Although a one-to-one correspondence has not been established, 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 high 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) within 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 extrapallial 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/ CaCO^ ratio within the 
specific shell region. The end-product of this process, from a strictly structural 
viewpoint, is one growth increment. 
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