Amami-Oshima Island Research Articles

Deformation mechanisms and fluid conditions of mélange shear zones associated with seamount subduction

Lithologic heterogeneity and the presence of fluids have been linked to seamount subduction and collocated with slow earthquakes. However, the deformation mechanisms and fluid conditions associated with seamount subduction remain poorly understood. The exhumed Chichibu accretionary complex on Amami-Oshima Island preserves mélange shear zones composed of mudstone-dominated mélange and basalt–limestone mélange deformed under sub-greenschist facies metamorphism. The mudstone-dominated mélange contains sandstone, siliceous mudstone, and basalt lenses in an illitic matrix. The basalt–limestone mélange contains micritic limestone and basalt lenses in a chloritic matrix derived from the mixing of limestone and basalt at the foot of a seamount. The basalt–limestone mélange overlies the mudstone-dominated mélange, possibly representing a submarine landslide from the seamount onto trench-fill terrigenous sediments. The asymmetric S–C fabrics in both mélanges show top-to-SE shear consistent with megathrust-related shear. Quartz-filled shear and extension veins in the mudstone-dominated mélange indicate brittle failure at near-lithostatic fluid pressure and low differential stress. Microstructural observations show that deformation in the mudstone-dominated mélange was accommodated by dislocation creep of quartz and combined quartz pressure solution with frictional sliding of illite, whereas the basalt-limestone mélange was accommodated by frictional sliding of chlorite and dislocation creep of coarse-grained calcite, with possible pressure solution creep and diffusion creep of fine-grained calcite. The mélange shear zones formed in association with seamount subduction record temporal changes in deformation mechanisms, fluid pressure, and stress state during megathrust shear with brittle failure under elevated fluid pressure, potentially linking tremor generation near subducting seamounts.

Coordinated development of island ecotourism: A comprehensive evaluation of Amami-Oshima Island

Island tourism has garnered growing attention especially with the rapid expansion of ecotourism in island destinations. While past research has explored the integrated development of island tourism, environment, and economy, there exists a demand for a more comprehensive exploration that incorporates social aspects. This study aimed to establish a comprehensive evaluation index system for island ecotourism using Amami-Oshima Island as an example. It uses four tourism subtypes, including environment, economic, social, and other-sector tourism, within the framework for island ecotourism. We employed a Coupling Coordination Degree Model and a Grey Verhulst model, utilizing data from Amami-Oshima Island from 2010 to 2021 as a basis for quantitative analysis of the current status of island ecotourism coupling coordination development and for making predictions. The research outcomes reveal the following: (1) Between 2010 and 2021, Amami-Oshima Island exhibited an overall upward trajectory in the comprehensive development of ecotourism. The environment-tourism subsystem demonstrated robust performance, the social-tourism subsystem displayed fluctuating growth, and the economic and the other-sector tourism dimensions generally progressed in tandem. (2) The coupling degree and coordination level of Amami-Oshima Island ecotourism showed an overall upward trend, with coordination evolving from a state of minimal coordination to an intermediate level. However, opportunities remain to enhance high-quality coordination through mutual influences and interdependencies. (3) Predictions indicate a slight increase in ecotourism coupling coordination on Amami-Oshima Island over the next nine years (2022–2030). Based on these findings, this paper presents several policy recommendations, serving as a guideline for ecotourism development on Amami-Oshima Island. The research results carry significant implications for understanding the present status and future trends of ecotourism on Amami-Oshima Island and offer valuable policy suggestions for the development of ecotourism on other islands.

Analysis of the occurrence of the fall armyworm, Spodoptera frugiperda, in the winter season on the southwestern islands of Japan using the insect's strontium radiogenic isotope ratio (87Sr/86Sr)

AbstractSpodoptera frugiperda, an invasive pest insect that targets maize and other crops, first arrived in Japan in the summer of 2019. This species occurs year‐round in East Asian subtropical regions such as southern mainland China and the island of Taiwan, where the mean air temperature in the coldest month is above 10°C. Adults are similarly found throughout the year on the southwestern islands of Japan. Trap monitoring there showed continuous or intermittent S. frugiperda catches in the three winter seasons since 2019. However, it was difficult to distinguish between immigrants arriving from these neighbouring areas and local individuals occurring on each Japanese island. In this study, the possible natal origin of captured insects on five small islands (Yonagunijima, Taramajima, Okinawajima, Amamioshima and Tanegashima) was determined by investigating the strontium radiogenic isotope ratios (87Sr/86Sr) and comparing them with those of reference hosts and insects. Since trapping data and the 87Sr/86Sr values of trapped insects didn't support S. frugiperda's winter breeding on the northernmost island, Tanegashima, further analysis was limited to the four southern islands. The 87Sr/86Sr values of reference host plants and reared insects on the four islands ranged from 0.70929 to 0.71009, while those of catch insects ranged from 0.70885 to 0.71090. The 87Sr/86Sr values of the catch insects and the reference on the four islands did not differ significantly. In addition, the monthly averages of daily mean air temperature in January and February 2020–2022 were above 10°C, and the wind direction at the surface was mostly from the northeast or northwest. These pieces of evidence, together with winter host availability, suggested that S. frugiperda occurs year‐round on the islands. In other words, the year‐round occurrence area of S. frugiperda in East Asia extends to the Japanese southwestern islands below Amamioshima Island.

Fertilizer Management, Soil Chemical Properties, Leaf Mineral Contents, and Fruit Quality in Passion Fruit Orchards on Amami-Oshima Island

Fertilizer Management, Soil Chemical Properties, Leaf Mineral Contents, and Fruit Quality in Passion Fruit Orchards on Amami-Oshima Island

Observation of air-exposure behaviour in the reef hermit crab, Calcinus elegans (H. Milne Edwards, 1836) (Decapoda, Diogenidae)

Abstract Many species of hermit crabs, such as Calcinus and Clibanarius spp., live on the rocky shores of tropical and warm temperate regions. During low tides, some of these species climb out of the tidal pools onto rock surfaces exposed to the air. In the genus Calcinus, however, such ‘Air-exposure (AE) behaviour’ was only known previously from Calcinus laevimanus. During a field study of the hermit crab fauna on Amami Oshima Island, Kagoshima, Japan, the AE behaviour of Calcinus elegans (Decapoda, Diogenidae) was observed on rocky shores. Moreover, the repeatability of the AE behaviour was confirmed. Therefore, we documented the AE behaviour of C. elegans as a second record of AE behaviour among Calcinus species by providing videographic evidence. The findings indicate that the AE behaviour has evolved in a broader range of Calcinus species than was previously assumed.

京都府の街路樹モミジバフウにおけるアカオビアザミウマの発生

The red-banded thrips Selenothrips rubrocinctus (Giard) was originally distributed throughout northern South America. This thrips attacks tropical fruits such as mango, cocoa, and guava in North America, Asia, and Central and South Africa. In Japan, it has been observed on mango and guava in Okinawa Prefecture, Amami-Oshima Island in Kagoshima Prefecture, the Ogasawara Islands in Tokyo Metropolis, and Kochi Prefecture; on Actinidia rufa in Miyazaki Prefecture; and on maple trees in Kyoto Prefecture. In late September 2022, damage caused by S. rubrocinctus was observed in sweet gum in Kyoto Prefecture, with gray or dark brown leaves. This is the first record of S. rubrocinctus occurring on sweet gum in Japan. The degree of damage to the leaves and the occurrence of adults and larvae of the thrips were investigated in sweet gum lining streets and road facilities in Kyoto Prefecture. Damage caused by the thrips to sweet gum was observed in areas south of Kameoka, and adults and larvae were detected in areas south of Nantan, Kyoto Prefecture.

The effect of temperature and irradiance on the photosynthesis of a seagrass, <i>Halodule uninervis</i>, from Amami-Oshima Island, Kagoshima

分布北限に近い奄美大島産海草ウミジグサの光合成における光と温度の影響を明らかにした。15℃と24℃における光合成光曲線は，最大純光合成速度が15℃で顕著に抑制された。総光合成速度と最大量子収率による光合成温度曲線（8–40℃）は，それぞれ30.5℃と23.9℃で最大となった。15℃と24℃，光量300と1000 μmol photons m－2 s－1の組み合わせによる光と温度の複合応答では，低温·強光条件で量子収率が顕著に低下し，その後の暗馴致でも回復しなかったことから，低温光阻害が示唆された。

Geographic Variation in Skull Morphology of the Japanese Crocodile Newt, Echinotriton andersoni (Amphibia: Urodela: Salamandridae), Inferred from Geometric Morphometrics

The Anderson's crocodile newt, Echinotriton andersoni, is an endangered species distributed in the islands of Central Ryukyus, Japan. This species is well known to show high intraspecific genetic variation among islands, but its morphological variation has not been explored. We examined the skull variation in this species including enigmatic specimens collected from Taiwan by using geometric morphometrics, and found sexual differences and geographic variation mainly between the Okinawa group (Okinawajima Island) and the Amami Group (Amamioshima and Tokunoshima Islands). We further found close resemblance between the Taiwanese specimens and the female specimens from Amamioshima Island.

An assessment of Ryukyu Archipelago's coral communities over a wide latitudinal range

Japan's coral reefs have changed dramatically over the last several decades due to climate change and anthropogenic impacts. Due to its dynamic location, the islands of Ryukyu Archipelago offer a unique environmental gradient to study the interactions between environmental variations, connectivity, resilience, climate change and adaptation of marine biodiversity along a wide latitudinal gradient. To obtain detailed baseline assessments of the reefs of Ryukyu Islands, we carried out Point Intercept Transect surveys to compare the assemblages of benthic communities in six regions of Ryukyu Archipelago, examining the influence of latitude gradient and emphasising on regional features. A total of 69 individual reef sites were surveyed for this study. The overall hard coral cover for Ryukyu archipelago was 22.41% ± 7.98 (Mean ± SD) from pooled dataset. The most abundant benthic assemblage for Ryukyu Archipelago was turf algae (39.72% ± 7.21) with all regions except Amami Oshima Island having overall cover above 40%. A total of 2607 individual colonies of hermatypic corals were recorded and identified up to genus level for this study where 55 genera of corals belonging to 17 families were recorded for Ryukyu Archipelago. The overall relative abundance was highest for Acroporidae family (32.22%), followed by Merulinidae family (27.69%) and Poritidae family (14.38%). Our results highlight the current condition of the reefs of Ryukyu archipelago and offers baseline data on broad scale ecosystem where future ecological change could be compared.

Middle Miocene forearc alkaline magmatism in Amami-Oshima Island, central Ryukyu Arc: implications for paleoreconstruction of Shikoku Basin

During the middle Miocene, forearc alkaline magmatism occurred in the Outer Zone of Southwest Japan and the northern Ryukyu Arc, resulting in the formation of forearc alkaline basaltic rocks with ocean island basalt (OIB)-like chemical signatures. In contrast, subduction-related magmatism was present in the central Ryukyu Arc. However, the southwestern margin of the forearc alkaline magmatism was poorly constrained in the Ryukyu Arc. We examined two basaltic dikes in the Chichibu accretionary complex of Amami-Oshima Island, the central Ryukyu Arc. The dikes cut massive basalt, reddish chert, varicolored shale, and grey chert. The chemical compositions of basaltic dikes are characterized by the enrichment of incompatible trace elements, possibly representing a low degree of partial melting from a deep mantle source. 40Ar/39Ar dating analyses indicate that the basaltic dikes yield ages of 16.37 ± 0.14 Ma and 16.51 ± 0.10 Ma. The paleomagnetic analyses on stepwise thermal demagnetizations allowed extracting the direction of stable magnetizations with unblocking temperatures of 450–575 °C for the two dikes [(Dec, Inc) = (138.1°, − 13.3°), (124.0°, − 24.9°)]. The magnetization could be primary, acquired either as part of a secular variation or a geomagnetic excursion during reversed polarity chron/subchron. The 40Ar/39Ar ages and paleomagnetic directions within the error range imply that they intruded almost simultaneously during C5Cn.2r (16.532–16.434 Ma). The age and trace element patterns of the basaltic dikes are comparable to those of Miocene alkaline basaltic rocks, which resulted from forearc alkaline magmatism during the initial subduction of the young and warm Shikoku Basin. The discovery of alkaline basaltic dikes on Amami-Oshima Island suggests that the distribution of middle Miocene forearc alkaline magmatism may extend to the central Ryukyu Arc. Hence, the northern end of the Kyushu-Palau Ridge (i.e., southern end of Shikoku Basin) could have been located south of Amami-Oshima Island around 16.5–16.4 Ma, then moved eastward to the current location.Graphical

A New Record of Hypsugo pulveratus on Amami-Oshima Island, Japan and Comparison with H. alaschanicus

Three vespertilionid bats were captured from Amami-Oshima Island, and had an average forearm length of 35 mm and average body weight of 8 g. From the side of the bat skulls, the ridge of the crown was slightly curved at the base of the snout; the second upper premolar (P2) in the bats was relatively large; the male baculum was shaped like an elongated, 3-mm-long spear tip and was very similar in both shape and size to that of Hypsugo pulveratus. The phylogenetic tree inferred from the mitochondrial COI gene sequences revealed that these bats clustered with H. pulveratus. Therefore, the bats from Amami-Oshima Island were identified as H. pulveratus. The present paper lists and depicts the morphological differences between H. pulveratus and H. alaschanicus so far known from the territory of Japan. Finally, genetic analysis based on the barcoding gene sequences revealed that these species formed distinct clades.

「奄美大島，徳之島，沖縄島北部及び西表島」—「島」の世界自然遺産の特徴

「奄美大島，徳之島，沖縄島北部及び西表島」—「島」の世界自然遺産の特徴

奄美マングースバスターズによる奄美大島の外来マングース根絶の快挙まであと少し

奄美マングースバスターズによる奄美大島の外来マングース根絶の快挙まであと少し

奄美大島・徳之島・沖縄島北部及び西表島の世界自然遺産登録の経緯とその課題

奄美大島・徳之島・沖縄島北部及び西表島の世界自然遺産登録の経緯とその課題

Diversity and distribution of air-breathing sea slug genus Peronia Fleming, 1822 (Gastropoda: Onchidiidae) in southern Japanese waters.

Species of the genus Peronia Fleming, 1822, are air-breathing onchidiid sea slugs that inhabit intertidal reef flats of temperate to tropical zones. In the Ryukyu Islands of southern subtropical Japan, Peronia species are a traditional food source for local people. To date, there have been three species recorded around Okinawajima Island; P. verruculata and P. peronii, along with recently described P. okinawensis, which was described as possibly endemic to Okinawajima Island. This study aimed to map the distribution ranges of these three Peronia species within the Ryukyu Islands using molecular analyses in order to understand the specific distribution of each species. Since Peronia species are generally indistinguishable by gross external morphology, a DNA barcoding approach was employed to identify specimens. The molecular data showed that there are four species present in the Ryukyu Islands. P. verruculata (unit #1 sensu Dayrat et al., 2020) was dominant at almost all locations, while P. peronii was present in much lower numbers than P. verruculata, but found across a relatively wide range in the Ryukyu Islands. We newly record P. okinawensis and P. setoensis from Amami Oshima Island and from several places around Okinawajima Island, and also identified high levels of genetic variation within P. setoensis. Peronia okinawensis and P. setoensis have been thought to be endemic to Okinawajima Island and to Honshu, mainland Japan, respectively. However, as both species were observed around Okinawajima and Amami Oshima islands, other islands of the Ryukyus are also likely to harbor these species, and their distribution ranges are wider than previously thought. Based on the results from molecular analyses, we provide general descriptions of each species. Sizes of specimens were consistently smaller for P. setoensis and relatively larger for P. peronii specimens. On the other hand, P. verruculata and P. okinawensis had similar size ranges, but P. okinawensis had comparatively much more distinct papillae. This study revealed that the Ryukyu Islands are the only region currently known with four sympatric Peronia species, and this work provides a basis for future research on these Peronia species throughout the northwest Pacific Ocean, representing the first step in more effective management of the local Peronia fisheries in the Ryukyu Islands.

Morphological and gonadal histological characteristics of the silver-phase male Indo-Pacific eel Anguilla marmorata.

Eight silver-phase male Indo-Pacific eels (giant mottled eels) Anguilla marmorata were collected from three small rivers on subtropical Amami-Oshima Island, Japan, and their morphological and gonadal-histological characteristics were examined. The total length, body mass and age of the silver eels were 543.8 ± 49.3 (496-659)mm, 376.0 ± 116.0 (282-660)g and 14.6 ± 2.3 (11-19)years old, respectively. The silvering-related characteristics (eye index, pectoral-fin index, gut-somatic index) and reproductive characteristics (gonadosomatic index, histological stage) of the silver eels were significantly advanced compared to those of yellow eels, which is similar to other anguillid species.

The first record of Anatrachyntis lunulifera ( ) (Lepidoptera: Cosmopterigidae) in the Ryukyus, Japan

Anatrachyntis lunulifera (Meyrick, 1935) (Lepidoptera: Cosmopterigidae) is distributed in the Pacific region, for example, in Fiji and the Marquesas Islands, and larvae of this species feed on the male inflorescence of Pandanus species. In our study, A. lunulifera was collected from Pandanus odorifer (Forssk.) Kuntze in Amami oshima Island, Okinawa-jima Island, and Miyako-jima Island, the Ryukyus, Japan. This article is the first report of A. lunulifera from Japan, North Pacific. In this study, we have redescribed and illustrated the adult, male and female genitalia of A. lunulifera, and illustrated those of A. incertulella (Walker, 1864), another Pandanus feeder occurring together with A. lunulifera in the Ryukyus, for comparison.

Luciogobius punctilineatus n. sp., a new earthworm goby from southern Japan.

Luciogobius punctilineatus n. sp. is described on the basis of 21 type specimens from Kyushu, and the Koshiki and Osumi Islands, southern Japan. It is also found in Kochi Prefecture (Shikoku) and Amami-oshima island (Ryukyu Islands), Japan, confirmed by examination of non-type specimens. The genus Luciogobius includes 15 valid and several undescribed species, and most of them inhabit interstitial spaces of stones and gravel in the intertidal zone. The new species is characterized by the following combination of characters: total second dorsal-fin rays 1012 (modally 11); total anal-fin rays 1214 (13); pectoral-fin rays 812 (10); vertebrae 1618 + 2224 = 3942 (17 + 23 = 40); pectoral-fin posterior margin slightly concave; pelvic fins united, forming a ventral disc; snout relatively short, length 3.14.3% of SL; anus to anal-fin origin (AAA) distance twice body depth at anus, 11.416.9% of SL; snout length less than 34.7% of AAA distance; pre-anus length less than 85.5% of pre-anal-fin length; single poorly defined black longitudinal line along mid-lateral body region from behind pectoral fin to caudal-fin base, indistinct anteriorly (line embedded, visible through semi-transparent muscle tissue in fresh or live specimens); black spots forming a single longitudinal row on mid-lateral body surface from behind pectoral fin to caudal-fin base (more distinct in preserved specimens).

Drifting Situation of Pumices Discharged from the 2021 Eruption of Fukutoku-Oka-no-Ba Volcano: Cases at Osumi Peninsula in Kagoshima Prefecture, Southern Part of Miyazaki Prefecture, Tanegashima and Amami Oshima Islands

Drifting Situation of Pumices Discharged from the 2021 Eruption of Fukutoku-Oka-no-Ba Volcano: Cases at Osumi Peninsula in Kagoshima Prefecture, Southern Part of Miyazaki Prefecture, Tanegashima and Amami Oshima Islands

Cyphellostereum ushimanum sp. nov. (Hygrophoraceae, Agaricales) described from Amami-Oshima Island (Kagoshima Prefecture, Ryukyu Islands), Japan, with ultrastructural observations of its Rhizonema photobiont filaments penetrated longitudinally by a central haustorium

Cyphellostereum ushimanum sp. nov. (Hygrophoraceae, Agaricales) described from Amami-Oshima Island (Kagoshima Prefecture, Ryukyu Islands), Japan, with ultrastructural observations of its Rhizonema photobiont filaments penetrated longitudinally by a central haustorium