons can be retracted.

Dimyrian: A valve having two adductor muscle scars; one anterior and one posterior.

Isomyrian: A dimyrian shell where two adductor scars generally equal in size.

Anisomyrian: Dimyrian shell where the two adductors are of unequal size; usually the posterior scar is the larger of the two.

Monomyrian: A shell having only one adductor scar; which is usually a centrally positioned posterior adductor scar.

Figure 4 - Interior Shell Markings

 

Feeding

Most bivalves are filter feeders, trapping suspended food particles as water passes through their gills. Only two groups, the nuculoids and cryptodonts, actively feed on organic material within the sediment and are thus true deposit feeders. Taxodont dentition is characteristic of deposit feeders.

Relations to Substrate

Bivalves have a variety of morphologic features that can be related to their particular life habit or mode of attachment to the substrate. We will examine several.

Infaunal
  • Burrowing: Shells are usually equivalved and isomyrian (or anisomyrian) with a distinct pallial line. They include: the nuculoid burrowing deposit feeders, the shallow burrowing non-siphonate forms lacking a pallial sinus, and deep burrowing siphonate forms identified by a distinct pallial sinus.
  • Boring: Shells are usually thick, equivalved, and cylindrical in cross section. Some forms are moderately ornamented with ridges and stout spines whereas others such as the "ship worms" are tubular in form.

Semi Infaunal
  • Byssally attached (endobyssate). Similar to many epifaunal byssate forms (see below), yet maximum shell width (inflation) is at mid-line of shell cross-section. Some forms can be elongated and fan-like with a reduced anterior area. Examples include pen shells, and the mussel-like modiolids, and some ark shells. The depth to which the bivalves are partially buried can often be deduced by looking for encrusting organisms that may have attached themselves above the sediment-water interface.

Epifaunal
  • Byssally attached (epibyssate). Shells can be either equivalved or inequivalved depending on their orientation to substrate during life. Usually, all epibyssate forms have a reduced anterior region. Some groups, such as the blue mussels, are similar to endobyssate forms except the maximum inflation is below the mid-line of the valves cross-section. Other forms may have a byssal notch and/or a well defined auricle, or, as in the case of some arks, have a gape along the ventral margin.
  • Reclining. Shells are commonly inequivalved with a larger lower (usually the left) valve which is more inflated or convex while the upper valve may be planar. Some also exhibit spines, especially on the lower valve, to aid in stabilization in soft substrates in a manner similar to some brachiopods. Many have a small attachment area at beak where earliest growth stages were cemented. The giant clam Tridacna, who has photosymbionts similar to hermatypic scleractinian corals, is a recliner even though it had a functional byssus during its earliest juvenile stages.
  • Swimming. Shells are usually equilateral but not equivalved. The lower (usually the left) valve is usually slightly larger. Swimming forms are typified by having a greater umbonal angle (greater than 105°) than similar-looking epibyssate forms. Furthermore, swimming forms typically have a single (monomyrian), large, centrally located adductor muscle.
  • Cementing. Shells are commonly inequivalved with the lower (usually left) valve assuming the form of the object to which it is cementing, a condition called xenomorphism. In such cases, both valves are usually highly variable in shape, as in the common oysters and other forms as well. Some groups such as the Cretaceous rudists could reach very large sizes and were able to form reefs mimicking corals in both morphology and ecology.

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