ASTM E2455-06(2013)

1.3.1 Freshwater mussels are bivalve mollusks belonging to the family Unionidae or Margaritiferidae (section 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. 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 1.4.4 Results of tests, even those with the same species, using procedures different from those described in 1.5 Summary of Results of Toxicity Tests Conducted with Freshwater Mussels: