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 repHcate 25 

 ml water samples per aquarium were v^thdrawn 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 Unear 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 In C^ 



(no. animals) x A T min) C^ 



where C and C 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'^. 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 weU 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 



