5.1 An important goal of aquatic toxicology is to determine the effects of toxic compounds on species that play a central role in aquatic communities. Rotifers have a major impact on several important ecological processes in freshwater and coastal marine environments. As filter-feeders on phytoplankton and bacteria, rotifers exert substantial grazing pressure that at times exceeds that of the larger crustacean zooplankton (1, 2).³ Rotifer grazing on phytoplankton is highly selective (2-4) and can influence phytoplankton composition, the coexistence of competitors, and overall water quality (5). The contribution of rotifers to the secondary production of many aquatic communities is substantial (6-9). In fresh water, rotifers often account for the major fraction of zooplankton biomass at certain times of the year (10, 11) . Rotifers and other zooplankton are a significant food source for many larval fish, planktivorous adult fish (12, 13), and several invertebrate predators (14-16) . The high metabolic rates of rotifers contribute to their role in nutrient cycling, which might make rotifers more important than crustaceans in certain communities (17, 18) .

5.2 In addition to their important ecological role in aquatic communities, rotifers are attractive organisms for toxicological studies because an extensive database exists on the basic biology of this group. Techniques have been published for the culture of many rotifer species (3, 19). The rotifer life cycle is well defined (20, 21), and the factors regulating it are reasonably well understood (22-25). Several aspects of rotifer behavior have been examined closely (26-29). The biogeography of many rotifer species has been characterized (30, 31), and the systematics of the group are well described (32, 33).

5.3 Toxicity tests with rotifers of the genus Brachionus are more easily performed than with many other aquatic animals because of their rapid reproduction, short generation times, sensitivity (34), and the commercial availability of rotifer cysts. Brachionus spp. have a cosmopolitan distribution that spans six continents (31), and they are ecologically important members of many aquatic communities impacted by pollution. The use of B. plicatilis in an acute toxicity test for estuarine and marine environments and B. rubens in fresh water has been described, as well as their sensitivity to several toxicants (35, 36).

5.4 The test described here is fast, easy to execute, sensitive, and cost-effective. Obtaining test animals from cysts greatly reduces some of the major problems in routine aquatic toxicological testing such as the limited availability of test animals and the inconsistency of sensitivity over time. Rotifers hatched from cysts are of similar age and are physiologically uniform, thus eliminating pre-test conditions as a source of variability in the toxicity test. Cysts can be shipped inexpensively world-wide, allowing all laboratories to use standard, genetically defined strains that have been calibrated with reference toxicants. The convenience of an off-the-shelf source of test animals that require no pre-conditioning is likely to permit new applications of aquatic toxicity tests.

5.5 Sensitivity to toxicants is compound and species specific, but the sensitivity of B. calyciflorus is generally comparable to that of Daphnia (37).

5.6 Rotifer cysts are commercially available, but they can also be obtained from natural populations and from laboratory cultures. Techniques for rotifer cyst production in laboratory populations have been described (24, 25, 38, 39). However, using a well-characterized rotifer strain is best since strains are known to have differing toxicant sensitivities.