growth, particularly in specimens sampled from high elevations (highly stressed 

 environments) within the intertidal zone. 



Lutz and Rlioads (26) have recently presented evidence that structural 

 changes witliin 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) witliin 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 tliese 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 moUuscan shell 

 in the form of small-scale growth increments and/or changes in shell 

 structure. 



167 



