Figure 158. — The average percentage of time open of two 

 specimens of 0. lurida at each hour of the day observed 

 over the '29-day period (solid circles). Average tem- 

 perature readings for each liour during the same period 

 (open circles). From Hopkins (1931), fig. 4, p. 6. 



for instance, has a tendency to close with the fall- 

 ing of temperatm-e and open with a rise of tempera- 

 ture (Hopkins, 1931). The sensitivity of this 

 oyster to temperature changes was reported to be 

 greater at the lower range. At 4° to 6° C. the 

 oysters remained closed a relatively high per- 

 centage of the time; at 6° to 8° C. they were open 

 only about 6 hours, while at the maximum of 

 about 15° C. they remained open over 23 hours 

 per day. In both cases the diurnal curve of shell 

 activity was parallel to the curve of temperature 

 fluctuation observed by Hopkins (1931) although 

 the percentage of time open in warmer water was 

 consistently higher (fig. 158). It would be of 

 interest to repeat these observations and compare 

 them with controls kept at a constant temperature 

 since Hopkins' temporary laboratory near Olym- 

 pia, Wash, lacked adequate ecjuipment for regula- 

 tion of temperature. He concluded that "change 

 of temperatui-e is more important in affecting the 

 length of time Olympia oysters remain open than 

 the degree of temperature itself." The results 

 of his observations on (\ virginica at Beaufort, 

 N.C., bear close resemblance to those described 

 above but, unfortunately, they were not accom- 

 panied by thermograph records and so are 

 not entirely convincing. His conclusions need 

 clarification. 



EFFECT OF LIGHT AND DARKNESS 



Periods of light and darkness have no apparent 

 effect on the closing or opening of the valves. 



Analysis of 103 daily records of shell movements 

 of oysters kept in the Bm'eau's Woods Hole lab- 

 oratory in running sea water at neai'ly constant 

 temperature (daily fluctuations ±0.5° C.) and 

 constant salinity sliows that of the total number 

 of 831 hours of inactivity (shell closures), 266 hours 

 or 32 percent occurred during the 8-hour period of 

 darkness and the balance of 565 hours, or two- 

 thirds of the total, took place during the remaining 

 two thirds of daylight (Galtsoff, 1928). During 

 the summer, from June to August inclusive, the 

 Long Island Sound oysters kept their shells open 

 for 94.4 percent of the total time during daylight 

 and 93.8 percent during the hours of darkness 

 (Loosanoff and Nomejko, 1946). These observa- 

 tions repudiate Nelson's conclusion that the 

 periods of inactivity (or closings) occur dm'ing 

 darkness (Nelson, 1921, 1923c). 



EFFECT OF TIDE 



There is no evidence that the opening and 

 closing of oyster valves is related to the stages of 

 tide. The idea that oysters living below the 

 low-water mark are relativel^y inactive during the 

 outgoing tide and that the times of cessation and 

 commencement of feeding are correlated to stages 

 of tlie tide, was several times expressed by Nelson 

 (1922, 1923a, 1923c, 1938) and without verification 

 was accepted by Orton in his article in Encyclo- 

 pedia Britannica (Orton, 1929). Loosanoff and 

 Nomejko (1946) analyzed the kymograph tracings 

 of shell movements of oysters kept under vu-tually 

 natural conditions on a platform installed on a 

 small oyster bed on the bottom of Milford Harbor 

 in Long Island Sound. They found that the shells 

 remained open on an average of 93.4 percent of 

 the time during the flood periods and 95.2 percent 

 of the time during the ebb periods. The tidal 

 changes in Long Island Sound are not accompanied 

 by the excessive changes in the temperature, 

 salinity, pH, and turbidity of water which fre- 

 f}uently take place in the tidal streams of the 

 southern Atlantic states and may influence the 

 shell movements of oysters. 



POWER OF THE ADDUCTOR MUSCLE 



Anyone who attempts to open a live oyster by 

 inserting and twisting a knife between the two 

 valves becomes aware of the considerable resistance 

 exerted by the mollusk. As a rule the valves of 

 healthy 03-sters just taken out of sea water are 

 difficult to pry apart. The power of the adductor 

 muscle, which is solely responsible for keeping 



THE ADDUCTOR MUSCLE 



175 



