AbstractCeriodaphnia dubiaRichard used in 7‐d chronic toxicity testing from six environmental testing centers were assayed for allozymic variability at 11 enzymatic loci. Ten of the eleven scored loci were monomorphic and homozygous in all six stocks. One stock was homozygous for an alternative electromorph, as compared to the other five, at the glucose‐6‐phosphate dehydrogenase (G6PDH) locus. Laboratory stocks comprise two morphologically distinguishable forms ofCeriodaphnia dubiathat may be environmentally determined.Ceriodaphnia lacustrisandCeriodaphnia reticulatafrom natural populations showed within‐ or between‐population allozymic variability at 8 of the 11 loci, suggestingCeriodaphniapossesses genetic variability equivalent to that of other cladocerans. The three species ofCeriodaphniaare closely related, having only one diagnostic electromorph that separatesCeriodaphnia dubiafrom the other two species. Possible causes of the monomorphism of the laboratory stocks are
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