VIABILITY OF GLOCHIDIA OF 2 SPECIES OF ANODONTA EXPOSED TO LOW PH AND SELECTED METALS

Mature glochidia were stripped from the gills of gravid mussels and exposed to low pH (3-6.5), aluminum (150-3000 mug/L), aluminum (300 mug/L) at low pH (3-6.5). zinc (5 - 30000 mug/L), cadmium (50-3000 mug/L), or copper (5-1500 mug/L) for 6 days. Viability was tested after 24, 48. 72. and 144 h by observing the ability of the glochidia to close when exposed to an irritant, 2.5 M KCl. Effective concentrations needed to reduce the closure response to 50% (EC50) under the experimental conditions were calculated using probit analysis. The relative toxicities (based on 48-h EC50s) of the metals to Anodonta cygnea glochidia were Cu (EC50 = 5.3 mug/L) > Cd (EC50 = 46.8 mug/L) > Zn (EC50 = 69.1 mug/L) > Al. The EC50 for pH was 4.34 for A. cygnea and 4.69 for Anodonta anatina. In 300 mug/L Al, the EC50 for pH for A. cygnea was 4.46. The ability of glochidia to close their valves is considered a reliable measure of viability, as this action is necessary for the larvae to complete the obligate ectoparasitic stage of their life cycle. Decrease in glochidial viability is suggested as a possible explanation for the disappearance of unionids from acid- and metal-contaminated waters.