356 
BULLETIN OF THE BUREAU OF FISHERIES 
SERIES VI 
With this specimen another attempt was made to obtain comparative data on 
the reactions to a series of salinities, the changes being made in relatively small 
steps. The summarized results are shown in figure 9 and table 1, series VI. After 
4 days in a salinity of 28.31 parts per mille, during which there was some variability 
in activity, the salinity was reduced to 23.57 parts per mille and left for 4 days. 
Adaptation appeared to be quite rapid on the first 2 days ; but on the fourth day, for 
unknown reasons, the value of F was lower than before. 
When the salinity was changed to 20.90 parts per mille adaptation was appar- 
ently complete on the second day. However, reduction to 16.31 resulted in an 
entirely different behavior, 
the specimen becoming adapt- 
ed within a few hours to a 
certain low level of pumping 
which was maintained with- 
out improvement during the 
entire period of the test (6 
days) . Examination of figure 
9, as well as the results of pre- 
ceding tests, leads to the sug- 
gestion that salinities below 
about 20 parts per mille be- 
come more and more strik- 
ingly unfavorable in their 
Figure 9. — Graph of daily averages of values of S (degree of openness) and F (rela- , 
five rate of flow) at different salinities. Series VI. 17°-19° C. Cn eCt . 
Whether adaptation would 
take place in such instances to the extent that water would be pumped at the same 
rate as at higher salinities may not definitely be stated, but it is clear that adaptation 
is extremely slow. 
SERIES VII 
In those experiments which have already been described it was indicated that 
when specimens are placed in water of a salinity of about 15 to 17 parts per mille 
the rate of pumping is greatly reduced and adaptation is very slow, possibly never 
reaching the level observed at higher salinities. It was also shown that a salinity 
as low as 10 to 11 parts per mille results not only in almost complete stoppage of 
the flow of water through the gills but also in a harmful effect from which recovery, 
after return to high salinity, is extremely slow. It would appear that the lower sa- 
linity limit which the oyster can tolerate lies somewhere between 10 and 15 parts 
per mille. This experiment was performed in order to locate this limit more 
exactly (series VII, table 1). 
Preliminary tests were carried on over a period of 9 days with a salinity of 
25.09 parts per mille before changing to a salinity of 13.00 parts per mille. 
During the first day at the lower salinity the valves did not open wide and there 
was no flow of water. On the next day it looked as if the specimen was going to 
become well adapted, but during the following days this rate of activity was not 
maintained. During 8 days of treatment in water of this salinity only a sluggish 
stream was pumped as compared with the activity observed during the preceding 
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