Southwestern Shikoku Research Articles

Population genetic structures of newly recruited and adult populations of pocilloporid corals in southwestern Shikoku, Japan

Population genetic structures of newly recruited and adult populations of pocilloporid corals in southwestern Shikoku, Japan

Previous outbreaks and current abundance of corallivorous gastropods in southwestern Shikoku

Previous outbreaks and current abundance of corallivorous gastropods in southwestern Shikoku

The spawning season of Drupella fragum in southwestern Shikoku

The spawning season of Drupella fragum in southwestern Shikoku

Middle Holocene relative sea‐level changes and vertical tectonic crustal movements on Shikoku Island near the Nankai Trough, Japan

AbstractThe Philippine Sea plate subducts beneath the Eurasia plate at the Nankai Trough, northwestern Pacific, causing crustal deformation, mega‐thrust earthquakes, and tsunami events. Shikoku Island, 150 km northwest of the trough, experiences both coseismic and interseismic deformation. Coastal sediments potentially record vertical crustal movements as relative sea level (RSL) changes. We studied sedimentary facies and microfossil ostracodes in core SKM from southwestern Shikoku Island for evidence of middle Holocene tsunami events and deformation. The core sediments included nine event layers corresponding to storm or tsunami events. Using modern analog techniques, we estimated RSLs from the ostracode assemblages of core SKM and 13 other cores from Shikoku Island and the surrounding region. Then, we subtracted RSL changes due to glacio‐hydro isostatic adjustment from the estimated RSLs to estimate vertical tectonic movement rates in these cores between 8.6 and 4.7 ka. The inferred RSL changes suggest that the Sukumo site has experienced both uplift and subsidence since 8.6 ka. Before 6.6 ka, rates of tectonic crustal movement were higher than the modern‐day rate, and its spatial distribution also differed. After 6.6 ka, tectonic crustal movement showed a similar spatial pattern and occurred at rates close to the modern day interseismic rate. The spatial pattern and rates of tectonic crustal movement could be caused by changes in rupture areas between the Eurasia and the Philippine Sea plates beneath the Shikoku Island and in stress condition of the asthenosphere. Some of the vertical displacements can be explained by the movements of local active faults.

Growth pattern of the siliceous skeletons of living Spumellaria (Radiolaria) from the Kuroshio Current, offshore southwestern Shikoku Island, Japan

Experimental culture studies of living radiolarians experience many difficulties because radiolarians are too sensitive to be maintained alive in artificial conditions. We here developed an improved experimental protocol in order to detect newly-formed parts of the polycystine radiolarian skeletons after they were labeled with fluorescent markers. Living samples were collected from surface seawater of the Kuroshio Current offshore from Kashiwajima Island, Kochi Prefecture on the island of Shikoku, Japan. The taxonomic composition of the Kashiwajima radiolarian samples varied markedly across the seasons and sampling sites. Spumellaria were usually abundant in samples collected near Kashiwajima, and had high diversity in shell morphology. Here, we performed experimental studies on living spumellarian radiolarians to clarify their siliceous skeletogenesis. Our culture-based research on spumellarians provided the following results.(A) The pattern of silica deposition on the skeleton of Spongaster tetras tetras varies depending on the growth stage. Three ontogenetic stages were identified: (1) young, (2) progressively-growing and (3) fully-grown. Furthermore, novel varied types of skeletal growth modes were determined as follows: (i) rapid growth on the outermost shell with enlargement of shell size and (ii) internal deposition on skeletal components without change in shell size. From measurements of the growth rate of cultured radiolarians, it is assumed that S. tetras tetras cell has at least a month-long life cycle.(B) Spherical spumellarians have their own shell-forming strategy that permits additional silicogenesis while maintaining the taxonomically-specific shape of their shell; the meshwork of their shell surface consists of polygonal-shaped pore-frames formed by heterogeneous patterns of added silica.Our direct observation with fluorescence markers detected a skeletogenesis process of Haliommilla capillacea possessing a fragile spherical shell. We thus confirm the bridge-growth hypothesis for morphogenesis of the polygonal frame as proposed in previous studies. Spongosphaera streptacantha, having extremely long spines, exhibited a special growth pattern of spine elongation in a centrifugal direction, without a change in width. This was achieved by a particular temporal pattern of silicification characterized by an alternating pattern of patchy addition of new silica on the entire part of each blade from the base to the tip. Moreover, a thin layer of new silica was added by a flowing deposition along the edge of the spine. Based on these culture-based studies using living radiolarians, we conclude that Spumellaria control and maintain a particular shell shape by regulating where silicification takes place on the skeleton, particularly depending on the growth stage.

Great earthquake at 7.3 ka inferred from tsunami deposits in the Sukumo Bay area, Southwestern Japan

AbstractTsunami deposits in Kyushu Island, Southwestern Japan, have been attributed to the 7.3 ka Kikai caldera eruption, but their origin has not been confirmed. We analyzed an 83‐cm‐thick Holocene event deposit in the SKM core, obtained from incised valley fill in the coastal lowlands near Sukumo Bay, Southwestern Shikoku Island. We confirmed that the event deposit contains K‐Ah volcanic ash from the 7.3 ka eruption. The base of the event deposit erodes the underlying inner‐bay mud, and the deposit contains material from outside the local terrestrial and marine environment, including angular quartz porphyry from a small inland exposure, oyster shell debris, and a coral fragment. Benthic foraminifers and ostracods in the deposit indicate various habitats, some of which are outside Sukumo Bay. The sand matrix contains low‐silica volcanic glass from the late stage of the Kikai caldera eruption. We also documented the same glass in an event deposit in the MIK1 core, from the incised Oyodo River valley in the Miyazaki Plain on Southeastern Kyushu. These two 7.3 ka tsunami deposits join other documented examples that are widely distributed in Southwestern Japan including the Bungo Channel and Beppu Bay in Eastern Kyushu, Tachibana Bay in Western Kyushu, and Zasa Pond on the Kii Peninsula as well as around the caldera itself. The tsunami deposits near the caldera have been divided into older and younger 7.3 ka tsunami deposits, the younger ones matching the set of widespread deposits. We attribute the younger 7.3 ka tsunami deposits to a large tsunami generated by a great interplate earthquake in the Northern part of the Ryukyu Trench and (or) the Western Nankai Trough just after the late stage of the Kikai caldera eruption and the older 7.3 ka tsunami deposits to a small tsunami generated by an interplate earthquake or Kikai caldera eruption.

Hyperxiphia hirashimai, comb. n. (Hymenoptera, Xiphydriidae) from southern Japan: remarkable sexual dimorphism revealed by DNA barcodes and new distribution records.

Based on a study of the type material, Genaxiphia hirashimai Okutani, 1965, a southern Japanese xiphydriid showing remarkable sexual dimorphism, is newly combined with the genus Hyperxiphia Maa, 1949. The previously unknown female is described, and the male is redescribed with additional material. The sexes are associated by examination of specimens of both sexes that apparently emerged from the same piece of branch and by a molecular analysis using mitochondrial COI sequences. Hyperxiphia hirashimai, originally described from Amami-oshima Island, is newly recorded from Miyake-jima Island, southwestern Shikoku, southern Kyushu, Yakushima, Kuroshima and Okinawa-jima Islands.

Radiocarbon age offsets of Plants and Shells in Holocene sediments from the Sukumo plain, Southwest Coast of Shikoku, Southwest Japan

AbstractTo identify chronological changes in the marine reservoir effect in southwest Japan, the radiocarbon ages of eight pairs of marine shells and terrestrial plants were measured from the same horizons of one core of Holocene sediments. This core was obtained from the northern part of the Sukumo Plain in southwestern Shikoku Island, which faces the warm Kuroshio Current. The drilling site is located in an area of subsidence associated with the convergence of the Philippine Sea Plate and the Eurasian Plate. On analyses of lithology, molluscan assemblages, and radiocarbon dating, we interpreted, from oldest to youngest, nine units: basement rock, a braided river channel, a meandering river channel, an estuary, a transgressive inner bay, Kikai-Akahoya volcanic ash, a deltaic inner bay, a tidal flat, and artificial soil. Changes in the depositional environment were mainly associated with sea-level rise after the end of the last glaciation. The reservoir ages of the eight pairs, 330 ± 70 yr, were obtained from a transgressive inner bay to deltaic inner bay sediments, formed during 4100–9200 cal BP. The chronological change in the reservoir effect allows us to correlate the Sukumo Core sediments with previous results from southwestern Japan, the Korean Peninsula, and Taiwan Island.

Mass occurrence of the enigmatic gastropod Elmira in the Late Cretaceous Sada Limestone seep deposit in southwestern Shikoku, Japan

Elmira is a medium-to-large gastropod of uncertain systematic affinity, which has so far been reported only from a presumably Eocene methane-seep deposit in Cuba. This study reports a mass occurrence of Elmira shimantoensis Kiel and Nobuhara sp. nov. from a Late Cretaceous hydrocarbon-seep deposit in Shikoku, Japan, called the Sada Limestone. Its paleoecology is reconstructed based on its mode of occurrence, carbonate petrology, and stable carbon isotope analyses. The fauna of the Sada Limestone in general is characterized by an abundance of large chemosymbiotic bivalves of the family Thyasiridae and of “Serpula” tubes. The mass occurrence of Elmira shimantoensis was found in a lens-shaped carbonate body with a flat top and a concave base, 6.5 m in length and less than 2 m in thickness, consisting of multiple layers of shell accumulations, which were formed by shell-winnowing and the filling of a depression in slope mud. The scarcity of Elmira shimantoensis elsewhere in the Sada Limestone suggests that it formed locally from a gregarious population in the vicinity of the depression, possibly on hard ground. The matrix of the mass occurrence is rich in dolomite and ankerite, and is less depleted in 13C (δ13C values of calcite: −5.3 to −2.4 ‰; of dolomite: −8.3 ‰) than the matrix of the enclosing thyasirid-rich and tube-rich limestones. This suggests that the gastropod mass occurrence was cemented below the sulfate reduction zone and has thus undergone little anaerobic methane oxidation. Therefore, Elmira shimantoensis is reconstructed here as a bacteria grazer on a hard substrate such as exposed carbonate mounds rather than as a species that relied on chemosynthetic symbionts for nutrition.

Borniopsis mortoni sp. n. (Heterodonta, Galeommatoidea, Galeommatidaesensu lato), a new bivalve commensal with a synaptid sea cucumber from Japan.

The Galeommatoidea is a bivalve superfamily that exhibits high species diversity in shallow waters. Many members of this superfamily are associated commensally with burrowing marine invertebrates in benthic sediments. The genus Borniopsis is known only from eastern Asia and exhibits high host diversity (e.g., mantis shrimps, crabs, holothurians, sipunculans and echiurans). A new species, Borniopsis mortoni sp. n., is described from mud flats at the mouth of the Souzu River, southwestern Shikoku Island, Japan. This species has elongate-ovate shells covered by a tan to dark brown periostracum, and lives attached by both its foot and byssal threads to the body surface of the synaptid sea cucumber Patinapta ooplax. Several individuals of Borniopsis mortoni are often found on the same host, but sometimes more than 10 individuals can occur together. Borniopsis mortoni is one of the smallest species in this genus. Probably, its small body size is an adaptation to the mode of life in a narrow host burrow. Until now, only two other Borniopsis species were known to have commensal associations with synaptids. Thus, this is the third example of a synaptid-associated species from this genus. In addition, we briefly review the galeommatoideans commensal with apodid sea cucumbers.

Occurrence of Cicadulina bipunctata (Hemiptera: Cicadellidae) in southwestern Shikoku, Japan and comparisons of gall-inducing ability between Kyushu and Shikoku populations

The maize orange leafhopper Cicadulina bipunctata (Melichar) is an insect pest of cereal crops in tropical and subtropical regions of the Old World. This leafhopper induces gall symptoms characterized by stunted growth and swollen leaf veins on various Poaceae. Damage by C. bipunctata has been reported from Australia, the Philippines, China, Taiwan, and Japan. In Japan, C. bipunctata occurs in the central and southern parts of Kyushu. Because the leafhopper is a potential pest of various cereal crops, we conducted field surveys in Shikoku and the southern part of the Kii Peninsula (a part of Honshu), where the climate seems to be suitable for the establishment of C. bipunctata. As a result, we found C. bipunctata at some localities in Ehime and Kochi Prefectures. This is the first record of this leafhopper from Shikoku. Our laboratory experiment revealed that a Kochi population of C. bipunctata had a gall inducing ability similar to the Kumamoto population. More attention should be paid to assessing the risk of further expansion of C. bipunctata populations in Shikoku.

Vegetation history since MIS 15 based on a pollen record from the Uwa Basin, southwestern Shikoku, Japan

Vegetation history since MIS 15 based on a pollen record from the Uwa Basin, southwestern Shikoku, Japan

Comparative Analysis of Prickles on Rubus sieboldii (Rosaceae) between Grazed and Ungrazed Areas in Southwestern Shikoku, Japan

Kashima Island (32o57?N, 132o27?E) is a very small island (45.1 ha) in southwestern Shikoku, Japan with a high density (ca. 38.5/km 2 ) of Sika deer ( Cervus nippon ). To examine induced defences of plants against the Sika deer, we conducted morphological analyses of prickles of Rubus sieboldii Blume (Rosaceae) between grazed (Kashima Island) and ungrazed areas (neighbour locations) in southwestern Shikoku. The length and density of prickles on leaves and around stems were measured. The prickles of the plants on Kashima Island were significantly longer and denser than those of the other areas, implying that the increased length and density of prickles were an induced defence of R. sieboldii on Kashima Island.

Seasonal occurrence of adult Haliplus sharpi Wehncke in paddy fields of southwestern Shikoku, Japan

Seasonal occurrence of adult Haliplus sharpi Wehncke in paddy fields of southwestern Shikoku, Japan

Comparative analyses of hairless-leaf and hairy-leaf type individuals in Aster hispidus var. insularis (Asteraceae)

The distribution of Aster hispidus Thunb. var. insularis (Makino) Okuyama is limited to the coastal areas of south-western Shikoku. This species has been described as having fatter stems, thicker leaves, and glabrous stem and leaves ciliate on margin; however, we found many individuals that ranged from having no hair to many hairs under the stems leaves in this variety. We measured the number of leaf hair, degree of herbivory, and stomatal density of leaves of A. hispidus var. insularis. Two types of A. hispidus var. insularis for the leaf hair (presence and absence) were accepted. The degree of herbivory did not differ significantly between these 2 types; however, the stomatal density was lower in the hairy-leaf-type individuals than in the hairless-leaf-type individuals.

Whole rock chemistry of shale and sandstone from the Northern Shimanto belt in southwestern Shikoku

Whole rock chemistry of shale and sandstone from the Northern Shimanto belt in southwestern Shikoku

U-Pb zircon ages of syenitic and granitic rocks in the Ashizuri igneous complex, southwestern Shikoku: Constraint for the origin of forearc alkaline magmatism

., 1989). The results range from 10 to 16Ma, which suggest that the igneous activities of theAshizuri complex were generally coeval with those of theOuter Zone Granitic Rocks. Relatively large scatter in thereported age, however, obscures the strict temporal rela-tionship between the magmatism of Ashizuri complex andthose of the Outer Zone Granitic Rocks.We report U–Pb ages of the zircons separated fromtwo felsic samples of the Ashizuri complex using laserablation ICP mass spectrometry (LA-ICP-MS) to clarifythe timing of this peculiar magmatism. Implications tothe origin of the alkaline magmatism in the forearc re-gion will also be discussed.

Petrologic Study and Fission-track Age of Middle Pleistocene Hirano Volcanic Ash in Northern Nomura Town, Southwestern Shikoku, Japan

Petrologic Study and Fission-track Age of Middle Pleistocene Hirano Volcanic Ash in Northern Nomura Town, Southwestern Shikoku, Japan

Coral recruitment patterns in high-latitude coral communities in southwestern Shikoku, Japan

造礁サンゴの群集構造と動態に重要な役割を果たしている幼生加入について，四国南西海域の高緯度造礁サンゴ群集で周年にわたる季節変動を調査した。2007年と2008年に，本海域内の須ノ川，西泊，桜浜の3地点で，設置期間を2ヶ月とする定着板の設置と回収を繰り返した。造礁サンゴ幼生の定着は，5月から9月に設置した定着板で観察された。定着量のピークは，2007年は7-9月，2008年は6-8月で，それぞれ定着板1セットあたり1.7個と2.1個であった。種類別では，ハナヤサイサンゴ科が72.1％で最も多く，次いでハマサンゴ科が12.6％，ミドリイシ科が6.0％であった。これは，本海域の造礁サンゴ群集ではミドリイシ属が優占することと一致しなかった。幼生の定着パターンは種類によって異なっていた。ハナヤサイサンゴ科では，定着は5-7月から9-11月までのおよそ半年にわたったが，本海域での幼生放出期に当たる期間前半で多かった。ハマサンゴ科も同様の期間で定着がみられたが，季節変動は不明瞭であった。ミドリイシ科の定着は，本海域でのミドリイシ科の産卵時期である7月から8月の短期間に集中した。本海域での造礁サンゴの幼生加入は，サンゴ礁域に比べて少なくハナヤサイサンゴ科の割合が高いという，高緯度造礁サンゴ群集に特有の特徴が見られた。

Record of juvenile Siberian Blue Robin Luscinia cyane on Mt. Sasa, southwestern Shikoku

Record of juvenile Siberian Blue Robin Luscinia cyane on Mt. Sasa, southwestern Shikoku