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 suppressed 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 fluctuations 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 and so mix the genes of individuals inhabiting brackish and freshwater bodies.
In species with last sperm precedence, post-mating guarding behaviour by males is effective for avoiding sperm displacement by additional males. However, for males to guard their mates under all circumstances may be undesirable, particularly when the population density of conspecific rival males is low. To better understand the effects of rival density on mate guarding behaviour, we compared post-copulatory behaviour in males artificially subjected to different male densities using Aquarius paludum. Our results indicate that males of A. paludum modulate their post-copulatory behaviour based on previously experienced rival densities. and Atsuo Matsueda, Kenji Fujisaki.