Evaluation of the sensitivity of three cladoceran species widely distributed in Mexico to three reference toxicants.

Acute toxicity effects of sodium dodecyl sulfate (SDS), sodium pentachlorophenate (SPCP), and hexavalent chromium (Cr(VI)) were determined in Ceriodaphnia dubia, Daphnia pulex, and Simocephalus mixtus. Neonates were obtained from parthenogenetic females of different ages to establish the relative sensitivity of these species, in order to propose them as test organisms in the Mexican environmental standards. For C. dubia the average LC(50) (48 h) values were 8.59 +/- 1.08, 0.92 +/- 0.16, and 0.23 +/- 0.04; for D. pulex, 5.34 +/- 0.90, 1.49 +/- 0.29, and 0.28 +/- 0.06; whereas for S. mixtus, they were 4.50 +/- 0.41, 1.31 +/- 0.16, and 0.12 +/- 0.03 mg/L for SDS, SPCP, and Cr(VI), respectively. SDS and SPCP acute toxicity results depicted coefficients of variation of <or= 40 % that were not related to the age of mothers. The same was observed for Cr(VI) with C. dubia, whereas for S. mixtus and D. pulex a reduction in neonates' sensitivity was determined for progeny produced by females older than 23 and 25 days, respectively; this situation establishes limits to the useful period of the culture for the supply of test organisms from these cladocerans. The less toxic reference toxicant was SDS. S. mixtus was the most sensitive cladoceran to SDS and Cr(VI), whereas C. dubia was the most sensitive to SPCP. Although differences existed in the culture conditions and in managing the production of newborns, results suggest the feasibility of using D. pulex and S. mixtus as test organisms in acute toxicity assessment as surrogates of D. magna, an exotic cladoceran in Mexico. Considering its short life-cycle, C. dubia could be used in chronic toxicity tests, as well as in acute toxicity tests, having in both situations the advantage of requiring lower volumes of test samples.