The equivalent median diameters of the natural sediment (minerals) 

 and the Fuller's earth could not be verified by the light microscope, 

 since many of the particles were beyond the detectable size limits 

 of that instrument. However, the particles of Si02 and Fuller's 

 earth were subjected to particle population analyses using a Coulter 

 Model T counter equipped with a 100-micrometer orifice. This counter 

 verified our observations with the light microscope. 



The larger proportion of numbers of particles was present for both 

 of these materials at Coulter volume sizes less than 8 micrometers, 

 even though their median equivalent spherical diameter sizes (by 

 weight) were different by a factor of more than 30 (<0.5 micrometer 

 for Fuller's earth versus 17 micrometers for Si02) . 



We have no evidence to show that selective grasping of particles 

 was or was not occurring. However, reduced ingestion of food cells 

 with increasing sediment concentrations, as demonstrated, supports 

 the hypothesis that nonselective filtering occurred in our experiments. 



c. Effects of Patuxent Silt (500 milligrams 1"^) on Feeding 

 Activity With Increasing Time . The short-term feeding experiments 

 for A. tonsa and E. af fin-is (5 and 10 minutes, respectively) were 

 extended to 125 minutes (Table 3) and 240 minutes (Table 4) for 

 exposure to 500 milligrams 1"-^ Patuxent River silt. 



The decrease in the number of radioactive cells ingested during 

 exposure to the sediment treatment stayed constant for as long as 

 125 minutes. This reduction (77.5 percent) approximates the reduction 

 in carbon uptake in Table 2 for 500 milligrams 1"-^ natural sediment 

 (72 percent) . The decrease in cell numbers filtered in both control 

 and sediment treatment over the time of this experiment may indicate 

 that cell concentration was becoming low enough to be a limiting 

 factor. 



The feeding activity pattern of E. af finis (Table 4) under the 

 500 milligrams 1"-^ sediment treatment is more clearly defined over the 

 4-hour feeding period, excluding the serious errors of excretion and 

 respiration of the carbon tracer (Conover and Francis, 1973). The 

 control feeding activity peaked at 5, 45, and 120 minutes. Feeding 

 activity with 500 milligrams 1"-^ silt added was more erratic than 

 the control pattern but generally follows it and remains significantly 

 depressed. An increase in activity at 3.75 hours was tested and was 

 not significant. 



These data for sediment effects for exposures up to 4 hours lend 

 support to our contention that these two species do not discriminate 

 food from nonfood. The sediment treatment response may be indicative 

 of the diluting effect of sediment on radioactive cell concentrations. 

 Since the guts of these organisms were full even during the 4-hour 



32 



