Temporal and spatial variation of shellmicrostructure of Polymesoda Caroliniana (Bivalvia: Heterodonta)

Temporal and spatial variation of the microstructure of inner surface of shell, condition index
and organic content of shell of the Carolina marsh clam Polymesoda caroliniana (Bose) in three different
Mississippi habitatsare described and discussed in relation to one another and environmental
conditions. Microstructure of the inner shell surface distal to the pallial line showed distinct seasonal
variation but little spatial variation. Pseudospiral microstructure, on inner surface of shell undertucked
by the periostracum, predominated over "normal" crossed-lamellar microstructures in cooler seasons.
Presence and seasonal frequency of occurrence of complex crossed-lamella one inside the pallial line
reflected habitat differences. It was consistently present in submerged clams, present only in June
and September in wild clams, and absent in exposed clams. Survival and condition index of transplanted
clams in submerged area were higher than those clams in areas often exposed to air. Condition index showed seasonal and spatial variation, while organic content did not.