EFFECT OF CRUDE OIL POLLUTION ON OYSTERS 
179 
gravity ranged from 1.0231 to 1.0273 (17.5° C.). The average rate of flow during 
treatment is 97.0 percent of the rate before treatment. It woidd appear that 5 
percent soluble fraction is no more toxic to oysters than 1 percent, over a period 
of 24 hours. However, on closer examination we find an interesting situation in 
this group. Of the seven experiments, four show an average increase of 11.2 percent 
during treatment, comparable to 12.0 percent shown by controls. The remaining 
three experiments have an average rate of 78.2 percent during treatment, comparable 
to the rate obtained with 10 percent solutions. 
In considering factors which might be responsible for the effect, or lack of it, a 
comparison was made between the conditions of the two groups of experiments. 
The average temperature of the four experiments in which an increased rate of 
flow resulted from treatment is 26.7°. The average for the three experiments whose 
flow was reduced during treatment is 26.9°. This difference of 0.2° between the. 
averages for the two groups is less than the fluctuations generally found during the 
course of an experiment. It is obvious that temperature differences could have no 
connection with the disparity in effect of the soluble fraction solution. 
Also, the effect of the soluble fraction has no apparent relationship with the 
specific gravity of the laboratory sea water. In experiment 182, the low specific 
gravity of 1.0231 was accompanied by an increase of 18 percent in rate of flow during 
treatment. In experiment 144, the high specific gravity of 1.0272 was associated 
with an increase in rate of flow of 13.9 percent during treatment. Consequently, 
there is no evidence that an increase or a decrease in rate of flow during treatment 
has any connection with a high or low specific gravity of the sea water, though this 
statement is intended to apply only to the limits of salinity occurring in these 
experiments. 
The difference in specific gravity between the laboratory sea water and the soluble 
fraction was, in most experiments, not over 0.0002. Where the difference exceeded 
this figure, notably in experiment 144, in which the difference was 0.0007, the effect is 
negligible compared with fluctuations in experiments where differences in specific 
gravity were not present. For example, experiments 106 and 107 were carried on 
simultaneously. They received soluble fraction prepared from the same sample of 
oil and sea water, were treated for the same length of time and the soluble fraction 
was adjusted to the same specific gravity as the laboratory sea water. Yet oyster 
106 increased the rate of flow during treatment by 5.9 percent, while oyster 107 
reduced its flow 19.5 percent. 
Since there is no intergrading effect between the two groups of oysters used in 
these experiments, the inference can be drawn that 5 percent solution exerted a 
definite effect on one group of them which comprised specimens more sensitive than 
the others. In other words, this concentration may be regarded as a threshold of 
inhibitory action. 
These results are based on treatments extending only for 18 or 24 hours. It is 
possible that the same depression of the efficiency of ciliary motion may be reached 
with smaller concentrations acting over a longer period of time. 
Ten percent soluble fraction solution very definitely inhibited ciliary activity 
and resulted in an average decrease in rate of pumping during treatment of 23.8 
percent. (Table 11.) 
There is no relationship between the increase in the specific gravity of the 
undiluted oil extract and the effect on rate of flow during treatment. In experiment 
