12 
BULLETIN OF THE BUREAU OF FISHERIES 
between 6 and 7 p. m., it occurs at 11 a. m. to 12 m. It was observed that at about 10 
a. m. direct sunlight struck the aquaria in which the specimens were immersed and 
that immediately, or within a few minutes, the oysters opened. It was thought that 
the bright light was responsible for the reaction; but, although light may have been 
concerned also, it would appear that rise in temperature was the primary stimulating 
agent. If it be assumed for convenience that these diurnal waves of shell activity 
are the effect of temperature fluctuation, as seems justified by comparison with the 
Olympia oysters, there are two sources of temperature variation which must be 
considered. 
In the first place, those factors in the laboratory, such as room temperature and 
direct sunlight, produce temperature change in the water in the aquaria. In the case 
of the specimens in nonrunning water (fig. 9), the most favorable temperature condi- 
tions occur at about 12 o’clock noon. On the other hand, the water in the tank 
A. M. p. m 
Figure 9.— Average day’s record of four specimens of 0. virginic a in nonrunning 
water over a period of 39 days, showing percentage of time open at each hour 
reaches optimum temperature (figs. 7 and 8) at around 6 to 7 p. m., because direct 
sunlight continues to warm the water all day. 
In Figures 7 and 8, not only can the effect of the latter be seen, but the former, 
namely, laboratory temperature, can also be seen. Comparison of the portions of 
these curves between 9 a. m. and 3 p. m. with Figure 9 shows that the sharp rise in 
the curves in Figures 7 and 8 at this time, appearing as humps, is strikingly similar 
to the comparable portion of Figure 9. 
It must be admitted that these records do not constitute as conclusive proof of 
the influence of temperature as the evidence cited for the Olympia oyster. However, 
by analogy it appears logical to assume that, in view of the similarity of the curves in 
the two cases, the eastern oyster behaves in a manner very similar to that of the 
Olympia oyster. The only other probable factor which might influence oysters in 
this diurnal manner is light, and there is some evidence that this may have been the 
source of some of the fluctuation in reactions. 
