Bull. Environ. Contam. Toxicol. (1989)43:271-279 ~Environmental 9 1989 Springer-Verlag New York Inc. ~Contamination |and Toxicology Gender-Dependent Problems in Toxicity Tests with Ceriodaphnia dubia G. Jayne Haynes, Arthur J. Stewart, and Bret C. Harvey Oak Ridge National Laboratory, Environmental Sciences Division, P.O. Box 2008, Oak Ridge, Tennessee 37831-6351, USA The 7-d, three-brood Ceriodaphnia chronic toxicity test developed by Mount and Norberg (1984) is used to assess biological quality of water and wastewaters. The test is becoming widely accepted because i t involves simple procedures, uses easily measurable and biologically meaningful endpoints (survival and fecundity), and uses a microcrustacean known to be relatively sensitive to various toxicants. The test uses individual animals as true replicates, and is relatively powerful statistically. As with other cladocerans, Ceriodaphnia is parthenogenetic under favorable environmental conditions, and males are rare. When overcrowded or when they experience changes or inadequacies in diet, photoperiod, and/or water temperature, however, cladocerans may produce males and sexual females (Hutchinson 1967; Allan and Goulden 1980; Wetzel 1983). In the sexual phase of their life cycle, female cladocerans can produce males which mate with sexual females. If fertilized, ephippial eggs produced by sexual females can give rise to parthenogenetic females, which re-establishes the parthenogenetic phase of the l i f e cycle (Hutchinson 1967; Wetzel 1983). The factors triggering the sexual phase of Ceriodaphnia's life cycle are not yet known, and although male C eriodaphnia are rarely observed in lakes or ponds (Frenzel 1977, Burgis 1967), laboratory populations can include parthenogenetic females, males, and sexual females (Berner 1986). Early instar male Ceriodaphnia dubia closely resemble juvenile females and can be distinguished from parthenogenetic or sexual females only after they are 4 or 5 d old. Then, males can be identified by their more restless swimming behavior, by body shape (males have more posteriorally tapered bodies and are slightly smaller than females of the same age), and on the basis of coloration (males tend to be more reddish-pink, Berner 1986). Because male and female Ceriodaphnia neonates cannot be readily distinguished from one another, a 7-d C. dubia test started with neonates produced by females from stressed cultures may include an i n i t i a l l y indeterminate number of males. The presence of males (which can be ascertained with certainty only when the test is nearly complete) can affect interpretation of toxicity test results in two ways. First, males reduce the number of animals (replicates) producing offspring, which reduces the power to Send reprint requests to G.J. Haynes at the address above 271