The duration of the pollution event should be considered relative to the 
mean life 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 life span that is long relative to the duration of the pollution event. If the 
ecological stress is sublethal, a record of growth 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 ot 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. Asa 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 thickness from 10° to 10" 
ju. 
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 lines 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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