6 
BULLETIN OP THE BUREAU OP FISHERIES 
From the above two cases it appears that change of temperature is more impor- 
tant in affecting the length of time which Olympia oysters remain open than the degree 
of temperature itself. In Figure 3 the records of two specimens are reproduced in 
brief to illustrate the direct effect of changes in temperature of the water. 
The figure represents the record of two specimens as heavy, solid lines when the 
shells were open and light, broken lines when closed. The hours of the day are 
shown as abscissae and the degrees of temperature as ordinates. During the first 
day the temperature was fairly high and the specimens were mostly open, but near 
the end of the second day (Mar. 7) the temperature fell from about 16° C. to 7° or 8° C., 
due to stopping of the heater, and the specimens closed. On the following day (Mar. 8), 
the specimens opened for a while following a rise of a few degrees in temperature of the 
water, but did not remain open. On March 9 they acted similarly, but remained open 
Figure 4.— Graph showing shell activity of the same two specimens as in Figure 1. The per- 
centage of time open at each hour of the day over the 29-day period is plotted. Average 
temperature readings for each hour during the same period are plotted to show the regular 
daily temperature fluctuation 
due to the sharp rise in temperature. During the next four days a repetition of this 
occurred, as shown in the figure. 
Many examples such as this are at hand, but their presentation is unnecessary. 
The closing of the oyster, as a result of lowering of temperature and opening following 
rise in temperature, both appear to be responses to the stimulation of temperature 
change, as is discussed further below. 
DIURNAL VARIATION IN SHELL BEHAVIOR 
It was pointed out by Nelson (1921) that oysters ( 0 . virginica ) are open less at 
night than during the day, but he did not consider that this might be due to tempera- 
ture. Galtsoff (1928) was able to observe no diurnal variation in shell activity, but 
his temperature records showed no considerable diurnal variation. 
In the present investigation not only was there a marked diurnal variation in the 
amount of time oysters ( 0 . lurida ) remained open, but also this could be directly 
correlated with temperature fluctuation. 
