growth, particularly in specimens sampled from high elevations (highly stressed 
environments) within the intertidal zone. 
Lutz and Rhoads (26) have recently presented evidence that structural 
changes within the shells of certain bivalve species may reflect periodic 
dissolution and “reworking” of primary depositional structures during periods 
of extreme environment stress. Here, alternating periods of aerobic and 
anaerobic metabolism provide the driving forces for shell deposition and 
dissolution, respectively. Parallel annually-formed sub-layers of nacre and 
simple aragonitic prisms (24, 25) within the inner shell layer of the Atlantic 
ribbed mussel, Geukensia demissa (Figure 12-1), for example, were interpreted 
as reflective of seasonal metabolic changes. In populations from Gulf of Maine 
waters, nacre deposition was restricted to the relatively warm months of the 
year (24, 25). During both the fall and spring, nacreous tablets on the inner 
shell layer growth surface became smaller and less regular, showing visible signs 
of erosion in the form of marked pitting and “hollow crystals”, as well as 
increased proportions of Fine-grained structures. Differential dissolution of 
calcium carbonate and organic material was also often observed at the inner 
layer growth surface during these seasons (Figure 12-2). During the colder 
months of the year (January — March, with water temperatures below 3°C), 
shell erosion became visible to the naked eye, the entire inner shell surface 
often presenting a chalky white appearance. Ultrastructurally, this surface 
appeared uniformly fine-grained or “homogeneous” (Figure 12-3). Similar 
visible erosion has been reported in Mercenaria mercenaria after long periods of 
valve closure (11). The ability of G. demissa to endure anaerobiosis for 
extended periods has been well documented (20, 22), as has the relative 
increased efficiency in this species of some of the citric acid cycle enzymes in 
an anaerobic direction (16). The observed shell erosion may well be a reflection 
of buffering of acid end-products from anaerobic metabolism during the colder 
months, when oxygen transport into the cells should theoretically be reduced 
relative to that occurring at higher temperatures (21). Wibur (37) has suggested 
that during periods of “adverse environmental conditions”, shell decalcification 
may predominate over growth. The often-seen gradation in fractured, as well as 
polished and etched, vertical shell sections of G. demissa nacreous laminae into 
finely grained structures (suggestive of massive erosion), instead of regular 
prisms, (Figure 12-4) tends to support this view. 
SUMMARY 
(1) Environmental and biological events are recorded in the molluscan shell 
in the form of small-scale growth increments and/or changes in shell 
structure. 
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