Study of 95 Western Atlantic bivalve mollusk species representing 29 families has demonstrated that morphologic features of the bivalve shell other than hinge type and microstructure primarily reflect life habits and habitat preferences. Many morphologic features represent potentially powerful tools for paleoecologic and evolutionary interpretation.
Byssally attached groups living with the sagittal plane vertical are characterized by elongation and flattening of the ventral margin. Non-burrowing, byssally attached arcids and carditids, for example, are more elongate than closely related burrowing species. Epifaunal mytilids can be distinguished from related infaunal and semi-infaunal species by cross-sectional shape, degree of anterior reduction, and configuration of the byssal musculature and sinus. Tridacnids show special adaptations for exposure of algae-infested siphonal tissue to sunlight.
Byssally attached groups living with the sagittal plane non-vertical are characteristically inequivalve, with the lower valve flattened for maximum substratum contact. Pectinids and pteriids utilize a special auricle-sinus configuration for firm byssal anchorage. The posterior “wing” of many pteriids apparently shelters the exhalent current to increase its discharge distance.
Epifaunal groups attached by cementation are commonly thick-shelled. The spines of certain cemented groups apparently serve a defensive function.
Many reclining taxa employ a thick shell and a flattened upper valve for stability. The lower valve is commonly convex, if partly buried in life position, and nearly flat, if lying at the substratum surface.
Most swimming species have thin shells. Pectinids with a free-living adult stage differ from those with a byssally attached adult stage in their high degree of auricle symmetry and large umbonal angle. Their shape is adapted for improved swimming ability; it serves to increase the forward component of propulsive forces, the volume of water expelled, and the aspect ratio of the shell.
For most burrowers in soft substrata, approximate burrowing orientation can be determined from simple geometric relationships. Angle of rocking movement and direction of substratum penetration show a general relationship to degree of shell elongation. Nearly all rapid burrowers are either (1) inhabitants of shifting sand substrata or (2) migratory deposit feeders. Most rapid burrowers have become streamlined by developing shells that are crudely disc-like, blade-like, or cylindrical, rather than spheroidal. Elongate and pointed shell anteriors among burrowing species often coincide with rapid-burrowing habits. Strongly ornamented and thick valves are employed primarily by sluggish shallow burrowers for stability” near the sediment-water interface; radial, divaricate, and oblique ornamentation also aid certain species in burrowing. Life orientation of burrowers living with the sagittal plane vertical is often reflected by features of the siphonal region and lateral-view outline. A non-vertical life position is usually associated with an inequivalve condition; in many tellinids, these two features are adaptations for siphonate deposit feeding. Most infaunal species that characteristically inhabit substrata with a mud fraction exceeding 25 percent are small, and many are thin-shelled, for flotation. Tube dwellers and deep burrowing groups with large siphons tend to have distinctive shell forms and are generally confined to cohesive substrata of fine-grained or slightly muddy sand.
Most groups that bore into hard substrata are similar in gross form to Mya-like deep burrowers with large siphons, but many possess special shell ornamentation for substratum abrasion. Species that nestle in pre-existing cavities in hard substrata tend to have irregular shell shapes.
Evolutionary divergence, convergence, and homeomorphy support functional interpretations and provide strong arguments for the adaptive nature of shell form in the Bivalvia. Divergent representatives of the Mactracea, for example, show remarkable convergence in form and habit with species belonging to other superfamilies.