the number usually varied from 50 to 75, while in the experimental 

 set-up, after the addition of oil, it fluctuated between 15 and 25, 

 No more detailed study of this was made because no sample of water 

 from the mixing chamber could be removed without interfering with the 

 operation of the constant level tank. This observation suggests, 

 however, that the presence of substances leached from crude oil may 

 exert some repelling effect on copepods# 



Three tests, lasting 16, 33, and 15 days respectively, were per- 

 formed during July and August of 1948, The conditions of the test, 

 so far as temperature, salinity, rate of renewal of water, and amount 

 of oil used, were almost identical. The results are summarized in 

 tables 21, 22, and 23 and are presented graphically in figures 12, 13, 

 and 14* 



The reactions of the oysters to the water contaminated with oil can 

 be easily observed in the changes in the muscular movement and in the 

 rate of pumping. A very distinct case of almost immediate response is 

 shown in figia*e 15, which represents photographic reproduction of the 

 original record. The following changes become apparent shortly after 

 the introduction of oil into the sea water syateraj (1) Partial con- 

 traction of the adductor muscle (1st line) euid the ensuing reduction 

 in the degree of opening of the valves and restriction of the access 

 Of water to the mantle oavityj (2) increase in the number of the 

 oontraotions of the adductor and decrease in the extent of each con- 

 traction, shown by the vertijaal excursion of the up and down strokes 

 of the lever, (3) marked decrease in the rate of pumping of water 

 (2d line). No such changes take place in the control oyster (figure 16) 

 which continues to pump water at a very high rate. Sometimes these 

 changes eire less pronounced and become apparent only in about one hour 

 after the addition of the oil -contaminated water (figure 17). In all 

 three experiments, however, a change from pure to oil -contaminated water 

 sooner or later resulted in a significant decrease in the percentage of 

 time the oysters remained open and in a marked reduction in the rate of 

 pumping. 



The depressing effect of the substance or substances leached from 

 oil lasts for a long time. This can be seen by examining the data 

 summarised in tables 21, 22, and 23, and shown graphically in figures 

 12, 13, and 14. In the first two experiments (figures 12 and 13) there 

 was, apparently, a common factor which affected both the experimental 

 and the control oysters and was responsible for the rise and fall in 

 the rate of pumping after the eighth day of the test. The nature of 

 this factor has not been definitely established but it is reasonable to 

 postulate that it was correlated with the prespawning condition of the 

 oysters. In the summer of 1948,. oysters kept in laboratory tanks spawned 

 several times between July 8 and 31. Mass spawning was observed on July 24, 

 tiie day which corresponds to the eighth day of the second experiment 

 (figTire 13). Our previous experience in studying spawning of the oyster 

 shows that the maximum rate of pumping by the oysters may be expected 

 at the time they are ready to shed their sex products. It is therefore 

 reasonable to infer that gradual increase in the rate of pumping 

 observed in our tests was due to this cause. This tentative explanation 



39 



