Chronic toxicity of silver to the sea urchin (Arbacia punctulata)

The chronic toxicity of silver to the sea urchin (Arbacia punctulata) was determined in 30 parts per thousand salinity seawater during a three-part study: A fertilization test (1-h sperm exposure), a 48-h embryo test, and a 30-d adult test. Combined data from the three tests resulted in a lowest-observed-effect concentration of 19 mu g/L, a no-observed-effect concentration of 8.6 mu g/L, and a maximum acceptable toxicant concentration of 13 mu g/L, based on measured concentrations of dissolved silver. The 96-h median effective concentration was 40 mu g/L, and the acute to chronic toxicity ratio was 3.1. During the tests, measured concentrations of free ionic silver (Ag+) were only 0.0027 to 0.0046% of dissolved silver concentrations, as predicted by ion-speciation theory. Some measured Ag+ concentrations were lower than predicted, indicating the presence of other ligands in the seawater test media. These strong sulfide ligands were exuded by the exposed sea urchins into the seawater (where Ag-sulfide complexes formed) in amounts that increased in direct proportion to the silver concentration during the toxicity test. This suggests a toxicity-defense mechanism that functioned by modifying the chemistry of the surrounding external medium.