Evaluation of ability of reference toxicity tests to identify stress in laboratory populations of the amphipod Hyalella azteca

Standard methods for conducting toxicity tests imply that the condition of test organisms can be established using reference toxicity tests. However, only a limited number of studies have evaluated whether reference toxicity tests can actually be used to determine if organisms are in good condition, at the start of a test. We evaluated the ability of reference toxicants to identify stress associated with starvation in laboratory populations of th e amphipod Hyalella azteca using acute toxicity tests and four reference to xicants: KCl, CdCl2, sodium pentachlorophenate (NaPCP), and carbaryl. Stres s associated with severe starvation was observed with exposure of amphipods to carbaryl or NaPCP but not with exposure to KCl or CdCl2 (i.e., lower LC 50 with severe starvation). Although the LC50s for NaPCP and carbaryl were statistically different between starved and fed amphipods, this difference may not be biologically significant given the variability expected in acute lethality tests. Stress associated with sieving, heat shock, or cold shock of amphipods before the start of a test was not evident with exposure to c arbaryl or KCl as reference toxicants. The chemicals evaluated in this stud y provided minimal information about the condition of the organisms used to start a toxicity test. Laboratories should periodically perform reference toxicity tests to assess the sensitivity of life stages or strains of test organisms. However, use of other test acceptability criteria required in st andard methods, such as minimum survival, growth, or reproduction of organi sms in the control treatment at the end of a test, provides more useful inf ormation about the condition of organisms used to start a test compared to data generated from reference toxicity tests.