(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 cUmatic 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 Gallaglier, 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. 

 Malacologja 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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