C/) 



100 n- 



<i: 



^ 80H 



6^ 

 CD 





50- 

 50- 



40- 



30- 



d 20- 





O 

 O 



oo 



10- 



8- 

 7- 

 5- 

 5- 



:r 4 



END OF EXPERIMENT, 96 HOURS 



^ 



THRESHOLD 



CONCENTRATION 



FOR FOOT IMMOBILIZATION 



120 ^00 300 400 600 sio 

 POTASSIUM CONCENTRATION mg/l 



Fig. 1. — Foot-immobilization response 

 curve. The 5-hour EC50 of approximately 

 600 mg of potassium per liter of water is 

 an extrapolated value, .^fter 5 hours foot 

 immobilization had occurred in fewer 

 than 50 percent of the clams exposed to 

 the highest concentration of potassium 

 (458 mg/liter). All other EC50's plotted 

 on this graph were determined by inter- 

 polation, i.e., more than 50 percent of the 

 clams had responded at the high concen- 

 trations and fewer than 50 percent had 

 responded at the lower concentrations. 



cept.s the concentration axis. The threshold concentration 

 of potassium for the foot immobilization response of 

 Corbkula m. is 120 mg per liter of water. Fig. 2 shows 

 that gaping also occurs within 48 hours of expo.sure to 

 potassium and that the threshold concentration is prob- 

 ably close to 190 mg of pot;issium per liter of water. The 

 gaping response curve appears to be reaching an asymp- 

 tote at 190 mg/liter after 96 hours of exposure, but a 

 longer exposure period than the 96 hours used in our 

 experiments would be necessary to confirm the threshold. 

 It is important to note that 85 percent of the clams ex- 

 posed to 458 mg of potassium per liter of water did not 

 j gape; rather, they clamped tightly shut, sometimes pinch- 

 I ing the extended and immobilized foot (Fig. 3) . 



The foot-immobilization response is considered signifi- 

 cant because it results in direct exposure to the toxicant 

 of a portion of the clam's body. Gaping likewise permits 



contact between the tissues of the clam and chemicals in 

 the surrounding water. One of the problems in attempts 

 to control the .\siatic clam has been it.s ability to with- 

 stand slug dosing of chemicals. The resistance is in part 

 attributed to the clam's habit of tightly closing its shell 

 when exposed to an irritant. Thus, a high concentration 

 of a toxicant and a long exposure time are required to 

 control the adult clam effectively. Although additional 

 research is needed, the foot-immobilization and gaping 

 respon.ses might prove useful in the control of the clam. 

 Exposing the clam to potassium could be accompanied 

 or followed by exposure to a mollu.scacide or other con- 

 trol agent, and thus a shorter exposure time or a reduced 

 concentration of the toxic agent might be sufficient. 



The 96-hour LC50 was 225 mg of jxita.ssium per liter 

 of water. The 95-perccnt confidence limits for this value 

 were 157-322 mg of potassium per liter of water; the 



