1.3.1 Freshwater mussels are bivalve mollusks belonging to the family Unionidae or Margaritiferidae (section 10.1). Adults are sedentary animals, spending their entire lives partially or completely burrowed in the bottoms of streams, rivers, or lakes. Adult mussels are filter feeders, using their gills to remove suspended particles from the water column. The microscopic, juvenile stage uses foot (pedal) feeding to some degree for the first several months of their lives, feeding on depositional materials in pore water of sediment, including bacteria, algae, and detritus. Freshwater mussels have an unusual and complex mode of reproduction, which includes a brief, obligatory parasitic stage on fish or other host organisms called glochidia (Fig. 1).

1.3.2 The successful transfer of mature glochidia to a suitable host constitutes a critical event in the life cycle of most freshwater mussels. Once the glochidia are released from the female, the glochidia need to attach to the gills or the fins of an appropriate fish host and encyst to complete development. Although glochidia may survive for months during brooding in the female mussel, glochidia typically survive for only a few days after release unless the glochidia reach a compatible host. Encystment on the host occurs by overgrowth of host tissue. Metamorphosis of juvenile mussels on the fish host occurs within days or weeks, depending on species and temperature. Host fish specificity varies among mussels. While some mussel species appear to require a single host organism, other species can transform their glochidia into juvenile mussels on several species of host fish. Following proper host infestation, glochidia transform into microscopic juveniles and excyst (drop off) and settle into suitable habitat to survive. The transformation of glochidia to juveniles results in the development of internal organs necessary for self-sustained existence as a benthic organism.

1.3.3 Newly-transformed juvenile mussels have a life style different from adult mussels. Transformed juvenile mussels may be at the sediment-water interface or may burrow several centimeters into sediment and rely on water percolating between substrate particles of sediment for food and oxygen. Newly-transformed juvenile mussels feed using ciliary currents on the foot and mantle. Older juvenile and adult mussels likely use different food types when living in different microenvironments. Given that glochidia and juvenile mussels are ecologically and physiologically different from adult mussels, protection of habitat quality of adult life stages may not be protective of glochidia or juvenile life stages of freshwater mussels. Distributions of adult mussels are dependent both on the presence of host fish and on microhabitat conditions. Efforts to assess effects of contaminants on mussels need to evaluate potential exposure to host fish in addition to exposure to each unique life stage of freshwater mussels.

1.4.1 Section 4 provides a summary of conditions for conducting toxicity tests with glochidia and juvenile mussels. Annex A1 provides guidance for conducting water-only toxicity tests with glochidia and juvenile mussels. Recommended test conditions for conducting these toxicity tests are based on various published methods outlined in Table A1.1 and Table A1.4 in Annex A1 and are based on the conditions used to conduct an inter-laboratory toxicity test with glochidia and juvenile mussels (section 16.5). Glochidia and juvenile mussels are only available on a seasonal basis. Section 10 describes procedures for collecting adult female mussels from the field to obtain glochidia for conducting toxicity tests or for obtaining glochidia to propagate juvenile mussels using a host organism.

1.4.2 In the field, mussels may be exposed to contaminants in water, sediment, or food. This standard only addresses effects associated with exposure of mussels to contaminants in water.

1.4.3 Guide E724 describes procedures for conducting acute 48-h toxicity tests with embryos or larvae of saltwater bivalve mollusks. Endpoints measured in Guide E724 include survival or shell deposition. Procedures outlined in Guide E724 may be useful in helping to design studies for conducting toxicity tests with freshwater mussels as outlined in Annex A1.

1.4.4 Results of tests, even those with the same species, using procedures different from those described in Annex A1 may not be comparable. Comparison of results obtained using modified versions of these procedures might provide useful information concerning new concepts and procedures for conducting toxicity tests with aquatic organisms. If tests are conducted with procedures different from those described in this standard, additional tests are required to determine comparability of results. General procedures described in this standard might be useful for conducting tests with other aquatic organisms; however, modifications may be necessary.

1.5.1 Keller et al (2005) (7) summarized results of acute laboratory toxicity tests conducted with glochidia and juvenile mussels described in 16 published studies. Freshwater mussels tended to be less sensitive in exposures to some pesticides and other organic compounds compared to other commonly-tested aquatic organisms. In contrast, Keller et al (2005) (7) concluded that U.S. Environmental Protection Agency (USEPA) water quality criteria (WQC) for some metals and ammonia may not be protective of freshwater mussels.

1.5.2 Augspurger et al (2003) (6) evaluated ammonia toxicity data generated for glochidia and juvenile of freshwater mussels in laboratory toxicity tests. Specifically, these toxicity data were used to estimate concentrations that would not likely be harmful to mussels in acute and chronic exposures and were used to evaluate the protectiveness of the WQC for ammonia. Results of acute toxicity tests (24 to 96 h) for 10 species in 8 genera were used to calculate genus mean acute values (GMAVs) ranging from 2.56 to 8.97 mg/L (total ammonia as N at pH 8 at 25°C). The freshwater mussels are at the sensitive end of the range when added to the GMAVs from the database used to derive the acute WQC for ammonia. Recalculation of the criteria maximum concentration (CMC) including these mussel data resulted in a CMC 75 % lower than the CMC of 5.62 mg/L total ammonia as N at pH 8 at 25°C (for application when salmonids absent). No chronic ammonia toxicity data (for example, 21 to 28-d exposures) were available for freshwater mussels; however, when a range of acute to chronic ratios were used to estimate a criteria continuous concentration (CCC), the estimated CCC for mussels was 20 to 75 % less than the CCC of 1.24 mg/L total ammonia as N at pH 8 and 25°C. Hence, Augspurger et al (2003) (6) concluded that the acute and chronic WQC for ammonia may not be protective of freshwater mussels.

1.5.3 Milam et al (2005) (10) conducted a series of 24-h acute toxicity tests with glochidia of six freshwater mussel species, Leptodea fragilis, Utterbackia imbecillis , Lampsilis cardium, Lampsilis siliquoidea, Megalonaias nervosa, and Ligumia subrostrata, and with two commonly-tested organisms, Ceriodaphnia dubia and Daphnia magna. Chemicals selected for testing (carbaryl, copper, 4-nonylphenol, pentachlorophenol, permethrin, and 2,4-dichlorophenoxyacetic acid [2,4-D]) represented different chemical classes and different toxic modes of action (Dwyer et al 2005a,b) (11, 12). No single chemical elicited consistently high or low toxicity; however, carbaryl and 2,4-D were generally the least toxic to the species tested. Milam et al (2005) (10) concluded that the toxicity data generated with C. dubia and D. magna were relatively protective of the range of sensitivities exhibited by glochidia of the mussels species tested. However, toxicity data generated with the commonly-tested U. imbecillis were not always protective of the range of sensitivities exhibited by the other mussel species tested.