Rationale

ASTM E2455-22 includes methods for conducting acute 96-h toxicity test, short-term chronic 7-d test, and chronic =28-d test with juvenile mussels. The USEPA is developing new short-term chronic toxicity test methods with additional test species, including a freshwater mussel (Lampsilis siliquoidea). Currently, a collaborative effort led by USGS-Columbia, Missouri, and USEPA- Cincinnati, Ohio, is revising the ASTM short-term 7-d test method within E2455 to comply with USEPA test method requirements. A major update during the development of the USEPA mussel method is to use length gain or weight gain as an index for growth, rather than final length or final weight as recommended in the current ASTM method. This is because (1) not all final length and weight represent growth (there are length and weight at the start of the test) and (2) actual growth of control mussels during the 7-day test is relatively small (typically ~30-60% length increase or ~1.0- to 2.5-fold weight increase from initial sizes; Wang et al. 2021). Thus, the difference between final length or weight and true growth cannot be negligible (see A1.3.3 below). For example, a 25% reduction in final weight would actually be a 50% reduction in growth when the final weight is only double the initial weight. In addition, the use of final length is often not sufficient to estimate a 25% reduction in growth during the 7-day test. In addition, text for the number of replicates in E2455-22 was focused on acute toxicity tests and did not consider the need for chronic tests. Acute 50% effect concentrations (EC50s) are recommended to be calculated based on mussel mortality (empty shell or decomposed tissue) plus immobility (no foot movement within 5 minutes). Thus, the test acceptability criteria in summary Table A1.1 needs to be updated for consistency. A typo in is also being corrected.