Sodium and chloride regulation in freshwater and osmoconforming larvae of Culex mosquitoes.

In this study, we examined aspects of Na(+) and Cl(-) regulation in mosquito larvae of the genus Culex, a group that includes species that tolerate high salinity as well as other forms that are restricted to fresh water. When the euryhaline osmoconformer C. tarsalis was acutely transferred from 30 % to 50 % sea water, the patterns of hemolymph Na(+) and Cl(-) regulation were similar. The underlying regulatory mechanisms for these two ions have very different characteristics. In C. tarsalis, Na(+) efflux was significantly elevated compared with the rates measured in the freshwater-restricted C. quinquefasciatus, while Cl(-) influx was relatively lower. The modulation of Na(+) efflux and Cl(-) influx allowed C. tarsalis to avoid a potential salt load and ionic disturbance in the hemolymph during an acute increase ( )in salinity. The observed adjustment of NaCl regulation departs from that determined for other euryhaline organisms and is integral to the osmoconforming response. At the other extreme of the salinity spectrum, we observed that C. tarsalis faces difficulties in ion regulation in habitats with low NaCl levels because of its inability to reduce ion efflux and adjust ion absorption rates to maintain hemolymph ion balance. In contrast, C. quinquefasciatus exhibited a reduced ion efflux and the ability to upregulate Na(+) uptake, traits necessary to extend its lower salinity limit.