Yonaguni Island Research Articles

Back‐Arc Opening in the Western End of the Okinawa Trough Revealed From GNSS/Acoustic Measurements

AbstractWe measured seafloor movement using a Global Navigation Satellite Systems (GNSS)/Acoustic technique at the south of the rifting valley in the western end of the Okinawa Trough back‐arc basin, 60 km east of northeastern corner of Taiwan. The horizontal position of the seafloor benchmark, measured eight times between July 2012 and May 2016, showed a southeastward movement suggesting a back‐arc opening of the Okinawa Trough. The average velocity of the seafloor benchmark shows a block motion together with Yonaguni Island. The westernmost part of the Ryukyu Arc rotates clockwise and is pulled apart from the Taiwan Island, which should cause the expansion of the Yilan Plain, Taiwan. Comparing the motion of the seafloor benchmark with adjacent seismicity, we suggest a gentle episodic opening of the rifting valley accompanying a moderate seismic activation, which differs from the case in the segment north off‐Yonaguni Island where a rapid dyke intrusion occurs with a significant seismic activity.

Present status and changes over the past 38 years in marsh vegetation in Yonaguni Island

Present status and changes over the past 38 years in marsh vegetation in Yonaguni Island

Vincetoxicum yonakuniense, a New Combination of Asclepiadaceae and its New Distribution Record on Senkaku Islands, Ryukyu Archipelago

Cynachum yonakuniense, a perennial twining herb, endemic to Yonaguni Island, the Ryukyu Archipelago has never been recorded since its original materials collected there. We rediscovered it on the isla

Small Mammalian Remains from the Late Holocene Deposits on Ishigaki and Yonaguni Islands, Southwestern Japan.

Yuichiro Nishioka, Ryohei Nakagawa, Shin Nunami, and Satoshi Hirasawa (2016) Small mammalian remains were newly discovered from the Late Quaternary sediments of the Yaeyama region (Ishigaki and Yonaguni Islands) in the southwestern-most part of Japan. We examined these materials based on taxonomical and chronological approaches, in order to reconstruct the past fauna in this region. Accelerator Mass Spectrometry radiocarbon dating indicates that the mammalian assemblages from Ishigaki and Yonaguni Islands are composed mainly of late Holocene fossils. The fossil assemblage from Ishigaki Island comprises five species of small mammals belonging to Soricomorpha (Suncus murinus), Chiroptera (Pteropus sp., Hipposideros turpis, and Rhinolophus perditus), and Rodentia (Niviventer sp.). One rodent bone, collected from the fissure sediments near Sabichi-do Cave, Ishigaki Island, is dated at 230 ± 20 yBP (ca. AD 1,700), and is considered part of the recent remain. Furthermore, the fossil assemblage from Umabana-zaki Fissure, on Yonaguni Island, is dated at 1,760 ± 20 yBP (ca. AD 300) and comprises three species of small mammals belonging to Chiroptera (Pipistrellus cf. abramus) and Rodentia (Niviventer sp. and Mus musculus). Niviventer is not currently distributed in Japan. Only domestic rats (Rattus rattus and R. norvegicus) live on Ishigaki and Yonaguni Islands at the present time. However, the fossil assemblage from Umabana-zaki Fissure is dominated completely by Niviventer sp. Most fossils of small mammals found from the late Holocene Ishigaki and Yonaguni Islands represented species that are currently endemic to the Yaeyama region. Niviventer sp. from these islands is unique because this form has never been found from neighboring regions, such as Taiwan and Miyako Islands, and because it likely existed in the late Holocene. These discoveries support the hypothesis that the Yaeyama region had been isolated zoogeographically from the continent even during the Last Glacial Maximum, when the sea level had drastically fallen.

PCR detection of Babesia ovata from questing ticks in Japan

Babesia ovata is a tick-transmitted hemoprotozoan parasite of cattle. In the present study, we analyzed tick DNA samples (n=1459) prepared from questing ticks collected from various cattle pastures in Hokkaido (Shibecha, Taiki, Otofuke, Memuro, and Shin-Hidaka districts) and Okinawa (Yonaguni Island) prefectures of Japan for B. ovata. When all the tick DNA samples were screened by a previously described B. ovata-specific apical membrane antigen-1 (AMA-1) gene-based polymerase chain reaction (PCR) assay, none of the DNA samples was positive. Therefore, we developed a PCR assay based on the protozoan beta-tubulin (β-tubulin) gene to detect B. ovata from ticks in Japan. In the specificity test, the PCR assay amplified the expected 444-bp target gene fragment from B. ovata DNA. No PCR products were amplified from DNA samples from other blood pathogens, bovine blood, or ticks. In addition, the PCR assay detected 100fg of B. ovata-genomic DNA extracted from an in vitro culture of the parasites. Subsequently, when all the tick DNA samples were screened by this new PCR assay, 18 were positive for B. ovata. Positive samples were found only in the Yonaguni and Memuro areas. In Okinawa, where all the ticks were identified as Haemaphysalis longicornis, 9.7% of the samples were PCR-positive, while a single tick (Ixodes ovatus) from Memuro was infected with B. ovata. When the nucleotide sequences of the PCR amplicons were phylogenetically analyzed, they formed a separate clade containing a previously described β-tubulin gene sequence from B. ovata (Miyake strain), confirming that the PCR assay had detected only B. ovata from the tick DNA samples. This is the first report that describes the PCR detection of B. ovata in ticks. The findings warrant transmission experiments to evaluate I. ovatus as a potential vector of B. ovata.

Development and the condition of horse farming in Yonaguni island, Okinawa

Development and the condition of horse farming in Yonaguni island, Okinawa

Rickettsiae in Ticks, Japan, 2007–2011

Rickettsiae in Ticks, Japan, 2007–2011

Genetic diversity of Theileria orientalis in tick vectors detected in Hokkaido and Okinawa, Japan

In the present study, we investigated the possible tick vectors that can transmit Theileria orientalis in eastern Hokkaido, Japan. Questing ticks collected from three different districts, Taiki, Otofuke, and Shin-Hidaka, of Hokkaido included Ixodes persulcatus, Haemaphysalis megaspinosa, Haemaphysalis douglasi, and Ixodes ovatus, while all the ticks collected from Yonaguni island of Okinawa were identified as Haemaphysalis longicornis. When the ticks were screened by polymerase chain reaction (PCR) for T. orientalis, the parasite was commonly detected among all tick species. Genotype-specific PCR assays revealed that all tick species in Hokkaido were predominantly detected with type 2, while ticks collected from Okinawa (H. longicornis) were predominantly detected with type 1. Consistent with the genetic diversity of T. orientalis in ticks, genotyping PCR assays from cattle grazed in the same Hokkaido sampling locations identified type 2 as the most prevalent genotype. This study provides the first identification of I. persulcatus, H. megaspinosa, H. douglasi, and I. ovatus as possible tick vectors of T. orientalis, and finds that the variety of vectors apparently capable of transmitting T. orientalis is wider in Japan than expected. The authors suggest that tick control strategies should be modified in Hokkaido based on the seasonal activities of ticks identified in the present study.

Detection of the New Ehrlichia Species Closely Related to Ehrlichia ewingii from Haemaphysalis longicornis in Yonaguni Island, Okinawa, Japan

We collected a total of 206 Haemaphysalis longicornis ticks by flagging in pastures in Yonaguni Island, Okinawa, Japan, in April 2008. Four of the 206 tick DNA samples tested were positive in a polymerase chain reaction (PCR) screening for the 16SrRNA gene of Anaplasmataceae. Partial sequences of 4 PCR products were identical to each other. Longer sequences of the 16SrRNA gene were successfully determined in 2 of the 4 tick samples, and the obtained 1,392 bp and 1,300 bp sequences revealed high similarity to the 16SrRNA gene sequences of the validated Ehrlichia species, including Ehrlichia ewingii, E. chaffeensis, and E. canis (98.3-98.6%). We also sequenced 1,304 bp of the groEL gene from the 2 tick samples, and found that these had the highest similarity to sequences of E. ewingii (94.0-94.4%) in the validated ehrlichial species. Based on the 16SrRNA and groEL gene sequences, the ehrlichial agents detected in this study were similar to the Ehrlichia species detected in Asia and may compose a new Ehrlichia species with other Ehrlichia species detected in Asia.

Genotypic Diversity of Theileria orientalis Detected from Cattle Grazing in Kumamoto and Okinawa Prefectures of Japan

Theileria orientalis is a benign protozoan species that is widely distributed in Japan, yet sometimes causes serious economic losses in the livestock industry. In this study, we conducted a molecular survey based on genes encoding the major piroplasm surface protein (MPSP) and p23 for T. orientalis detected in cattle grazing in southern areas of Japan, consisting of 2 farms in Kumamoto prefecture (Aso and Kuma districts) and 3 farms in Okinawa prefecture (Ishigaki, Iriomote, and Yonaguni Islands). High prevalence rates of T. orientalis infection were shown in all the cattle populations using the diagnostic MPSP- and p23-PCR assays. Phylogenetic analyses revealed 4 MPSP genotypes and 3 p23 genotypes. Furthermore, MPSP genotype-specific PCR methods were developed in this study and wide distributions of 5-district genotypes of T. orientalis were observed for the examined farms. Our results indicate that at least 5 types of T. orientalis exist in Kumamoto and Okinawa prefectures of Japan and that genotype-specific PCR assays are highly applicable for the quarantine of transported cattle and for epidemiological surveys of bovine theileriosis in Japan.

Establishment and host records of Icerya aegyptiaca (Douglas) (Hemiptera: Coccoidea: Monophlebidae) in the Sakishima Islands of the Ryukyu Archipelago, Japan, with notes on its worldwide distribution

The monophlebid scale insect Icerya aegyptiaca is reported for the first time in the Ryukyu Islands of Japan, where it was found in four islands: Kurima Island, Kuro Island, Miyako Island and Yonaguni Island. Icerya aegyptiaca was collected from 36 plant species belonging to 23 families. Information on the distribution and natural enemies of I. aegyptiaca and a key to separate known species of Icerya occurring in Japan are provided.

Salarchaeum japonicum gen. nov., sp. nov., an aerobic, extremely halophilic member of the Archaea isolated from commercial salt

Strain YSM-79(T) was isolated from commercial salt made from seawater in Yonaguni island, Okinawa, Japan. The strain is an aerobic, Gram-negative, chemo-organotrophic and extremely halophilic archaeon. Cells are short rods that lyse in distilled water. Growth occurs at 1.5-5.3 M NaCl (optimum 2.5-3.0 M), pH 5.0-8.8 (optimum pH 5.2-6.3) and 20-55 °C (optimum 40 °C). Mg²⁺ is required for growth, with maximum growth at 200-300 mM Mg²⁺. Polar lipid analysis revealed the presence of phosphatidylglycerol, phosphatidylglycerophosphate methyl ester, sulfated diglycosyl diether-1 and five unidentified glycolipids. The G+C content of the DNA was 64 mol%. On the basis of 16S rRNA gene sequence analysis, strain YSM-79(T) was determined to be a member of the family Halobacteriaceae, with the closest related genus being Halobacterium (94 % sequence identity). In addition, the rpoB' gene sequence of strain YSM-79(T) had <88 % sequence similarity to those of other members of the family Halobacteriaceae. The results of phenotypic, chemotaxonomic and phylogenetic analysis suggested that strain YSM-79(T) should be placed in a new genus, Salarchaeum gen. nov., as a representative of Salarchaeum japonicum sp. nov. The type strain is YSM-79(T) ( = JCM 16327(T) = CECT 7563(T)).

Distribution of Two Distinct Genotypes of Citrus Greening Organism in the Ryukyu Islands of Japan

Asian type Candidatus Liberibacter asiaticus (Las, citrus greening organism) is severely dam- aging citrus production in Asia including Japan. Our previous study suggested that the bacteriophage- type DNA polymerase region (DNA pol) would be useful for molecular differentiation in different Southeast Asian Las isolates. Moreover, Las isolates originated from most of the Southeast Asian regions harbor the DNA pol gene, whereas Japanese ones lack this region. These preliminary findings lead us to a hypothesis that all Japanese isolates lack DNA pol. To try this hypothesis, we collected citrus leaf samples infected with Las throughout the Ryukyu Islands, and examined them by a duplex PCR that could simultaneously amplify two DNA fragments of DNA pol and nusG-rplK operon of Las. The duplex PCR was applied to the collection of 65 Las isolates. Both DNA pol and nusG-rplK operon were successfully amplified from nine isolates, whereas only nusG-rplK operon was amplified with the other 56 isolates. These nine isolates with DNA pol originated from the Hateruma, Irabu, Kohama, Miyako, Tarama, and Yonaguni Islands, which are geographically close to Taiwan. The nucleotide sequence of DNA pol of these nine isolates was identical, and was also the same as four Taiwanese isolates reported previously. These results suggest that Japanese Las isolates comprise at least two distinct genotypes, and the genotype that had DNA pol is highly homogeneous.

Enhancement of phytoplankton primary productivity in the southern East China Sea following episodic typhoon passage

The enhancement of primary productivity (PPenh) in the southern East China Sea (ECS) following 16 typhoon passages was investigated using ocean color data and a primary productivity model. PPenh tended to be higher when typhoons traversed slowly with trajectories that allowed strong southerly winds to prevail over Yonaguni Island. Such long‐lasting southerly winds were believed to push the Kuroshio current axis shelfward, enhancing the upwelling of nutrients, hence promoting new productivity (NP). The importance of long‐lasting southerly winds as a proxy for physical perturbations underlying PPenh was expressed by an empirical equation by which 88% of PPenh variation could be explained. Applying this equation, we assessed that typhoon passages accounted for a minimum of 0.6–11.8% of the ECS summer–fall NP, suggesting that typhoon passage over the southern ECS is an important phenomenon supporting NP in the ECS.

Survey of Japanese Encephalitis Virus in Pigs on Miyako, Ishigaki, Kume, and Yonaguni Islands in Okinawa, Japan

Serum specimens were collected from 125 pigs on Miyako Island, 112 pigs on Ishigaki Island, and 42 pigs on Kume Island from 2005 to 2007, and 54 pigs on Yonaguni Island from 2006 to 2007. Their sera were tested for Japanese encephalitis virus (JEV) antibody by hemagglutination inhibition (HI) assay. Five serum samples (4.5%) from Ishigaki Island were positive for HI antibody, and 4 of the 5 samples were positive for 2-mercaptoethanol- sensitive antibody (IgM Ab). All samples from Miyako, Kume, and Yonaguni Islands were negative for HI antibody. Our results indicate that JEV transmission activity was extremely low on Miyako, Ishigaki, Kume, and Yonaguni Islands. The JEV genome (JEV-RNA) was detected from the sera of one pig on Ishigaki Island. The partial gene of the E region (151 nt) was analyzed phylogenetically. The analysis showed that the new JEV-RNA belonged to genotype 3 and was closely related to JEV strains isolated in Taiwan from 1985 to 1996. It was suggested that JEV previously introduced from Taiwan had been maintained on Ishigaki Island.

Movement of the Kuroshio axis to the northeast shelf of Taiwan during typhoon events

We investigated the movement of the Kuroshio axis on the northeast shelf of Taiwan associated with the passage of typhoons, using sea surface current data observed by the ocean radar system on Yonaguni and Ishigaki islands. First, we examined daily Kuroshio axis variation on the northeast shelf of Taiwan during typhoon events. The ocean radar data showed that the Kuroshio axis moved onto the shelf after passages of typhoons. The Kuroshio moved onto the shelf and stayed there after the passage of Typhoon Hai-Tang; while the Kuroshio maintained this pattern, southerly wind blew continuously for 4 days. The mean current speed northeast of Taiwan after the typhoon's passage increased by 18 cm s −1. In addition, the sea level difference between two satellite altimetry tracks east of Taiwan increased by 14.4 cm. These results suggest that coastal upwelling east of Taiwan caused by the southerly wind generated an east–west sea level difference that, in turn, generated a northward geostrophic current. This current could have enhanced the Kuroshio east of Taiwan, and pushed it onto the shelf.

An Analysis of the Responses to the Travel Brochure of a Solitary Island

A travel brochure of Yonaguni Island—the points of interest of which were defined in terms of two main dimensions, i.e., the island's “peculiarity” and its “adventurous and exciting nature”—was presented to 125 subjects. After reading the brochure, the subjects were asked to write an essay on their impression about it.Text mining analysis was performed on the obtained answers.The summary of the result is as follows:(1) Reading the brochures, the subjects actually had an impression of its “peculiarity” and “exciting nature.”(2) On the other hand, there were also subjects who felt a sense of “recreation” and “relaxation” in relation to Yonaguni Island. This impression was considered to stem from the influence of the preconceived images about south islands that the subjects held, prior to reading the brochures.(3) The essays contained many descriptions about the butterfly (moth). The result suggests the effectiveness of using “peculiar” creatures, which inhabit only that particular area, and visually “exciting” images in a brochure.(4) There were many descriptions about a TV drama. It was surmised that marketing the island as a filming location of a TV show is effective in its appeal to the people.(5) Although the brochure highlighted not only the beautiful but also the stimulative aspect of the sea around Yonaguni Island, the descriptions were more disposed toward the former, and there was almost no description about the latter aspect.

Reproductive biology of blue marlin Makaira nigricans around Yonaguni Island, southwestern Japan

We studied the spawning seasonality and gonadal development of blue marlin Makaira nigricans using specimens captured around Yonaguni Island in southwestern Japan between February 2003 and February 2006. The mean (±SD) lower jaw–fork length of females (234 ± 24 cm) was greater than that of males (191 ± 12 cm). The smallest mature female and male were 183 and 160 cm, respectively. Most of the 717 females had immature ovaries. However, in March and from May to September, the ovaries of 26 females contained oocytes with yolk globules, hydrated oocytes, or postovulatory follicles. Most males had testes with a large amount of spermatozoa throughout the year. The occurrence of mature blue marlin at Yonaguni Island suggests that spawning occurs here. The mean condition factors (fatness of the fish) of both sexes decreased from March to June or July, presumably as they expended energy to reproduce. We discuss our results in the context of migration theory for blue marlin in the western North Pacific Ocean.

Genetic structure of island populations of the endangered bat Hipposideros turpis turpis: implications for conservation

AbstractEfforts for the conservation of the endangered bat species Hipposideros turpis turpis in southern Japan are hampered by a lack of information about its biology and natural history and by the increasing effect of human activities. In an attempt to address some of the conservation challenges faced by this species, we studied the genetic structure and dispersal of intra‐ and interisland populations using six species‐specific microsatellite markers. In particular, we sought to establish the relationship between island populations and to define effective management units for conservation. Pairwise co‐ancestry index (FST) analysis, analysis of molecular variance, and Bayesian clustering suggested the presence of significant genetic differentiation between islands but little differentiation within them. The small Yonaguni Island population appeared to be not only geographically isolated, but also genetically isolated. This population is at the greatest risk of extinction, considering its size and low genetic variation. The larger populations on Iriomote and Ishigaki Islands are genetically related to each other to a greater degree and exhibit higher genetic variation than the Yonaguni Island population. This suggests that these two island populations should be included in a single management unit, while bats from Yonaguni Island should be managed independently and given higher priority for conservation. Actions such as defining vegetation corridors between colonies, as well as building gates at the entrance of the largest known colony, should be included in the conservation agenda of this still poorly known species.

Detection of Anaplasma bovis and Anaplasma phagocytophilum from cattle on Yonaguni Island, Okinawa, Japan

DNA fragments of Anaplasma bovis and Anaplasma phagocytophilum were detected in cattle on Yonaguni Island, Okinawa, Japan. Although the pathogenesis and epidemiology of these pathogens have not yet been clarified, this is the first detection of their presence in cattle from Japan.