Table 1 Toxicity of Cyclodienes and their Photoisomers to Some Freshwater Animals Insecticide Bluegills LD50 Minnows (p.p.m.)Asillus Mosquitoes f Aldrin 0.26 0.08 0.003Photoaldrin 0.09 0.04 0.0005 Dieldrin 0.17 0.024 0.006Photodieldrin 0.03 0.010 0.003 Heptachlor 0.013 0.10 0.005Photoheptachlor 0.008 0.06 0.002 Isodrin 0.012 0.019Photoisodrin 0.025 0.058 Endrin 0.010 0.017Mortalities recorded after 24 h. t Data from refs. 1-3 for Aedes aegypti larvae.The toxicity of all these photoisomers to freshwater animals which constitute food chains has not been reported. Because cyclodiene insecticides have been used extensively, the hazards of their persisting residues may be magnified by concentration of the lipophilic metabolites of their photoconversion products in food chains (Georgacakis and Khan, unpublished work). We report here the results of a preliminary investigation on the toxicity of these photoisomers to some freshwater animals. We used animals collected from natural conditions or purchased from local suppliers. The fish were acclimated at 65 F in an environmental room for at least 5 days in mixtiues of habitat water and distilled water in the following respective proportions: 2:1, 1:2, 1:8, and distilled water only. The distilled water was aged at this temperature for about 2 weeks with constant aeration (oxygen concentration 10 p.p.m.). Fish were exposed in 1 pint mason jars containing 200 ml. of constantly aerated water. Each jar contained no more than four fish, tadpoles or crayfish and about five replicates (twenty animals) were used for each concentration. Microcrustaceans were exposed in 25 ml. of water in plastic Petri dishes (five to ten animals per dish) with at least two replicates. Each experiment was repeated at least twice and the average values of the two used to determine LC50 or LT50 values4. Assays were per- formed at 65 F using the distilled water in which the fish were acclimated. Six graded concentrations (logarithmic dosages) between predetermined dosages which caused zero and 100% mortality were used to determine LC50 values.Table 1 shows LC50 values of the cyclodienes and their photoisomers to fish, Crustacea and insect. To the fish isodrin and endrin are the most toxic insecticides. Photoisodrin is about two times less toxic than isodrin to all these animals. Photoaldrin, photodieldrin and photoheptachlor are more toxic than their parent compounds aldrin, dieldrin and hepta- chlor, respectively. Photoaldrin seems to be specially toxic to mosquito larvae1. The onset of toxic symptoms leading to death is much faster with photoaldrin, photodieldrin and photoheptachlor than their parent compounds as judged by LT50 values for a number of aquatic animals (Table 2). Again, photoisodrin seems to be less toxic than isodrin or endrin; tadpoles, fish, isopods and Daphnia are more sensitive than Gammarus, crayfish and plannarians. Toxicity of photodieldrin has been reported for only a few animals6"8, where it has been found to be two to four times more toxic to rats, mice, guinea-pigs and pigeons, equally toxic to beagles and less toxic than dieldrin to the domestic fowl. To the freshwater animals tested here, however, photoaldrin, photodieldrin and photoheptachlor are more toxic than their parent compounds. Photoisodrin is less toxic than isodrin or endrin. Because the first three photoisomers can be converted to more toxic lipo- philic ketones3>4, the toxicity of these photoisomers produced by sunlight1-3'8 or by microorganisms9 may be magnified by the biological concentration10 of lipophilic ketones in animal food chains. The possible hazards of such a magnification are obvious, and research on the analysis of these photoisomers and their ketones in the environment, especially in the Great Lakes region, is badly needed. Table 2 Toxicity of Cyclodienes and their Photoisomers to Various Freshwater AnimalsLT50: min of continuous exposure AnimalAldrin Photoaldrin Dieldrin Photodieldrin Endrin Photoisodrin CrustaceaFreshwater flea, Daphnia pulex (1) 290 205 280 220 180 380 Freshwater isopod, Asillus spp (1) 160 75 80 50 35 55Freshwater amphipod, Gammams spp (1) 2,200 1,200 2,200 180 150 350 Freshwater crayfish, Cambarus spp (1.8) 1,140 390 1,000 200 200 1,000Planaria V Freshwater flatworm Dugensia spp (1) 400 300 300 800Amphibia Tadpoles (1) 50 30Fish Guppies, Gambia affinis (1) 90 35 130Minnows, Pimephalus promelas (0.7) 70 100 90 55 130 Bluegills, Lepomis macrochirus >360f loot >360f 70f 360| 440|Values in parentheses show the concentration to which the animals were exposed. t Concentration was 0.24 p.p.m.J Concentration was 0.2 p.p.m. This work was supported by grants from the National Science Foundation to Professor E. B. Hadley and from the University of Illinois at Chicago Circle Research Board. We thank Professor Hadley for facilities, and Mr W. P. Braker and Mr R. Lent for fish.