Sensitivity of the glochidia (larvae) of freshwater mussels (Bivalvia: Unionida: Hyriidae) to cadmium, cobalt, copper, lead, nickel and zinc: Differences between metals, species and exposure time.

Freshwater mussels (Bivalvia: Unionida) are among the most threatened freshwater faunal groups worldwide. Metal contamination is one threat that has been linked to declining mussel population distribution and abundance. This study determined the sensitivity (valve closure) of the glochidia (larvae) of six species of Australian freshwater mussels to cadmium (Cd), cobalt (Co), copper (Cu), lead (Pb), nickel (Ni) and zinc (Zn), key metal contaminants impacting urbanized coastal rivers in south-eastern Australia (home to ~50% of the population), in a soft reconstituted freshwater (hardness 42mgCaCO3 L-1 ; alkalinity 22mgCaCO3 L-1 and pH7.0) over 72h. The sensitivity of each mussel species to each metal increased 2.5-fold with increasing exposure time from 24 to 72h. The most sensitive mussel species (Cucumerunio novaehollandiae), across all metals and exposure times, was ~60% more sensitive than the least sensitive species (Velesunio ambiguus). The relative sensitivity of glochidia to the six selected metals, across all mussel species and exposure times, was: Cu>Cd>Pb>Co=Ni>Zn. Glochidia were most sensitive to Cu and least sensitive to Zn. Quantitatively, the toxicity of Cu was 3-fold more than Cd, 8-fold more than Pb, 14-fold more than Co or Ni and 16-fold more than Zn. The cell surface binding affinities (conditional log K values) of Cd (range 6.65-6.94), Co (6.04-6.29), Cu (7.17-7.46), Ni (6.02-6.29), Pb (6.24-6.53) or Zn (5.96-6.23), pooled for all mussel species after 72h exposure, were positively related to metal sensitivity. The chronic no effect concentrations (NECs) of Cu, Ni and Zn were below (i.e. glochidia were more sensitive than) their national freshwater guideline values, indicating that freshwater mussels may not be adequately protected for these metals in urbanized coastal rivers within south-eastern Australia.