Comparative Study of Salinity Tolerance an Oceanic Sea Skater, Halobates micans and Its Closely Related Fresh Water Species, Metrocoris histrio

This study aims, first, to examine the limit for tolerance to lower salinityby an oceanic sea skater, Halobates micans, and , second, to make it clear whether exclusively fresh water Halobatinae species, Metrocoris histrio has salinitytolerance. Adults of H. micans were collected using NeustonNet from the starboard side of R/V MIRAI on a fixed station at 8°S, 80°E, whereas those of M. histrio were collected from a small pond filled with a spring fresh water in Kochi (33°N, 133°E), Japan. Time in survival was measured in starved condition under several salinityconditions: 0‰, 2‰, 4‰, 6‰, 8‰, 9‰ and 10‰ for H. micans; 0‰, 5‰, 10‰, 12.5‰, 15‰ for M. histrio. Half of adults were in coma due to lower salinityunder 10‰ and time in survival was less than 10 hours under less than 4‰ for H. micans. Time in survival was half at 5‰ of 80 hours on average at 0‰ as a control and less than 10 hours at 10‰ or higher salinityfor M. histrio. Relatively flexible osmo-regulation ability by H. micans would be related to wide variety of salinitycondition of surface oceanic water, whereas very limited tolerance even to lower salinityof 5‰ may be permitted by the no chances to be exposed to brackish waterin natural habitats of M. histrio. This study showed that salinitytolerance of Halobatinae species would reflect, directly, the salinitycondition of their habitats.