![]() Nearly all rapid burrowers are either (1) inhabitants of shifting sand substrata or (2) migratory deposit feeders. Angle of rocking movement and direction of substratum penetration show a general relationship to degree of shell elongation. 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.įor most burrowers in soft substrata, approximate burrowing orientation can be determined from simple geometric relationships. 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. The lower valve is commonly convex, if partly buried in life position, and nearly flat, if lying at the substratum surface. Many reclining taxa employ a thick shell and a flattened upper valve for stability. The spines of certain cemented groups apparently serve a defensive function. The posterior “wing” of many pteriids apparently shelters the exhalent current to increase its discharge distance.Įpifaunal groups attached by cementation are commonly thick-shelled. Pectinids and pteriids utilize a special auricle-sinus configuration for firm byssal anchorage. Tridacnids show special adaptations for exposure of algae-infested siphonal tissue to sunlight.īyssally attached groups living with the sagittal plane non-vertical are characteristically inequivalve, with the lower valve flattened for maximum substratum contact. 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. Non-burrowing, byssally attached arcids and carditids, for example, are more elongate than closely related burrowing species. ![]() Many morphologic features represent potentially powerful tools for paleoecologic and evolutionary interpretation.īyssally attached groups living with the sagittal plane vertical are characterized by elongation and flattening of the ventral margin. 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. ![]()
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