animals were then transferred to control conditions, and their filtration rates 
measured periodically to determine recovery. 
Gill filtration rate was measured indirectly by recording the rate at which 
the animals removed food particles from the surrounding water. Each cage of 
50 mussels was transferred from its exposure or recovery tank to individual 
glass aquaria containing equal and known quantities of Isochrysis galbana. The 
algal suspension was maintained by aeration. Subsequently, three replicate 25 
ml water samples per aquarium were withdrawn at intervals during a three-hour 
feeding period. The number of algal cells in each sample was counted on a 
Coulter electronic cell counter, and the average percent reduction through time 
recorded for each aquarium. 
Filtration rates were expressed both graphically and numerically. Feeding 
curves were generated by plotting percent food particles removed versus time 
to allow visual comparison between the feeding rates of the control and oiled 
mussels for each of the various exposure or recovery periods. Results were 
analyzed statistically by performing linear regressions on the natural log 
transformed data, and comparing the regression lines (9). Actual filtration 
rate—the rate at which a solution is pumped through the gills of the animal in a 
given time period—was determined numerically with the aid of the following 
formula (8): 
Filtration rate (ml/min/mussel) = voL solution ( ml > x ln £o 
(no. animals) x A T min) C t 
where C Q and C t represent food concentrations at the beginning and end of a 
particular feeding interval (AT). The solution is based on the assumption that if 
filtration rate remains constant over the feeding interval, then the rate at which 
particles are removed from suspension will decline exponentially, as described 
by the curve e‘ x . For a given group of mussels, filtration rate was finally 
expressed as the average of those values calculated separately for each interval 
during which the mussels were actively removing food particles. Averaging is 
necessary to correct for the fact that the calculation of filtration rate can vary 
slightly depending on the magnitude of the time interval selected, a result of 
the fact that particles are not always removed at an exact exponential rate. 
The flow-through oil exposure system is designed to dose marine animals 
with the water-accommodated fraction of No. 2 fuel oil at three nominal 
concentrations-0.01 ppm, 0.1 ppm, and 1 ppm. The W.A.F. contains finely 
dispersed oil as well as the water-soluble components, but does not include the 
whole oil slick. The system simulates an area of chronic petroleum 
hydrocarbon pollution, such as one that might exist near a sewage outfall, an 
oil refinery, or an area consisting of sediments that have been heavily 
contaminated with oil. 
114 
