New Zealand mudsnails (Potamopyrgus antipodarum) in Boulder Creek, Colorado: environmental factors associated with fecundity of a parthenogenic invader

New Zealand mudsnails (Potamopyrgus antipodarum (Gray, 1853)) are non-native snails that are increasingly spreading in freshwater systems in North America. Most invasive populations are parthenogenic and threaten native freshwater diversity. We observed variability of P. antipodarum fecundity each month for 16 months at a recently invaded site in Boulder Creek, Colorado. We collected 100 snails each month and dissected them to count embryos in the brood sac. We used a general linear model analysis to examine water-quality variables as predictors of the monthly variability in P. antipodarum fecundity. After dissecting 1600 snails, we observed four male individuals (<1%), brood sizes ranging from 0 to 70 embryos per snail, reproductively mature females at 3.2 mm in length or greater, and a significant relationship between snail length and embryo counts (r(2) = 0.38, p < 0.001). The model with the highest level of support for predicting variability in snail fecundity included water temperature, snail shell length, water hardness (calcium carbonate), and nutrient levels (total phosphate) (adjusted r(2) = 0.53, p < 0.01). These variables may be important for snail growth and promote increased rates of reproduction in this parthenogenic, invasive snail. These results can further inform efforts to model geographic areas at high risk of P. antipodarum establishment and rapid demographic growth.