The duration of the pollution event should be considered relative to the 

 mean Ufe span of individuals (turnover rate). Again, the ideal situation is one 

 where the species overlapping the affected area is one with a low turnover rate, 

 and a Ufe span that is long relative to the duration of the pollution event. If the 

 ecological stress is sublethal, a record of grovv1;h before, during, and after the 

 stress event, may be recorded within the living population, and can be 

 compared with that of the reference population outside the affected area. If 

 the stress results in high mortality, the death assemblage may be all that 

 remains to document the event. 



MOLLUSCAN GROWTH PATTERNS 



Environmental information is stored within the molluscan shell in the form 

 of either (1) microstructural growth increment sequences or (2) changes in the 

 shell structural type (e.g. nacreous, prismatic, crossed-lamellar, etc.) or relative 

 proportions of structures within the shell. These two distinct types of records 

 and their usefulness in ecological studies are discussed below. Much of this 

 discussion is taken directly from a recent article by Lutz and Rhoads (26). 



Microstructural Growth Patterns 



During the past decade, numerous workers (2-4, 13, 14, 30-32) have 

 described microstructural increments within the molluscan shell. As a result of 

 marked periodicity associated with many of these structures, they have proved 

 useful in geophysical studies for defining changes in the earth's rotational rate 

 (3, 29-31), in ecological and paleoecological studies for assessing the effects of 

 various biological and environmental stresses (9, 14, 18, 30, 32), and in 

 archaeological studies for reconstructing migration patterns of prehistoric 

 hunter-gatherers (6, 7, 19). When shells are viewed in cross-section (procedural 

 details outlined in Methods section below), these microstructural patterns are 

 seen as alternating bands of shell material ranging in tliickness from 10*^ to 10 



Many, if not all, microstructural periodicity structures within the molluscan 

 shell are a reflection of variations in the relative proportions of organic material 

 (conchiolin) and calcium carbonate (aragonite or calcite). Alternation of 

 calcium carbonate-rich layers and organic-rich regions or hnes has been well 

 documented for numerous recent and fossil species through detailed studies of 

 shell thin sections, acetate peels, and polished and etched surfaces, employing 

 polarizing, optical, and scanning electron microscopy (see Methods section). 

 "Daily" growth increments have been reported by several workers (2, 13, 14, 

 18, 30-32). These "daily" lineations were originally interpreted as reflections 

 of solar time (13, 14, 17, 30, 31). Recent studies, however, have revealed a 

 complex relationship between incremental growth, and lunar and solar cycles. 



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