Viability of glochidia of two species of <i>Anodonta</i> exposed to low pH and selected metals

Abstract

Mature glochidia were stripped from the gills of gravid mussels and exposed to low pH (3 – 6.5), aluminum (150 – 3000 μg/L), aluminum (300 μg/L) at low pH (3–6.5), zinc (5 – 30 000 μg/L), cadmium (50–3000 μg/L), or copper (5 – 1500 μg/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% (EC 50 ) under the experimental conditions were calculated using probit analysis. The relative toxicities (based on 48-h EC 50 s) of the metals to Anodonta cygnea glochidia were Cu (EC 50 = 5.3 μg/L) &gt; Cd (EC 50 = 46.8 μg/L) &gt; Zn (EC 50 = 69.1 μg/L) &gt; Al. The EC 50 for pH was 4.34 for A. cygnea and 4.69 for Anodonta anatina. In 300 μg/L Al, the EC 50 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.