The Distribution of the Oceanic Sea Skaters , Halobates inside and outside the Kuroshio

The purpose of this study was to clarify the distribution of oceanic Halobates in the area of the Kuroshio flowing near the southern shore in the direction of 100 ̊ 120 ̊, and also to compare the population density of Halobates between the area within or outside the area of the Kuroshio and also among seasons. This study was carried out during 8 cruises by R/V TANSEIMARU. The Kuroshio area south of the southern Japan coast (30 ̊00'N 35 ̊00'N, 130 ̊25'E 141 ̊04'E) was dominated by H. sericeus, and the averaged population-density of this species was significantly higher inside the Kuroshio than outside this current. On the Kuroshio, H. sericeus was dominant with the population density of 16,396.4 km ± 66,138.4 [26] (Mean ± SD [n]), whereas the density of H. germanus was 8,581.9 km ± 24,443.2 [26]. The two oceanic sea skaters, H. sericeus and H. germanus showed significant seasonal variation in the population density, with significantly higher density in October than other months, whereas there was no such significant October peak in the cosmopolitan oceanic sea skater, H. micans. The results of this study may suggest that H. sericeus could use the Kuroshio as a transportation tool to distribute a wide latitude area of from 10 ̊N to 40 ̊N in the western tropical, subtropical and temperate area in the Pacific Ocean.


Introduction
Ocean currents take an important role for the transportation of animals inhabiting there.For example, Hernández-León et al. [1] showed that the trophic T. Harada et al. scenario of mesozooplankton could be depicted for the oceanic, upwelling and eddy system in the Canary Current System.Namely, the tremendous increase in production in the coastal area off Northwest Africa, the coupling of this production with the oceanic area through filaments and eddies topographically formed in the coast or shed by the islands promote a continuous transport of organic matter towards the deep ocean in the Canary Current System [1].
Pearce et al. [2] performed the application of an individual-based particle tracking model to the migration of tropical fish larvae along the continental shelf between the Houtman Abrolhos Islands and Rottnest Island (Western Australia).This application has shown that there was a potential for the southwards advection of passive particles/larvae in the Leeuwin Current system.
Several animals have been reported to be transferred by the Kuroshio which takes important roles for them to make life histories and their distributions, serving several resources for life.For example, the diving behavior of the false killer whale, Pseudorca crassidens was recorded using a pop-up archival transmitting (PAT) tag: "The whale reaches the Kuroshio front in the morning on one day in October, as indicated by the drastic change in the temperature gradient.The whale frequently performed deep dives exceeding 200 m during the daytime in both the transition and the Kuroshio front regions" [3].The surface waters of the Kuroshio are generally regarded as nitrogen-deficient and oligo-trophic region [4].Many economically important pelagic fish species, however, are abundant in these waters such as the dolphinfish, Coryphaena hippurus with annual catches of 9,500 tons by Taiwanese fisheries [5].In the early and middle larval stages of round herring Etrumeus teres, seasonal variation in daily growth rates in Tosa Bay which is strongly influenced by the Kuroshio is largely determined by the sea temperatures experienced by hatch-date cohorts in the Pacific coastal waters off southern Japan [6].Based on monthly samples collected from January 2001 to December 2004, in total, 2558 mesopelagic fish larvae were sampled also in the Tosa Bay [7].Using the output of a high-resolution ocean general circulation model, particle-tracking experiments were performed to infer the distribution of larvae of the Japanese sardine (Sardinops melanostictus) and to detect effects of transport environment on sardine recruitment, and observed data of sardine spawning grounds during 1978-2004.By the 60th day following spawning, 50% of the larvae had been transported to the Kuroshio Extension [8].
The only insects that inhabit the open ocean are members of the genus Halobates, commonly known as sea skaters.They are included in the family Gerridae (Heteroptera), which also includes common pond skaters or water striders.Only six species have been reported from open oceans among the 47 species included in the Halobates genus [9].The six oceanic species are Halobates micans Eschscholtz, 1822, Halobates germanus White, 1883, Halobates sericeus Eschscholtz, 1822, Halobates splendens Witlaczil, 1886, Halobates sobrinus White, 1883 [10] and Halobates princeps White 1883 [11] which has been reported as a coastal species [12] in most cases.Another H. sp. is currently in the process of being described (Harada et al. unpublished).
Three species, H. sericeus, H. micans, and H. germanus, dominantly inhabit the tropical and temperate areas of the Pacific Ocean in the northern hemisphere, including the Kuroshio (Black Current), the East China Sea, and the Japan Sea [10] [13] [14] [15].Halobates micans is a cosmopolitan oceanic species which is mainly distributed in the latitude range of 20˚N to 20˚S in the Pacific, Indian and Atlantic oceans [16].This species has also been found up to 32˚N only in the area south of the southern Japanese coast, perhaps being transported there by the Kuroshio [12].The Kuroshio might be a transporting agent of oceanic Halobates, a point that remains to be studied.
Andersen and Cheng [12] presented a summary of data regarding the population density of oceanic Halobates.This review paper [12] shows that H. micans has a low population density of 2,000 individuals/km 2 in the eastern tropical Pacific Ocean, even at the low latitude of 10˚ -20˚N.Halobates micans and H. sericeus have average population densities of 3,000 and 7,000 individuals/km 2 , re- spectively, in the 13˚ -20˚N area of the western Pacific Ocean [17].Harada et al. [9] recently reported highly dense populations of H. micans of 10,000 individuals or more per km 2 in the Tropical Western Pacific Ocean at 0˚ -10˚N, 130˚ -135˚E.However, the eastern region of 147˚ -156˚E of the Tropical Western Pacific Ocean is less likely to be affected by freshwater flowing down from rivers in the tropical islands.The population density of H. micans there was only around 5,000 individuals/km 2 , half that at 130˚ -135˚E [9].In the area on the Kuroshio band of the eastern China Sea, all three species of H. micans, H. germanus, and H. sericeus have been collected at 7 sampling points from 27˚10'N, 124˚50'E to 29˚00'N, 129˚00'E, while the 127˚00' -129˚00'E area and the area north of the Kuroshio was mainly dominated by H. sericeus [15].However, few studies have been done on the role of the Kuroshio flowing near the southern shores of Japan islands for the distribution of Halobates.This study aims to examine the population density of oceanic Halobates in the area inside and outside of the Kuroshio flowing near the southern shore in the direction of 100˚ -120˚, and to discuss the role of the Kuroshio in the distribution of Halobates.

Materials and Methods
During the eight cruises, samplings were performed with one of two kinds of NEUSTON nets (small net: 0.57 m diameter, large net: 1.

Table 1(a) and Table 1(b)
show the number of individuals (total of juveniles and adults) and population density of oceanic sea skaters at 12 sampling points during the 8 cruises (Figures 1(a 2).
The population densities of H. sericeus and H. germanus were significantly higher and more than 15,000 ind.km −2 in October inside and around the Kuroshio (Mann-Whitney U-test: H. sericeus, z = −2.328,p =0.020; H. germanus, z = −2.483,p = 0.013) than the other seasons, whereas there was no significant seasonal differences in population density of H. micans (z = −0.770,p = 0.441 between October and the other months) (Table 3).In total, the population density inside the Kuroshio was 27,234.5 ind.km −2 (±71,486.7 [26]) (Mean ± SD [n]) on average and tended to be higher than the population density in the area outside the Kuroshio (8,661.6 individuals km −2 ) (±13,343.1 [19]) (Mann-Whitney U-test: z = −1.763,p = 0.078).At the beginning of December (Table 1(d)), no oceanic Halobates were collected even in the the Kuroshio where the sea surface temperature was 25˚C.

Discussion
Results of this study suggest that Halobates sericeus may be the dominant rider of the Kuroshio among the oceanic sea skaters studied.Andersen & Cheng [12] reported that the distribution of this species was limited to 13˚ -45˚N in the Pacific Ocean.However, estimated suggest was a moderate population density of 2,010 -7,100 individuals/km 2 in the tropical area from 0˚ -10˚N (including the area around the equator) in the Western Tropical Pacific Ocean (Harada et al. 2010)  [9].This species may have been actively transferred by several currents, including the Kuroshio, the North Equator Current, the

Figure 1 .
Figure 1.(a) The number of individuals (square of circles) and species ratio in September and October in relation with the current route of the Kuroshio at each sampling.Characters A, B, C and E correspond with Table 1(a)-A, B, C and E, respectively, in this manuscript.(b) The number of individuals (square of circles) and species ratio in June in relation with the current route of the Kuroshio at each sampling.No sea skaters were collected in D -D' in December.Characters D and F correspond with Table 1(a)-D and F, respectively, in this manuscript.(c) The number of individuals (square of circles) and species ratio in July and August in relation with the current route of the Kuroshio at each sampling.Characters G and H correspond with Table 1(b)-G and H, respectively, in this manuscript.(d) The number of individuals (square of circles) and species ratio in September and October in relation with the current route of the Kuroshio at each sampling.Characters, I and J correspond with Table 1(b)-I and J, respectively, in this manuscript.(e) The number of individuals (square of circles) and species ratio in September in relation with the current route of the Kuroshio at each sampling.Characters K and L correspond with Table 1(b)-K and L, respectively, in this manuscript.