Marine Biology Research Division

Five species of ocean skaters Halobates are the only insects that have successfully  colonized the ocean.  In the western Pacific Ocean, three species of Halobates, H. micans, H. sericeus and H. germanus, are known to occur over a wide area. We investigated the spatio-temporal features of Halobates during the three cruises of R/V Hakuho Maru in the western Pacific in 1994, 1995 and 1998. During these cruises, the area between latitudes 7^o N - 22^o N and longitudes 125^o E - 150^o E was surveyed in all cruises, and in this area H. germanus was absent while H. micans and H. sericeus were caught every year. In 1994 and 1995, H. micans was found almost to the exclusion of H. sericeus and in 1998 H. sericeus was found almost to the exclusion of H. micans, suggesting that these two species rarely co-occur, while their distribution ranges change temporally. We examined water surface temperature, prevailing winds and the El Nino event as possible factors responsible for the replacement of H. micans by H. sericeus in 1998. 0n the ocean surface, oceanic diffusion is constantly acting to disperse  Halobates in all directions. However, local distribution patterns of H. micans and H.  sericeus were highly clumped, suggesting that these species have the ability to aggregate against oceanic diffusion.