Although a one-to-one correspondence has not been estabhshed, the deposition 

 of increments in bivalves is highly correlated with shell valve movements (27, 

 30, 35, 36). As the valves of many species are generally closed during low tide, 

 and open during high tide, a high positive correlation also exists between the 

 number of increments and the number of tides to which an organism has been 

 subjected. While valve-movement rhythmicity is generally most pronounced in 

 intertidal individuals, subtidal specimens of at least one species {Mercenaria 

 mercenaria) exhibit biological rhythms in relative harmony with the tidal cycle. 

 There is general agreement among growth line workers that when the valves are 

 open and the organism is actively pumping, a layer is deposited which is rich in 

 calcium carbonate relative to adjacent shell material. The origin of alternating 

 layers or lines relatively rich in organic content has recently been theorized by 

 Lutz and Rhoads (26). The following few paragraphs summarize this theory 

 which is based on recent studies of molluscan anaerobiosis and mechanisms of 

 shell formation. 



During aerobic metabolism, molluscs deposit calcium carbonate, in the form 

 of either aragonite or calcite, together with organic material, resulting in shell 

 construction. Such metabolism is usually highly correlated with periods of 

 active pumping- during higli tide in well-oxygenated waters. As the 

 concentration of dissolved oxygen falls, such as in the microenvironment 

 created by the organism during periods of shell closure, anaerobic respiratory 

 pathways are employed and levels of succinic acid (or other acidic 

 end-products) within the extrapallial fluid rise. The acid produced is gradually 

 neutralized by shell calcium carbonate, leading to increased levels of Ca and 

 succinate (or other end-products) v^thin the extrapallial and mantle fluids (8). 

 As a result of this decalcification, the ratio of relatively acid-insoluble organic 

 material to calcium carbonate increases at the mantle-shell interface. One need 

 not invoke the complication of increased concentration of organic material in a 

 given volume, although a collapse of unsupported matrix structures or 

 movement of the mantle as a compensatory response to the increased 

 mantle-shell distance could result in increased concentrations of freed organic 

 material in specific regions of the extrapaUial fluid. With the return of 

 oxygenated conditions and resumption of aerobic metabolism, and assuming 

 shell deposition during this post-anaerobic period proceeds via a process similar 

 to that occurring immediately prior to anaerobiosis, deposition of calcium 

 carbonate and organic material within an area already containing organic 

 material should result in an increase in the organic/ CaCOo ratio within the 

 specific shell region. The end-product of this process, from a strictly structural 

 viewpoint, is one growth increment. 



161 



