EFFECT OF ORGANIC SUBSTANCES ON OYSTERS 



173 



THE OYSTER'S RESPONSE TO 



CARBOHYDRATES 

 ANALYSIS OF SHELL MOVEMENTS 



In analyzing the activity of oysters, as shown 

 earlier, it is necessary to consider separately shell 

 movements and pumping rates. First, we have 

 attempted to isolate and define the elements of 

 the relaxation and contraction of the adductor 

 muscle as reflected by recorded shell movements 

 so as to relate oyster activity to carbohydrates. 



Figure 4 illustrates representative shell move- 

 ments. A represents the full gape of the oyster 

 as recorded by the apparatus. This range of 

 "openness," or gape, can be divided into three 

 levels, or phases, which we designate as phases I, 

 II, and III. Each phase has its physiological 

 significance. 



We interpret phase I as resulting from the 

 activity of a single, special set of muscle fibers. 

 It is characteristic of these fibers that they relax 

 "all or none," and thus cause the almost instan- 

 taneous gape as typified by B. Notice that (his 

 set of fibers does not close the valves with a single 

 sweep but in steps which have been designated as 

 "treppe" (Galtsoff 1946). It is apparent that 

 there is a distinct mechanism involved in this 

 phase of shell movement. This phase is charac- 

 terized, both opening and closing, by a more 

 rapid response to external stimuli than are phases 

 II and III. 



We believe that phase II must be regarded as a 

 delayed phase III, since it is a transition between 



phases I and III. This phase represents a test- 

 ing period, and probably involves only the 

 promyal and cloacal passages, since the oyster 

 rarely will pass more than 6 or 7 liters of water an 

 hour during this phase. It is probable that the 

 musculature involved is the same as in phase III, 

 but that there is a repressor mechanism which 

 delays progress into phase III until certain envi- 

 ronmental requirements are satisfied. Normally 

 an oyster will not remain long in this phase. 

 Figure 4 makes clear why we refer to any open- 

 ing not going beyond phase II as a testing period. 



In phase III, an oyster attains maximum gape 

 and pumps the maximum amount of water. The 

 pumping rate varies with conditions, so that it is 

 essential to record both effluent and shell move- 

 ment. Phase III, then, represents the full degree 

 of valvular gape which under ideal conditions con- 

 tinues except for certain anomalies. This portion 

 of the curve is broken at various intervals with 

 partial closures indicated in figure 4 (lower) as Y. 

 These anomalous closures might be termed "expul- 

 sion." or "snap," movements of the valves, proba- 

 bly to void accumulations of inert solids or irri- 

 tating substances. These snap closures do not 

 normally enter the zone of phas<> 1 1, but when they 

 do, the rate of reopening is much slower than when 

 they do not. In figure 4 (lower), A r indicates an 

 abnormal closure resulting from a mechanical 

 disturbance to thcoyster. Note that the reopening 

 is much slower than from the Y closure. 



Each of the three phases has associated with it 



Figure 5. — The basic components of shell movements of oysters and their relation to pumping rates. Each cycle of the 

 measuring box represents a minimum of 500 milliliters; the Roman numerals on the pumping rate curve correspond 

 to the three phases demonstrated. Reproduction of actual record. Paper speed 2 inches an hour; vertical lines 

 represent quarter-hour intervals. 



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