Ammonia has become increasingly prevalent in freshwaters in recent years due to anthropogenic inputs such as agricultural runoff. The most toxic form of the chemical, called unionized ammonia (NH3), causes death to aquatic animals via gill damage, immune system repression, and kidney degeneration, and negatively impacts movement and feeding behaviors in several aquatic organisms. However, the sublethal effects to freshwater snails, one of the most highly imperiled groups of animals in North America, remain largely unknown. Laboratory experiments were used to test whether and how NH3 affects righting behavior (an important movement used to return a snail to the upright position) and oxygen consumption in two freshwater snails from Kentucky, USA: fine-ridged elimia (Elimia semicarinata) and Shawnee rocksnails (Lithasia obovata). Groups of snails were exposed to doses of NH3 ranging from 0.25-12.61 mg NH3-N/L for 24 hours. The time it took upturned snails to right themselves was determined before and after exposure to NH3, with a maximum time of 60 minutes. A second group of snails was divided into treatments exposed to 10.6-15.0 mg NH3-N/L for 24 hours before oxygen consumption was assessed for each snail, using a respirometer and polarographic electrodes.
Results/Conclusions
Exposure to NH3 caused an increase in righting time for fine-ridged elimia and a decrease in righting success for Shawnee rocksnails. The concentrations required to elicit these effects on snail behavior were magnitudes greater than any reported previously for freshwater gastropods. Within the range of tested concentrations, NH3 did not affect respiration as measured by oxygen consumption, suggesting that a different mechanism causes ammonia to impair righting behavior, at least during short term exposure. Ultimately, the high tolerance of these snail species to NH3 indicates that their natural populations are unaffected by current NH3 levels in most freshwater ecosystems in North America. These results highlight the importance of studying toxicant sensitivities across a wider array of species, as previous generalizations for snails would have been inaccurate in predicting sensitivities of these two species to ammonia.