AbstractLaboratory experiments were conducted to evaluate the mysid crustaceanHolmesimysis costataas a test organism for routine complex effluent toxicity testing. Three‐day‐old juvenile mysids, hatched in the laboratory from field‐collected gravid females, were used in repetitive 48‐h and 96‐h mortality tests. With zinc sulfate as a reference toxicant, mean 96‐h LC50 and no observed effect concentration (NOEC) values forHolmesimysiswere 97 and 56 μg/L, respectively, lower than previously reported values for other crustacea. Different laboratories conducting the 96‐h mortality test withHolmesimysisproduced significantly different results in one trial, but results from different laboratories were not significantly different in the second trial. Coefficients of variation between laboratories for LC50s and NOECs were 24 and 33, respectively, in the first trial, and 25 and 1, respectively, in the second trial. Juvenile mysids aged 3, 5, 7 and 9 d old responded similarly to zinc exposure, though control mortality differed among these groups. Gravid female mysids were less sensitive, and 1‐d‐old mysids had high control mortality; neither group was suitable for toxicity testing. The NOEC for zinc effects on growth was 18 μg/L in a 7‐d experiment. LC50 and NOEC values for copper were 27 and<11 μg/L (the lowest concentration tested), respectively. Effluents from four municipal sewage treatment plants (primary through secondary) produced LC50s ranging from 4.5 to 64.0 effluent, and NOECs ranging from 1.0 to 32 effluent, depending on the level
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