Low Specific Conductivity Limits Growth and Survival of the New Zealand Mud Snail from the Upper Owens River, California

—The New Zealand mud snail (NZMS), Potamopyrgus antipodarum (Gray), is an invasive species of aquatic snail that is becoming widespread in inland and coastal waters of the western United States. The New Zealand mud snail can have significant impacts on stream ecosystems, as they may consume a large fraction of available algae production and compete with and displace native invertebrates. Even though the distribution of this species is expanding, the habitat conditions conducive to invasion are incompletely understood. Surveys following the NZMS invasion in the Upper Owens River, California, indicated that the snail may be excluded from waters where dissolved solute content is low, so experimental studies were undertaken to evaluate survival and growth as a function of varied specific conductivity (SC) and calcium availability. Juvenile snails were collected from the Upper Owens River and reared in dilutions of natural river water adjusted to 10, 50, 100, 200 and 300 μS ⋅ cm–1 SC. Experiments were also conducted with newborn clones raised in river water dilutions ranging from 25 to 200 μS ⋅ cm–1 to examine mortality and growth at this sensitive stage of development. In addition, calcium-free artificial river water was prepared at 200 μS ⋅ cm–1 to test for the independent effect of limitation of this mineral ion required for shell-building. Significant reductions in survival and growth occurred among treatments diluting river water from 300 to 50 μS ⋅ cm–1. No growth was found at or below 25 μS ⋅ cm–1. Growth was also inhibited in calcium-free artificial water compared to natural river water with the same SC, showing that lack of this mineral impedes development. These results suggest that many streams in the range of 25–200 μS ⋅ cm–1 cannot support productive NZMS populations and that nuisance invasions may be most prevalent in waters above 200 μS ⋅ cm–1 where sufficient dissolved mineral content is present for growth.