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lABORATORY INVESTIGATIONS AQUATIC ORGANISMS During 96-hour toxicity tests, several species of freshwater and marine invertebrates and fishes died at chlorpyrifos concentrations between 0.035 and 0.58 ug/l. lC-50 (96-hour) values, in ug chlorpyrifos/l, for sensitive species tested were 0.035 for mysid shrimp, Mysidopsis bahia; 0.11 for amphipod, Gammarus lacustris; 0.13 for fathead minnow, Pimephales promelas; 0.38 for stonefly, pteronarcella badia; and 0.58 for striped bass, Morone saxatilis; (Mayer and Ellersieck 1986; Table 3). Toxicity was usually greater at elevated temperatures and at increasing pH levels (Johnson and Finley 1980). In general, arthropods were the most sensitive group assayed and molluscs the most tolerant (Borthwick and Walsh 1981; Table 3). The bullfrog (Rana catesbeiana) also appears to be comparatively tolerant to chlorpyrifos, as judged by a single oral lD-50 value of >400 mg/kg body weight (Hudson et a 1. 1984). Sublethal effects of chlorpyrifos exposure have been documented for many species of freshwater and marine fauna; they include inhibition of cholinesterase (ChE) activity levels in brain and hematopoietic organs, sluggishness, motor incoordination, delayed maturation and growth, reproductive impairment, and reduced feed intake (Rongsriyan et al. 1968; Thirugnanam and Forgash 1977; Marshall and Roberts 1978; Tagatz et al. 1982; Jarvinen et al. 1983; EPA 1985,1986; Goodman et al. 1985b; Norberg and Mount 1985; Hansen et al. 1986). Reproductive impairment, for example, was observed in Daphnia magna at 0.08 ug chlorp.yrifos/l (EPA 1985). Reduction in setting rate was shown in oyster larvae after exposure to 0.1 ug/l for 8 days (Tagatz et al. 1982). In fathead minnows exposed to 0.12 ug chlorpyrifos/l for 200 days, ChE activity was significantly reduced, fecundity was reduced, maturation delayed and, in second generation fish, growth and maturation were reduced (Jarvinen et al. 1983). Equilibrium loss was documented in 50% of brown shrimp (Penaeus aztecus) after exposure to 0.32 ug chlorpyrifos/l for 24 hours (Marshall and Roberts 1978). Growth of the California grunion (leuresthes tenuis) was reduced 20% in an early life stage during immersion in 0.5 ug chlorpyrifos/l for 35 days and 26% in fry after exposure to 1.0 ug/l for 26 days (Goodman et al. 1985b). Additional and more comprehensive data on sublethal effects of chlorpyrifos to aquatic biota are listed elsewhere (EPA 1986). 6