8 
BULLETIN OF THE BUREAU OF FISHERIES 
a long period, as in Figure 1, indicates that it is the rapidity of the change which 
determines the oyster’s reaction. From all of the foregoing results it may be con- 
cluded that a rise in temperature causes oysters to open, while a drop causes them 
to close. It is of importance to determine how sensitive the oyster is to temperature 
changes under different conditions, that is, does a certain temperature change at a 
high temperature produce the same reaction as the same change at lower temperatures? 
In an effort to answer this question partially from the existing results, a table 
(Table 2) was prepared, consisting of data similar to those shown in Figure 4. Several 
series, or portions of series, were selected to cover the widest available temperature 
range. The average hourly temperature values and percentages of time open are 
given in parallel columns for each series. Maxima and minima of the diurnal waves 
are italicized, except in one case in which the individual variation is too great to be 
sufficiently eliminated by the few days’ duration. The average temperatures of the 
several series range from about 5° to about 16° C. The maxima and minima are 
repeated at the bottom of the table, and the difference between them determined in 
each case. The average temperature difference between trough and crest is close to 
2° C. in most cases, so that 2° C. may be considered the standard change in temperature 
the effect of which it is desired to determine. In order to bring all the results to this 
basis, the difference between maximum and minimum of the time open wave, or the 
amount of increase in time open following the given temperature rise, was calculated 
by simple proportion to conform to a 2° C. difference, that is, in the first column the 
temperature difference is 2.87 and the time open difference 88.4 per cent. Then to 
correct the latter to a 2° C. change, 2:2.87 = x:88.4; x = 61.6 per cent, which may be 
employed as representing the reaction to a rise of 2°, although this may not be 
strictly accurate. 
In Figure 6 is a graph showing these results (solid points) as the increase in per- 
centage of time open following the 2° C. rise in temperature during the average day 
(ordinates) plotted against the maximum value of the temperature wave (abscissae). 
It will be observed that these results fall remarkably well, considering great individual 
variations, into alignment. At low temperatures a change of 2° C. is accompanied 
by a great change in the length of time the specimens remain open; at high tempera- 
tures (14° to 17° C.) the same temperature change produces scarcely any change in 
the percentage of time the specimens remain open. In brief, the extent of the oyster’s 
reaction to such a change in temperature of the medium is a function of the existing 
temperature, and is greater the farther the basic temperature from the optimum. On 
this curve there are not enough points to permit mathematical analysis. The ideal 
line may well be actually curved, especially as it approaches the optimum and also 
near the other end. 
In contrast to the above, on the same chart (fig. 6) the results of the same tests 
are plotted (circles) as the percentage of time the specimens were open during the 
entire time against the average temperature for the same period. If a direct relation- 
ship exists between temperature, as such, and the length of time open, these points 
should also fall into a definite line. This is not the case, for although the points 
might be considered as falling about such a line, the variation is tremendous, and also 
a certain amount of alignment would be expected, due to the difference in sensitivity 
at different temperatures. The contrast between the two sets of points, representing 
the same actual data, is striking, and demonstrates that the influence of temperature 
on shell movement of this type is more a matter of sensitivity to temperature change 
than of temperature as such. 
