Dispersal and reproductive responses of the water strider, Aquarius paludum (Hemiptera: Gerridae), to changing NaCl concentrations

Abstract

The responses of females of the water strider, Aquarius paludum, to changes in NaCl concentration were examined in the laboratory. The insects were sampled seasonally in 2002 and 2003 at two reservoirs in Kochi, Japan, connected by a waterway, one at the mouth of the river with high NaCl concentrations (the range in NaCl concentration during a year: 0.1-1.08%) and one 700 m upstream, where the salinity was lower (0.03-0.23%). Sudden increase in NaCl concentration, from 0.45 to 0.9%, after adult strider emergence supprosed reproduction and promoted flight activity, whereas a decrease in salinity did not affect either trait. In the field, A. paludum was univoltine in brackish and multivoltine in freshwater ponds. Thus, the number of breeding periods per year was limited by fluctuation in the NaCl concentration in brackish habitats. Our results suggest that A. paludum can breed in brackish waters when the NaCl concentration is below the limitation for reproduction and growth. Abrupt increases in NaCl concentration caused by seawater surges, such as those following typhoons, can trigger the migration of individuals, which move to areas of lower NaCl concentration so mix the genes of individuals inhabiting brackish and freshwater bodies.