cided with the maintenance of a steady cloaca! 

 current. 



The determination of the number of hours the 

 oyster remains open under average normal condi- 

 tions is of significance in studies of reactions of 

 the mollusk to changes in its envh'onment. Cer- 

 tain industrial wastes discharged in sufficient 

 concentrations into natural waters reduce the 

 time the oysters stay open. It was found, for 

 instance, that the red lirjuor which is the waste 

 product of pulp mills using acid tligestion of wood 

 and the black licjuor of pulp mills which employ 

 a sulfate process exert this effect on the Olympia 

 oyster (Hopkins, 1931) mid on C. rirginica (Galtsoff, 

 Chipman, Engle, and Calderwood, 1947). Any 

 condition that forces oysters to remain closed for 

 an abnormally long time deprives them from 

 taking in food and eventually may harm them. 



The percentage of time during each 24-hour 

 period that the oysters are open can be used as an 

 index of normal behavior, provided the shell move- 

 ments of the mollusk do not indicate pathological 

 conditions of the type shown in curves D-1 to 

 D-5. Failure to recognize the significance of this 

 type of shell movement while recording the time 

 the oyster remains open may lead to serious mis- 

 understandings and errors. Unfortunately there 

 are many published data in which the "time open" 

 was recorded without observing the character of 

 shell movements. 



The length of time C. rirginica remains open is 

 also influenced by temperature and by the state 

 of the oyster itself. Since the shell movement is 

 influenced by several external and internal factors, 

 it is not surprising that there is a gi-eat discrepancy 

 in the estimates of the average duration of "open 

 shells" reported by various investigators. 



In the Bureau's shellfisli laboratory at Woods 

 Hole from June 15 to October 15, 1926, 132 daily 

 records of 34 oysters observed gave an average of 

 17 hours 7 minutes for open shells. The tempera- 

 ture of the water during tiiis period ranged from 

 13° to 22° C, but daily fluctuations of temperature 

 were insignificant, never exceeding 1.5° C (Galt- 

 soff, 1928). Records of the three oysters kept by 

 Nelson under observation for 21 days in New 

 Jersey water indicated that the shells remained 

 open on tlie average of 20 hours per day at tem- 

 peratures varying between 22° and 25° C. (Nelson, 

 1921). For oysters kept in running sea water at a 

 Beaufort, N.C., laboratory, average time open in 

 October to November varied between 10 and 14 



hours (Hopkins, 1931). The temperature of water 

 was not recorded. Two hundred and one daily 

 records of 49 York River (Virginia) oysters kept 

 under observation in the laboratory at Yorktown 

 showed that the periods of opening varied from 19.2 

 to 24.0 hours a day (Galtsoft", Chipman, Engle, 

 and Calderwood, 1947). Within the temperature 

 range of 17.0° to 28.0° C. Long Island Sound oys- 

 ters were found to remain open for an average 

 period of 22.5 hours. The latter data are based 

 on 64 records of 18 oysters (Loosanoff and No- 

 niejko, 1946). 0. lurida of the Pacific Coast 

 remained open for an average of 20 hours a day 

 at the temperature range of 5° to 17° C. (Hopkins, 

 1931). 



A sample of oysters always includes several 

 individuals tliat may remain closed for 24 hours 

 or longer. One or two of them will reduce unduly 

 the average figure based on a small number of 

 observations. Furthermore, under identical con- 

 ditions of the normal environment (i.e., not 

 affected by pollution, dredging, or other disturb- 

 ances) an oyster may keep its shell open or closed 

 for varying periods of time depending on the 

 requirements of the organism for food and oxygen. 

 I found that immediately after spawning the 

 female oysters have a tendency to keep their shells 

 closed for several days. On the other hand, 

 oysters left overnight out of water open almost 

 immediately upon being returned to sea water. 

 It is reasonable to assume that they accumulated 

 an oxygen debt during the period of closure. 

 In view of these observations the differences in 

 the duration of periods of opening or closing 

 described for oysters of different localities have no 

 particular significance. Tlie average value may 

 be useful, however, in determining the adverse 

 effects of the changes in the population of oysters 

 in a given locality and in making a comparison 

 between the behavior of these individuals in clean 

 and polluted waters. 



EFFECT OF TEMPERATURE 



Temperature as such has no direct influence on 

 the diu-ation of shell opening. There was no 

 significant difl'erence in the length of time the 

 Woods Hole oysters remained open when kept at 

 temperatures varying from 15° to 30° C. (Galtsoff, 

 1928). It is rapid change in temperature, often 

 occurring in those laboratories where sea water is 

 subject to wide diurnal fluctuations, that has a 

 pronounced effect on shell movements. O. lurida, 



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