Lee, Carol Eunmi*; Piermarini, Peter; Reid, Nicole: INDEPENDENT INVASIONS OF FRESH WATER: LOW-SALINITY TOLERANCE AND ACTIVITY AND EXPRESSION OF NA,K-ATPASE IN LINEAGES OF THE COPEPOD EURYTEMORA SPP.

The invasion of fresh water has occurred at least eight times independently in the sibling species complex Eurytemora affinis. We investigated the osmoregulatory role of an ion-pumping enzyme, Na, K-ATPase, in invasive and non-invasive lineages of E. affinis and in a non-invasive congener E. americana. We measured enzyme activity and expression at salinities ranging from 0.25 to 25PSU. We also measured development time and survival in response to salinity. At low salinities, survival declined, development time increased, and activity of Na, K-ATPase increased by about 1.5 fold for E. affinis. In contrast, for E. americana, Na, K-ATPase activity declined with salinity and survival was 0% at the lowest salinities. A positive correlation between Na, K-ATPase activity and survival at lower salinities suggests that this enzyme plays a role in freshwater survival. However, the modest 1.5-fold increase in enzyme activity is unlikely to account for the required ion uptake. Immunostaining for Na,K-ATPase was localized to the ventral nerve cord, and did not appear to vary among populations, species, or salinities. Moreover, we were unable to find a distinct epithelial-based organ or gland that could potentially function for ion excretion or ion absorption. Our results suggest that Na,K-ATPase may not be an important ion regulatory enzyme for maintaining hemolymph concentration in copepods invading low salinities, unlike the scenario assumed for most invertebrates.