Comparison of the Population Density of Oceanic Sea Skater of Halobates (Heteroptera: Gerridae) among Several Areas in the Tropical Pacific Ocean and the Tropical Indian Ocean

Abstract

The purpose of this study was to clarify the population density of oceanic Halobates in the western area of the tropical Pacific Ocean and the Indian Ocean around the equator (10°S-10°N). This study was carried out during 4 cruises (MR-06-05-Leg 3, KH-07-04-Leg 1, MR-08-02 and MR-09-04) by the Research Vessels Mirai and Hakuhomaru. Samplings were performed using Ocean Research Institute (ORI) nets and Neuston nets. Halobates micans showed to have a high population density of 10,000 individuals/km^2 or higher in the western area of the tropical Pacific Ocean at 0-10°N. In the relatively eastern region of this area which is less likely to be affected by freshwater flowing down from rivers of the tropical islands, the population density was around 5,000 individuals/km^2 and split between 147-156°E and 130-135°E. The distribution of H. sericeus has been reported to being limited to 13-45°N in the Pacific Ocean. However, a relatively moderate population density of between 2,010 to 7,100 individuals/km^2 was found in the four areas (including the area around the equator) of the western tropical Pacific Ocean. This species may have been transferred by several different currents including the Kuroshio Current, North Equatorial Current, Mindanao Current and the North Equatorial Counter Current throughout the large area of the western Pacific Ocean. In this study, the high population density of H. germanus was estimated to range between 13,000 and 60,000 individuals/km^2 throughout the longitudinal area of 130-156°E (0-10°N) of the western tropical Pacific Ocean. Halobates princeps (Halobates sp. 1) which has been reported as a shore species inhabiting widely Indonesia and might be a new oceanic specimen, was collected at 2°N, 130°E during the MR-06-05-Leg 3. The body length of males of H. sp. 1 (apical edge of the head to the penis like an arrow) is about 6 mm, and the reach of the mid-legs is 27 mm in a static posture on a flat surface. Individuals of H. sp. 2 were collected in the tropical Pacific Ocean during the three cruises, MR-06-05-Leg 3, MR-08-02, and MR-09-04. Halobates sp. 2 may be another new oceanic species in Halobates. Its size is similar to H. micans, while the body shape (morphometry) and color are very similar to H. sp. 1, which is much larger than that of H. sp. 2.