and the micro-growth lines are obscured. In addition, it is difficult to avoid the 

 formation of bubbles beneath the coverslip which obscure features. Equipment 

 for preparing tliin sections is available in rock preparation laboratories found in 

 most geology departments. Alternatively, there are commercial firms (e.g., 

 Rudolf von Huene, Pasadena, California) that will prepare thin sectioned 

 material. 



Ecological Applications 



Several workers (9, 14, 18, 30-32) have suggested that information about 

 physiological and environmental conditions may be recorded and stored in 

 molluscan shells. Various studies in wluch workers have used microstructural 

 increments within shells to extract such information are discussed below under 

 appropriate sub-headings. 



Seasonal Cycles 



Seasonally caused annual growth rates and patterns are observable in all 

 bivalves collected in climatic zones, ranging from cold-temperate to 

 sub-tropical. In many species, as winter approaches, there is a gradual slowing 

 down of the deposition rate, and the microstructural increments become 

 gradually thinner. This slowing down of growth in the autumn culminates in a 

 marked depositional break at the time of the first freeze. These depositional 

 breaks are characterized by indentations of the outer shell layer, a dark band of 

 organic-rich shell material extending downward from tlie base of the 

 indentation, small daily growth increments on either side of the break, and a 

 change in the shell structure near the break (9). These winter breaks may not 

 be as marked in specimens Uving subtidally (32), although Mercenaria 

 mercenaria from water depths of eight meters clearly show a distinct winter 

 break (9). 



Through careful examination of microstructural patterns, Farrow (13) 

 found that part of a population of the shallow sub tidal cockle, Cerastoderma 

 edule, from the Thames estuary in England, stopped growing during winter due 

 to sub-zero temperatures. Tevesz (34) observed that Gemma gemma grew very 

 little in the winter. Growth increments were very closely spaced and the inner 

 shell layer had a brownish hue. During the summer, G. gemma grew rapidly; 

 microstructural increments were v^dely spaced, and the inner shell layer was 

 clear and translucent in appearance. 



Through an examination of numerous acetate peels, Evans and LeMessurier 

 (12) were able to demonstrate striking winter growth rate differences between 

 two sympatric species of bivalves. They found winter growth of the 

 rock-boring clam, Penitella penita, to be approximately 75 percent of the 



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