(2) Alternating aerobic-anaerobic metabolic cycles are proposed as the 
physiologic mechanism for forming shell periodicity structures. Aerobic 
respiration is associated with shell calcification. Shell closure, 
accompanied by anaerobic metabolism, results in shell decalcification; 
acidic end-products are neutralized by dissolution of shell calcium 
carbonate. 
(3) Shell growth patterns can be easily studied by preparing shell thin 
sections or acetate peel replicas of acid-etched shell sections. Scanning 
electron microscopy of fracture or polished and etched shell sections 
can also be employed. 
(4) Patterns of growth increment sequences and shell structural changes are 
related to seasonal climatic cycles and, on shorter time scales, to lunar 
and solar periodicities. Semiperiodic or random events, such as storms, 
sedimentation events, or biological events (e.g., reproduction) are 
superimposed as “noise” on the geophysical cycles. Causal effects for 
this “noise” can be deduced by detailed studies of the growth record. 
(5) Shell growth patterns have proven useful in paleoecologic 
reconstructions. Detailed analysis of these patterns also promises to be 
an efficient manner in which to conduct after-the-fact or retrospective 
monitoring studies of pollution events. 
ACKNOWLEDGEMENTS 
Parts of this manuscript are a summary from a document prepared by Dr. 
Josephine Yingst; a contributor to the E.P.A. shell-growth manual. We thank 
A.S. Pooley, E. Tveter Gallagher, and A. Krishnagopalan for technical 
assistance with the scanning electron microscopy; W.C. Phelps for preparation 
of specimens; and W.K. Sacco for his assistance in photograph reproductions. 
This research was supported in part by Environmental Protection Agency grant 
R804-909-010 and NOAA grants 04-6-158-44056, SGI-77-17, and 
04-7-158-44034. Contribution number 108 from the Ira C. Darling Center, 
University of Maine, Walpole, Maine, 04573. 
REFERENCES 
1. Barker, R.M. 1964. Microtextural Variation in Pelecypod Shells. 
Malacologia 2: 69. 
2. . 1970. Constituency and Origins of Cyclic Growth Layers in 
Pelecypod Shells. Ph.D. Thesis, University of California, Berkeley, 265 p. 
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