Study of the Effects of Pb and Hg Toxicity Using a Chronic Toxicity Reproductive 5-Day Test with the Freshwater Rotifer Lecane quadridentata Saraı´ Herna ´ndez-Flores, Roberto Rico-Martı´nez Departamento de Quı´mica, Centro de Ciencias Ba ´ sicas, Universidad Auto ´ noma de Aguascalientes, Avenida Universidad 940, Aguascalientes, Ags. C.P. 20100, Me ´ xico Received 12 January 2006; revised 4 April 2006; accepted 16 April 2006 ABSTRACT: We have developed chronic toxicity reproductive 5-day tests to study the effects of Pb, as Pb(NO 3 ) 2 , and Hg, as HgCl 2 , exposure in the freshwater rotifer Lecane quadridentata. We used \r" (the in- stantaneous growth rate) as the endpoint. The test was performed using EPA medium at two food levels (10 5 and 10 6 cells/mL of Nannochloris oculata) at a controlled temperature of 258C and an L/D cycle of 16:8 h. We monitored the levels of both metals, using atomic absorption, at days 0, 2, and 5 of each experiment. Our results showed that rotifers fed at higher food concentrations were capable of withstand- ing higher levels of metal toxicity than those fed at lower food concentrations (EC50 ¼ 0.704 versus 0.664 mg/L 1 for lead, P < 0.05; EC50 ¼ 0.057 versus 0.054 for mercury, P < 0.05). Our atomic absorption anal- ysis showed that although 66% of lead nitrate can be taken up by algal cells efficiently and removed from the medium, rotifers also play an important role removing additional lead from the medium (up to 44% in some treatments). In the case of mercuric chloride, most of the mercury is bound by the salts contained in EPA medium or discarded by the organisms, and the remainder is removed by N. oculata in <48 h. # 2006 Wiley Periodicals, Inc. Environ Toxicol 21: 533–540, 2006. Keywords: metal toxicity; rotifer; lead; mercury; detoxification mechanisms INTRODUCTION The data obtained from life cycle toxicity tests represent a true approximation to assess ecological risk. However, var- iations on toxicant concentration during these tests are rarely measured (Gu ¨ven et al., 1999). This despite that dur- ing the test there could be variations in the metal concentra- tion induced by several factors such as temperature (Tsui and Wang, 2004) or acquisition of metals through diet (De Schamphelaere et al., 2004), and contaminated sediments (Gillis et al., 2005). Aquatic invertebrates have been suc- cessfully used as model organisms for toxicity tests, given the fact that they have short generation times and its small size that does not require of a big and expensive setup (Snell and Moffat, 1992). Although rotifers are an impor- tant component of the aquatic food webs, and they are very sensitive toward many toxicants (Burbank and Snell, 1994), their properties as organisms capable of detoxifying metals have been overlooked. Lead has been listed by the USEPA as one of the 129 priority contaminants: number 2 (mercury is number 3) in the 2003 ATSDR top 20 hazardous substances list (http:// www.atsdr.cdc.gov/cxcx3.html). Lead concentrations of up to 25 mg/L 1 have been reported from wastewater in the United States (US Environmental Protection Agency, 1986). All the effects of lead on the organisms are adverse, including those related with survivorship, growth, learning, reproduction, development, and metabolism (Irwin et al., 1998). Lead is the most ubiquitous toxicant metal, and it has been detected in practically all phases of the environ- ment. Lead can substitute calcium and probably zinc in ion dependent phenomena in the synapses, causing damages in Correspondence to: R. Rico-Martı ´nez; e-mail: rrico@correo.uaa.mx Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/tox.20218  C 2006 Wiley Periodicals, Inc. 533