exposures to malathion in concentrations of 0.002 and 0.02 mg/£. Chironomid 

 and mayflies also decrease considerably (Kennedy and Walsh 1970). 



Lesnikov (1974) suggests that the most sensitive indicator of the ef- 

 fects produced by organophosphorus compounds is an increase of both popula- 

 tion and biomass of aquatic organisms (Table 5). 



TABLE 5. DYLOX TOXICITY (mg/£) FOR SELECTED AQUATIC ORGANISMS 



The hazards of organophosphorus pesticides are even greater since ani- 

 mals demonstrate poor avoidance reactions to these chemicals. Some inverte- 

 brates do not avoid Dylox at all ( Hirudo medicinalis ), or some such as 

 Asellus aquaticus and Stephocephalus torvicornis avoid it only in concentra- 

 tions of 250 to 1000 times higher than their 48-hour LC50 (Flerov and 

 Lapkina 1976; Tagunov and Flerov 1978; Flerov and Tagunov 1978). Guppies 

 demonstrate avoidance reactions to Dylox at concentrations equal to the 48- 

 hour LCioo (Flerov 1979). 



Shrimp ( Palaemr"^^-'-:, pugio ) fail to avoid malathion (Hansen, et a1 . 

 1973), and mosquito fish avoid it only in acutely toxic concentrations 

 (Hansen, et al_. 1972). 



The toxicity of organophosphorus compounds is attributed to their 

 ability to inhibit acetyl cholinesterase irreversibly, which in turn, de- 

 pends upon the particular enzyme system in the animals. 



Thus, the two species of gastropods ( Limnaea stagnalis and Planorbis 

 corneus ) differ in resistance to Dylox by 100 times. The nervous ganglia of 

 these forms contain enzymes of the acetyl cholinesterase type, that 

 hydrolyze the same substrates, but differ in quantity, electrophometic 

 mobility and sensitivity to the toxicant. In vitro experimentation with 

 the sensitive species ( Limnaea stagnalis ) showed concentrations of 10"^ 

 to 10"^M Dylox completely inhibited enzyme activity, lO'^M inhibited by 

 97 percent, and lO'^M inhibited by 61 percent. In the resistant species, 

 Planorbis corneus , even concentration of lO'^^M of toxicant did not inhibit 

 the enzyme completely, although the enzyme content in ganglia of this 

 species is much lower than in Limnaea stagnalis (Figure 1 and 2). 



The correlation between the resistance of organism to the toxicant and 

 the sensitivity of an enzyme to it has also been observed in fish. The 

 roach, Rutilus rutilus , and the blue bream, Abramis ballerus , are poorly 

 resistant to Dylox. Their blood sera contains an enzyme of the cholinester- 

 ase type which is absent in more resistant fish, such as the carp, Cyprinus 



87 



