Figure 9-4. 24-Hour Recovery: Comparison of Filtering 
Activity of Mytilus edulis After 24 Hours of Recovery from 
Two Weeks Exposure to W.A.F. No. 2 Fuel Oil. 
Gradual improvement of all groups was observed as the animals remained in 
clean water. After two weeks, recovery was almost complete (Figure 9-5). The 
control and 0.1 ppm groups both Filtered at an accelerated rate of 30.9 ml/min, 
and the 1.0 ppm group Filtered at a rate of 17.2 ml/min, characteristic of 
pre-exposure rates. The higher Filtration rates observed may reflect increasing 
ambient water temperatures at the time of testing. Temperatures increased 
from 11°C at the time of pre-exposure testing, to 19°C during this latter 
testing period. Due to a laboratory failure resulting in reduced water flow, and 
subsequent anaerobic conditions in the recovery tank which held the 0.01 ppm 
exposure group, it was necessary to discard these mussels without 
demonstrating their complete recovery. However, since mussels at higher oil 
concentrations did recover, it seems reasonable to assume recovery for the 0.01 
ppm exposure group as well. There remained a signiFicant difference between 
the 1 ppm exposure group and the controls after two weeks in clean water; 
however, after one month of recovery, they actually fed slightly better than 
the controls. 
Based on the current investigation, it appears that the adverse effect of oil 
on Filtration rate of mussels is reversible, provided the stressed animals are 
returned to unpolluted conditions. However, the data also strongly suggest that 
recovery does not occur under conditions of continued exposure. Further 
investigation is currently in progress to determine the implications of reduced 
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