were captured individually from the culture flasks in a minimum amount 

 of water (usually less than 5 milliliters per individual). The captur- 

 ing tool was a glass tube fitted with a rubber-suction bulb at one end 

 for drawing the copepod into the tube. Ten copepods were placed in 

 each bottle. Bottle volume was adjusted to 125 milliliters with water 

 from the zooplankton culture flasks containing the phytoplankter_Af. 

 tutheri. In this way, phytoplankton cell concentrations (not radio- 

 active) in the bottles varied from 5,000 to 10,000 cells ml"'',. Zoo- 

 plankters were allowed to feed on these cells for 0.5 to 1 hour before 

 the start of the experiment. During this time they also became some- 

 what acclimated to the environment of the bottle. 



The feeding experiment was initiated by injecting a volume of 

 radioactive M. luthevi cell suspension into one bottle of a replicate 

 pair of bottles containing 10 zooplankters each. The other bottle in 

 this pair received an identical volume of the radioactive cells 5 

 minutes later. This cell suspension volume never exceeded 1 percent 

 (1.25 milliliters) of bottle volume, but it was sufficient to provide 

 a final radioactive cell concentration of close to 50,000 cells ml" . 

 Experiments were conducted at 20 ± 2°' Celsius under laboratory_c(DiLl^ 

 white fluorescent illumination (approximate intensity = 0.2 x lO** ergs 

 cm sec ) . Feeding activity was terminated by pouring the contents of 

 each bottle onto a stainless steel (type 304) wire sieve (325 mesh). 

 The bottles were rinsed with 50 milliliters of Whatman GFC filtered 

 Patuxent River water. This water was then poured through the sieve. 

 Preliminary studies showed that this 50-milliliter rinse effectively 

 washed away sorbed carbon-14 and any phytoplankton cells adhering to 

 the zooplankters which were trapped on the stainless steel mesh. 

 Immediately after rinsing, the sieve was placed under a dissecting 

 microscope and the live zooplankters were removed individually by 

 grasping an antenna with fine-pointed forceps. Each organism was 

 placed into a separate liquid scintillation vial containing I milli- 

 liter of Soluene-100 (Packard Instrument Company, Downers Grove, 

 Illinois), a rapidly acting tissue solubilizer (quarternary amine). 

 The vials were capped and digestion was allowed to proceed for at 

 least 2 hours at room temperature. With practice all zooplankters 

 (10 under optimal experimental conditions) could be efficiently re- 

 moved from the sieve and placed into liquid scintillation vials within 

 2 minutes of feeding termination. 



The zooplankters in successive replicate pairs of bottles repre- 

 senting longer feeding time intervals were handled in exactly the 

 same way (5-minute lag between bottles of a replicate pair). 



At the end of the digestion period, 15 milliliters of acidified 

 Instagel were added to each vial. Acidified Instagel was prepared as 

 a 1:9 (volume to volume) solution of 0.5 N HCl and Instagel (Packard 

 Instrument Company, Downers Grove, Illinois). The vials were placed 

 into a Packard Tricarb (Model 3320) liquid scintillation spectrometer 

 and counted at 9° Celsius after allowing 24 hours for reduction of 



20 



