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Index map of the Ryukyu Islands, Japanese Islands (Honshu, Hokkaido, Shikoku, and Kyushu), and Taiwan. Submarine contours are 200, 500, 1000, 2000, 3000, 4000, 5000 m. Base map from Vector Map (VMap) Level 0, National Geospatial-Intelligence Agency.
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In the Quaternary, the Ryukyu Islands evolved from a continental margin arc to an island arc by backarc spreading of the Okinawa Trough, accompanied by subsidence and isolation of the islands, a process that has continued to the present. Trough-parallel half grabens were filled with marine siltstone. Similar sediments filling orthogonal fault-contr...
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... Ryukyu island arc extends between Kyushu (southern Japanese islands) and Taiwan ( Figure 1). It consists of a chain of small islands that is convex (east) oceanwards in map view (Figure 1). ...
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... Ryukyu island arc extends between Kyushu (southern Japanese islands) and Taiwan ( Figure 1). It consists of a chain of small islands that is convex (east) oceanwards in map view (Figure 1). West of the arc, the Okinawa Trough is currently undergoing backarc spreading ( Letouzey and Kimura 1986;Park et al. 1998). ...
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... of the arc, the Okinawa Trough is currently undergoing backarc spreading ( Letouzey and Kimura 1986;Park et al. 1998). Because this opening is occurring in the absence of a buttress, the arc consists of small, subsided islands ( Figure 1). The Japan Sea ini- tially opened before 15 Ma, then stopped extending, and the uplift associated with continued subduction made the Japanese islands much larger than the Ryukyu Islands ( Figure 1; Osozawa 1997;Taira 2001). ...
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... this opening is occurring in the absence of a buttress, the arc consists of small, subsided islands ( Figure 1). The Japan Sea ini- tially opened before 15 Ma, then stopped extending, and the uplift associated with continued subduction made the Japanese islands much larger than the Ryukyu Islands ( Figure 1; Osozawa 1997;Taira 2001). Active backarc *Corresponding author. ...
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... Okinawa Trough has trapped a large amount of continentally derived clastic detritus, estimated to be up to 1000 m in thickness (Park et al. 1998), and mostly supplied by major streams, such as the Yangtze River (Figure 1). Yellow River detritus may flow in the northern Okinawa Trough through the Goto submarine canyon (Figure 1; e.g. ...
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... Okinawa Trough has trapped a large amount of continentally derived clastic detritus, estimated to be up to 1000 m in thickness (Park et al. 1998), and mostly supplied by major streams, such as the Yangtze River (Figure 1). Yellow River detritus may flow in the northern Okinawa Trough through the Goto submarine canyon (Figure 1; e.g. Oiwane et al. 2010). ...
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... marine transgression is recorded near the mouths of drainages that flowed in all directions, the westward-flowing drainages definitively record separa- tion of the landmass from the Chinese mainland to the west and creation of the island (Figure 2, 1 T a iw a n S tr a it OS: Osumi I. Figure 4. Shaded relief maps of Amami Oshima (A2), Okinawa-jima (O1), and Yaeyama (Y) islands (Ishigaki-jima and Iriomote-jima). Flat parts are covered by the Ryukyu Group. ...
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... most of the Ryukyu Islands, transgression continued, resulting in the deposition of Ryukyu limestone shortly after deposition of the earliest island-fringing sediments (Figure 2, 1.0 Ma). ...
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... summary of age constraints is given in graphical tabular form in Figure 5, and the marine siltstone age or basal limestone age is interpreted as the separation age based on the rationale given above. We also discuss the Tokara, Kerama, and Yonaguni gaps (Figures 1 and 3), which play a role in the isolation history (Figure 2), for they separate subgroups of the island chain from each other. ...
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... time when Kyushu, and therefore the rest of the Japanese islands were separated from Korean Peninsula and Asian mainland, corresponds to when the Tsushima warm current began to enter the Japan Sea. This geologic event is difficult to constrain from the geol- ogy of Kyushu because of the presence of two straits between Korea and Kyushu on either side of the island of Tsushima that has Palaeogene basement (Figure 1). The present Tsushima Current runs through both the Tsushima Strait (130 m depth) between Tsushima and Kyushu and the Korea Strait (230 m depth) between Tsushima and Korea. ...
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... Ma ( Sato et al. 1999; modified to 0.265 Ma by Sato et al. 2009). In addition, sediment core from the mouth of the Tsugaru Strait (Figure 1) records outflow of the Tsushima Current Figure 5. Graphical table of geochronologic constraints for the Shimajiri Group siltstone, Nakoshi Formation siltstone (time of isola- tion of island), reddish limestone, and Ryukyu limestone. Nannofossil datum planes correlated to the magnetic polarities are after Sato et al. (2009). ...
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... Watase line corresponds to the Tokara gap ( Figures 1 and 3), a narrow submarine valley connecting the Ryukyu trench to the Okinawa Trough that orthogonally truncates the Ryukyu arc chain and has a depth of 1500 m at the point of truncation. The Tokara gap divides the cor- respondingly named islands into northern and southern island subgroups. ...
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... Goto submarine canyon is also a fault-controlled valley (Figures 1 and 3; Oiwane et al. 2010), and together with the Tokara gap may represent part of the same sub- merged drainage system. If so, the Tokara gap may repre- sent the palaeo-mouth of the Yellow River (and the Huai River), reflecting when the Ryukyu area was a continental margin arc (cf., Figure 2). ...
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... island occupies the forearc high to the east of Amami Oshima (Figure 1). Its basement consists of marine siltstone of the Shimajiri Group, dated as planktonic foraminifera zone N22 (and older) ( Figure 5; Ujiié 1994), in agreement with the earlier work (Huang 1966). ...
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... Guga Formation fills these valleys. Upstream, these are terrestrial deposits, and downstream, they are shallow marine delta or fan deposits (Figure 2, 1.5 Ma), which were formed close to the present river mouth and shore- line. The maximum altitude of the marine Guga Formation is 100 m, and the Guga Formation is distributed both east- ern and western coast valleys of Okinawa-jima. ...
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... is because there was a large bay along the Kerama gap, with water deep enough to cover what became Kume-jima. Nakamura et al. (1999) postulated that the large-scale, cross-stratified sandstone (part of the Shimajiri Group) at Kume-jima represents deltaic sediments from the former Yangtze River (Figures 1-3). ...
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... present Kerama gap (Figures 1 and 3) is controlled by a graben (cf., Figure 2, 1.5 Ma). A series of normal faults trend NW-SE and dip SW in the area northeast of the axial part of the gap, cutting the Shimajiri Group and the Ryukyu limestone on both sides of the gap (Kimura 1996). ...
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... lies on the main Ryukyu Island chain (Figures 1 and 3), but lay originally at the eastern margin of the Chinese mainland based on the fact that the basement consists solely of the marine Shimajiri Group. Ryukyu limestone unconformably rests on the Shimajiri Group ( Figure 6E) and consists of basal conglomerate (part) and alternation of coral and detrital limestones (Honda et al. 1993). ...
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... isotopic compositions also reflect warm water (Lee 2000). There may be a branch of Kuroshio current passing through the Taiwan Strait (Figures 1 and 3). Owing to the shallow depth of the Taiwan Strait, it is likely that land bridges may have formed between Taiwan and the mainland during some glacial sea level lowstands. ...
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... the rifting of the Okinawa Trough began, a rift valley formed in the backarc of the Ryukyu arc (Figures 3 and 9, 1.5 Ma). The extension was con- trolled by NE-SW-striking normal faults, including the Nago fault (Osozawa and Watanabe 2011) on Okinawa- jima (O1). ...
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... small branch of extension (and also the Kuroshio current) might have existed between Tanega- shima (OS-1) and Yaku-shima (OS-2), as reflected by the fission track age of Tanega-shima (OS-1; Otsuka and Kuwayama 2000). The Tsushima gateway allowed inflow of the Kuroshio current to the Japan Sea at 1.552 ± 0.154 Ma ( Kitamura and Kimoto 2006), so the northern end of the rift valley likely extended to the Tsushima gate- way, through the Goto channel (Figure 9, 1.5 Ma). Marine deposits also record warm water through the Taiwan Strait, but such inflow would be unrelated to Okinawa Trough rifting. ...
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... deposits also record warm water through the Taiwan Strait, but such inflow would be unrelated to Okinawa Trough rifting. Japan, the Ryukyu Islands, and Taiwan separated from the continent at about the same time, 1.552 ± 0.154 Ma, through initial extension of the Okinawa Trough, for 1000 km along strike (Figure 9, 1.5 Ma). ...
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... the Tokara, Kerama, and Yonaguni gaps are NW-SE-trending normal fault valleys or probably N-S-trending fault valleys (Ujiié 1983;Kimura 1996;Lallemond et al. 2001) mostly orthogonal to the NE- SW-trending main rift valley, these gaps, particularly the former two, are also expected to be branches of the main rift, formed at 1.552 ± 0.154 Ma. The Kuroshio current entered through the Yonaguni gap, mixed with the western offshore warm current of Taiwan, and flowed out through the Tokara gap, with another branch separating to become the Tsushima Current through the Tsushima gateway (Figure 9, 1.5 Ma). ...
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... rifting was progressed, sea floor spreading followed to form the Okinawa Trough. Detrital sediments from the Yellow and Yangtze rivers were captured in the trough, and under the influence of the warm Kuroshio current, coral reefs formed around each island (Figure 2, 1.0 Ma). ...
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... Okinawa-jima (O1), how- ever, the subsidence rate accelerated at ca. 0.8 Ma, resulting in deposition of upper detrital limestone over reef lime- stone, before slowing. When the Ryukyu limestone was deposited, each island was much smaller than at present, and they could not have been connected to one another (Figure 2, 1.0 Ma). ...
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... the manner of a phylogenetic tree of biological species, we can graphically show the lineage/connection of each Ryukyu Island, including the Japanese Islands and Taiwan, compared to the mainland ( Figure 10). Our geological conclusion is that each island of Ryukyu formed since 1.552 ± 0.154 Ma, and as a consequence, new endemic species evolved on each island. ...
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Citations
... Bayesian tree with sequence of Neritina violacea used as outgroup showing (A) four lineages of Nerita yoldii and (B) their spatial distribution, the posterior probabilities (only values higher than 0.9 are shown) are indicated at the nodes. Map of East Asia showing the sampling sites of Nerita yoldii, dark-grey areas indicate current mainland and island configurations, and light-grey areas indicate palaeogeographic land of East Asia in the early Pleistocene(Osozawa et al., 2012). ...
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... Iriomote Island, in the southern Ryukyus, provides an excellent opportunity to investigate the intraspecific phylogeography of freshwater species. Covering an area of 289 km², this small continental island (Osozawa et al., 2012) is predominantly covered by state-owned forests, boasting minimal human disruption. Having approximately 40 rivers characterized by short, steep-gradient streams and small waterfalls, the island features natural barriers that may impede the dispersal of freshwater species. ...
Iriomote Island is a small continental island hosting approximately 40 rivers characterized by short, steep-gradient streams and small waterfalls. We examined the population genetic structure and diversity of the freshwater prawn Macrobrachium shokitaiFujino & Baba, 1973 (Palaemonidae) using the mitochondrial cytochrome c oxidase subunit I (COI) gene sequence. Specimens were collected from 19 sites along ten rivers on the island, covering the known geographic distribution of the species. Haplotype and nucleotide diversities were very low within each river on the island. Nearly all haplotypes were exclusive to a given river, confirming distinct population structures among rivers at a small geographical scale. Slight genetic differentiation was discerned between the upper and lower sites of waterfalls in only the Yuchin River. Phylogenetic tree reconstruction, haplotype network, and principle coordinate analysis classified five clades roughly correspond to geographical groups in the western, southern, central, and northern regions of the island. Extreme caution should therefore be taken when translocating individuals to other rivers to avoid introducing genetic disturbance, even during conservation and mitigation efforts.
... In the north, however, the plate boundary trends east-west along the Ryukyu Arc and is at a much lower angle to the convergent vector. Subduction of the Philippine Sea Plate beneath the Ryukyu Arc has also generated back-arc spreading in the Okinawa Trough, starting as recently as 1.5 Ma ( Fig. 1) (Osozawa et al. 2012;Sibuet et al. 1998Sibuet et al. , 2021. After a period of relative quiescence, ending ~1.0 Ma, extension in the backarc propagated westward to Taiwan (Sibuet et al. 1995). ...
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... The Japanese Archipelago is suitable for examining the relative contributions of these factors, including island-specific factors, for several reasons. Most of the islands in this archipelago are landbridge islands that have historical connectivity with the Eurasian continent (Osozawa et al., 2012). According to previous studies (Kawamura, 2007;Motokawa & Kajihara, 2017), these islands can be categorized into three main regions: Hokkaido (HKD), Honshu-Shikoku-Kyushu (HSK), and the Nansei Islands (NNS; Figure 1). ...
... NNS is separated from HSK by the Tokara Strait, which is recognized as a biogeographical border called the "Watase line." NNS is connected to the southern Asian continent and has been separated from the continent for longer time than the other regions (Motokawa & Kajihara, 2017;Osozawa et al., 2012). ...
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... Furthermore, the Ryukyu Islands may have been connected to the Chinese continent through Taiwan as a land bridge (1.6-1.0 Ma) (Kimura, 2000), and the connection between the Ryukyu Islands and the main Japanese Island was lost after 1.55 Ma (Osozawa et al., 2012), which both demonstrated a convenient dispersal route for fauna. ...
... Although the samples collected from the Goto Islands did not constitute a monophyletic group, the mtDNA phylogenetic analyses revealed that samples of N. cruciata from the Goto Islands and Tsushima/Iki Islands did not share a haplotype with the Kyushu Island lineage and therefore represented independent genetic groups (Figs 3, 4; Supporting Information, Fig. S1). The divergence time of these genetic groups was ~1 Mya (Fig. 4), which is similar to the time suggested for the formation of the Tsushima and Korean Straits (1.55 Mya; Osozawa et al. 2012). Genetic differentiation attributable to the formation of the Tsushima and Korean Straits has been reported previously; however, most of the associated research has been focused on genetic differentiation between the continental and island lineages (e.g. ...
The Japanese Archipelago consists of four major islands and numerous small islands. The Goto Islands are located near Kyushu, a major island in the Japanese Archipelago. We have previously reported that the population of the Japanese Genji firefly, Nipponoluciola cruciata, inhabiting the Goto Islands displays a unique flashing pattern that is used for pre-mating communication between males and females, indicating the possibility of genetic isolation of the Goto Islands population from neighbouring N. cruciata populations. In this study, we aimed to elucidate the gene flow between the Goto Islands N. cruciata population and its neighbouring island populations by molecular phylogenetic analyses based on the mitochondrial DNA regions of cytochrome c oxidase subunit II and ND5, also coupled to genome-wide analysis of nuclear DNA single nucleotide polymorphisms. Our results suggested that the Goto Islands N. cruciata population is genetically isolated from other populations of this species. Nuclear DNA-based population analyses indicated gene flow between the populations inhabiting the Kyushu and Iki Islands, situated at a distance approximately equal to that between the Goto and Kyushu Islands. Therefore, the genetic isolation of the Goto Islands N. cruciata population is largely affected by flashing pattern behaviour, underlining an ongoing speciation process.
... reveal mixing in various proportions with fine-grained sediment shed from other sources, including the Luzon and Ryukyu arcs and, subordinately, rivers of southeastern China (Fig. 15). A major role in the redistribution of clays in the western Philippine and northeastern South China seas is played by the northward flowing Kuroshio oceanic current (Fig. 1B;Liu et al., 2010b;Liu et al., 2011;Osozawa et al., 2012). ...
... Almost all islands are of continental origin. They formed on the eastern margin of the Asian continent in the past and underwent a complex geological history in the last two million years (Ota 1998;Osozawa et al. 2012; Government of Japan 2019), resulting in complex biogeography of lineages inhabiting the islands (e.g., Ota 1998; Kato and Yagi 2004;Kaito and Toda 2016;Hirano et al. 2019). The combination of all these aspects, i.e., the position of the islands, their climate, and their recent repeated connections and isolations from the continent and neighbouring islands, is the primary reason for the unique fauna and flora inhabiting the Ryukyu Archipelago. ...
We review the genus Anacaena Thomson, 1859 from the Ryukyu Archipelago, southern Japan. Three aquatic species are recognised: A. torikaii sp. nov. from Amami-ôshima Island, A. okinawana sp. nov. from Okinawa-jima Island and Kerama Islands, and A. kumejimana sp. nov. from Kumejima Island. All three species are very similar, with the morphology of the aedeagus being essential for a reliable identification. Dorsal colouration is also useful as a diagnostic character, despite some variation within species. We observe a possible geography-based variation between A. okinawana from Okinawa-jima I. and the neighbouring Kerama Is., but we treat both populations as conspecific based on genital morphology. Anacaena kumejimana and A. okinawana share many morphological characters possibly indicating their close relationship. We compare the endemism of aquatic Hydrophilidae in the Ryukyu Archipelago to that in other groups of aquatic beetles: the proportion of endemic species is higher in aquatic Hydrophilidae than in Dytiscidae, but much lower than in stream-inhabiting Hydraenidae and Elmidae. A list of Japanese species of Anacaena and a key to the Japanese species of the genus are provided.
... node 10 in Fig. 5 and Table 3], M. fruhstorferi on the Ryukyu Islands diversified. However, the past Ryukyu Islands stretched from north to south, and the separation of this archipelago began at approximately 1.55 Ma (Osozawa et al., 2012). The Meghimatium clades in this region may diversify prior to the separation of the islands. ...
East Asia, specifically the Japanese Archipelago, is a biodiversity hotspot of both vertebrates and invertebrates. Mollusks represent a burst of species diversity in this region due to the effects of biotic and abiotic factors on their morphological traits, such as shell shape and size. However, the evolutionary history of terrestrial slugs in East Asia remains unknown. In the present study, we investigated the molecular phylogeny of terrestrial slugs of the genus Meghimatium. This genus includes three described and eight undescribed species, and our study used all except for two. Based on phylogeny and the species delimitation tests, the genus Meghimatium was split into many putative species, suggesting higher species diversity than previously thought based on morphological and anatomical studies and that almost undescribed species may be inappropriate. Therefore, morphological traits, such as body size and colour, conventionally considered for classification may easily vary or be similar across geographic region. Moreover, the divergence time of this genus is almost concordant with the geographical time scale of the formation of the Japanese mainland. Our findings suggest that molecular phylogenetics helps classify Japanese Meghimatium slugs, but comprehensive taxonomic revisions using multi-locus analyses are needed.
... Ma) (Otsuka, 2002;Otsuka and Takahashi, 2000). However, dispersal of earthworms from the Ryukyus to the Japanese Islands may have been absent since the late Early Pleistocene-the connection between the Ryukyu Islands and the main Japanese Islands was lost after 1.55 Ma (Osozawa et al., 2011). ...
Megascolecid earthworms of the pheretimoid group are dominant detritivores of soil ecosystems in the Japanese Archipelago and East Asia. However, their diversity and phylogenetic relationships are poorly understood. We assembled whole mitogenome sequences for 197 megascolecid earthworms collected throughout Japan to study the phylogenetic relationships, phylogeography, divergence times, and diversification of important morphological characteristics among pheretimoid earthworms. Using 197 mitogenome sequences and 24 published mitogenome sequences from the East Asian mainland (221 sequences in total), we constructed a maximum likelihood tree and found that the pheretimoid earthworms currently assigned to Amynthas, Metaphire, Duplodicodrilus, and Manus are involved in the most senior genus Amynthas; thus, Amynthas can be treated as the sole genus encompassing all of the above genera. Within the Amynthas group, we identified three major lineages that led to four groups of endemic species in Japan. These lineages originated from different lineages on the East Asian mainland and Taiwan Island, indicating multiple colonization events from the East Asian mainland by different ancestral lineages, possibly after the Miocene. We also assembled nuclear ribosomal DNA sequences encompassing the 18S to 28S rRNA genes. The nuclear gene tree showed major groups consistent with the mitogenome tree except for different (and not well-resolved) relationships among major clades. Our molecular data covered 115-158 native and 7 non-native Amynthas group species in Japan in terms of DNA-based species delimitation. Our findings provide a basis for understanding the evolutionary relationships among diversified megascolecid earthworms in the Amynthas group in Japan and adjacent regions.
