Ishigaki Island Research Articles

Sciaphila sugimotoi (Triuridaceae), a new mycoheterotrophic plant from Ishigaki Island, Japan

A new species of Sciaphila (Triuridaceae), S. sugimotoi Suetsugu & T. Nishioka, is described and illustrated from Ishigaki Island, Okinawa Prefecture, Japan. The new species is similar to S. arfakiana in having a stipitate globose to ellipsoid knob without hairs in apices of all male perianth segments. However, it is clearly distinguishable by its shorter pedicel, filament-connective not extended and thick stem. A key to the Japanese Sciaphila based on the whole flower characteristics and stylar characteristics, are provided for easy identification of these rare mycoheterotrophic plants.

Thaumasiovibrio occultus gen. nov. sp. nov. and Thaumasiovibrio subtropicus sp. nov. within the family Vibrionaceae, isolated from coral reef seawater off Ishigaki Island, Japan

Two phylogenetically distinct Vibrionaceae strains C4II189T and C4V358T isolated from reef seawater off Ishigaki Island, Japan, in 2014 were studied with advanced genome-based taxonomy approaches. All aspects of phylogenetic (16S rRNA phylogeny, MLSA), phenotypic and genetic (ANI, DDH, AAI, and the number of core genes) cohesions between the two identified species were high enough to propose them as members of a new genus within the family Vibrionaceae. Consequently, an eighth genus Thaumasiovibrio gen. nov. is proposed that contains two new species Thaumasiovibrio occultus sp. nov. strain C4II189T (=DSM 101554T=JCM 31629T) (type species) and Thaumasiovibrio subtropicus sp. nov. strain C4V358T (=DSM 101555T=JCM 31630T). Thaumasiovibrio species were phylogenetically distinct from the other Vibrionaceae species based on pyrH gene sequences. The combination of catalase negative, sensitivity to vibriostatic agent O/129, and green colony formation on TCBS for the phylogenetically affiliated strains was the diagnostic features for the current tentative identification of this genus.

Assessment of Machine Learning Approaches for Bathymetry Mapping in Shallow Water Environments using Multispectral Satellite Images

This paper evaluates the performance of two proposed empirical approaches—random forest (RF) and multi-adaptive regression spline (MARS)—for bathymetry calculations in three diverse areas: the Alexandria harbor shallow coastal area, Egypt, as an example of a low-turbidity, silt-sand bottom water area with depths ranging from 4 m to 10.5 m; the Lake Nubia entrance zone, Sudan, which is considered a high-turbidity, unstable, clay bottom area with a depth of 6 m; and Shiraho, Ishigaki Island, Japan, a coral reef area with a depth of 14 m. Data from Landsat 8 and Spot 6 satellite images were used to evaluate the performance of the proposed models. The bathymetry results of the proposed models were compared with the corresponding results yielded from two conventional empirical methods: the neural network (NN) model and the Lyzenga generalized linear model (GLM). When compared with echo sounder data, the RF and MARS results outperformed Lyzenga GLM results. Moreover, the RF method produced more accurate results with average 0.25 m RMSE improvements range than the NN model. The RF algorithm produced the most accurate results proved to be a preferable algorithm for bathymetry mapping in the shallow water context.

Prevalence of skin infections caused by Panton-Valentine leukocidin-positive methicillin-resistant Staphylococcus aureusinJapan, particularly in Ishigaki, Okinawa.

The prevalence of Panton-Valentine leukocidin gene (pvl)-positive community-acquired methicillin-resistant Staphylococcus aureus USA300 clone, which is designated as the ST8-staphylococcal cassette chromosome (SCC) mec type IV (ST8-IV) lineage, is a major public health concern worldwide. Thus, to elucidate the prevalence and characteristics of pvl-positive community-onset MRSA in Japan, we conducted a molecular epidemiological analysis for 854 S.aureus isolates obtained from outpatients with skin infections during 2013 and 2014. The isolation rate of MRSA was 25.6% (219 isolates), and the ratio of pvl-positive MRSA was 13.2% (29 isolates). Notably, the proportion (93.8%) of pvl-positive isolates was particularly high among MRSA isolates from Ishigaki island in Okinawa. Pulsed-field gel electrophoresis and multilocus sequence typing showed that the pulsotype C isolates (11 isolates) were typical USA300 clones with arginine catabolic mobile element (ACME) type I-CC8-IV lineages and prevalent on the main island of Japan (Honshu). Pulsotypes A (11 isolates) and B (four isolates) consisted of ACME-negative CC8-IV clones and were specific for Ishigaki island. Both USA300 and Okinawa-Ishigaki specific clones were associated with deep-seated skin infections, such as furuncle and cellulitis. Pulsotypes D (two isolates) and E (one isolate) were ACME-negative clonal complex (CC) 59-IV clones and were related to superficial skin infections, such as impetigo. Our findings revealed that pvl-positive MRSA associated with deep-seated skin infections are spreading in Japanese communities, particularly in Ishigaki, Okinawa.

Population dynamics of the soldier crab Mictyris guinotae (Brachyura: Mictyridae) in Amparu Tidal Lagoon on Ishigaki Island, Japan

The life history of the soldier crab Mictyris guinotae in Amparu Tidal Lagoon on Ishigaki Island, Japan was investigated through population measurements with detailed size classifications. Surveys and measurements were carried out at 15 sampling points every 2 weeks during the 4-month incubation season starting in December 2005, and twice in April and in June 2006 when juveniles started and finished settlement, respectively, in the lagoon. Supplementary measurements were conducted every month from December 2006 to May 2007 to corroborate the results of the first experiment and to examine the growth rate of the juvenile population.The results showed that (a) the peak of incubation was from mid-January to mid-February, and the season of juvenile settlement was April to early June, which means that the duration of larval life in the ocean was about 2 months; (b) juveniles were distributed widely in the lagoon, and adults clustered to the west and south of the lagoon where the bed sediment was relatively fine with high ignition loss; (c) the survival rates of adults and juveniles in 1 year were 0.73 and 0.17, respectively, and the mean longevity of adults was about 3.7 years; (d) reproduction efficiency from eggs to juveniles was about 0.0040, which is much smaller than the survival rate of juveniles as well as adults, which means that the propagation of M. guinotae is mainly controlled by the condition that the larvae experience in the ocean.

Comparison of SWAT and GSSHA for High Time Resolution Prediction of Stream Flow and Sediment Concentration in a Small Agricultural Watershed

In this study, two hydrologic models, the Gridded Surface Subsurface Hydrologic Analysis (GSSHA) and the Soil and Water Assessment Tool (SWAT), were applied to predict stream flow and suspended sediment concentration (SSC) in a small agricultural watershed in Ishigaki Island, Japan, in which the typical time scale of flood event was several hours. The performances of these two models were compared in order to select the right model for the study watershed. Both models were calibrated and validated against hourly stream flow and SSC for half-month periods of 15 to 31 May 2011 and 17 March to 7 April 2013, respectively. The results showed that both models successfully estimated hourly stream flow and SSC in a satisfactory way. For the short-term simulations, the GSSHA model performed slightly better in simulating stream flow as compared to SWAT during both calibration and validation periods. GSSHA only gave better accuracy when predicting SSC during calibration, while SWAT performed slightly better during validation. For long-term simulations, both models yielded comparable results for long-term stream flow and SSC with acceptable agreement. However, SWAT predicted the overall variation of long-term SSC better than GSSHA.

Biomass of a mangrove forest and a sedge marsh on Ishigaki Island, South Japan

Biomass of a mangrove forest and a sedge marsh on Ishigaki Island, South Japan

Multidecadal oceanographic changes in the western Pacific detected through high‐resolution bomb‐derived radiocarbon measurements on corals

AbstractHigh‐resolution measurements of radiocarbon (14C) in corals can be used to reconstruct past variability in ocean conditions. Here we report seasonal Δ14C changes in coral from Ishigaki Island, Japan, and compare with previously reported data from Palau and Guam. Our data clearly indicate a significant increase in Δ14C from 1947 to 1998 related to atmospheric nuclear bomb testing. The three early Δ14C spikes related to the atmospheric nuclear bomb tests in the US Proving Grounds at Bikini and Enewerak atoll conducted in 1954, 1956, and 1958 were detected from the Ishigaki coral. After 1976, variability in the Mindanao Dome region related to North Equatorial Current (NEC) bifurcation latitude migration affected the Δ14C difference between Palau and Guam, whereas the difference between Ishigaki and Guam was not correlated with the bifurcation latitude. The Δ14C difference between Ishigaki and Guam may be due to mesoscale eddies in the Kuroshio area. On the decadal scale, the northward shift of NEC bifurcation latitude after 1976, the year as known as Pacific Decadal Oscillation regime shift from negative to positive, was concurrent with the abundant westward‐propagating mesoscale eddies in the Subtropical Countercurrent region and stronger Kuroshio transport off the east Taiwan, which may be represented by a smaller Δ14C difference between Ishigaki and Guam after 1976.

Analysis of Red Soil Sediment Yield in a Small Agricultural Watershed in Ishigaki Island, Japan, Using Long–Term and High Resolution Monitoring Data

This paper analyzes the temporal variability in sediment yield in a small agricultural watershed located in the southwest portion of Japan. The relationships between the rainfall, runoff, and sediment variables were analyzed, and the suspended sediment concentration–discharge hysteresis was used to interpret the mechanisms of sediment transport. A total of 227 flood events were derived from long–term high resolution discharge and sediment data that had been collected over the period of 2000–2015. Long–term analyses showed that there are highly variable hydrological and sediment responses on inter–annual, annual, seasonal scales. Sediment was strongly transported in the period of August–October, when the watershed was hydrologically and sedimentologically most active, followed by the February–April, May–July, and November–January periods. The sediment concentration for most flood events was relatively high, frequently exceeding 200 mg.L−1 and reaching as much as 1967 mg.L−1. The rainfall, runoff, and sediment variables showed significant correlations with each other. However, antecedent rainfall strongly influenced the efficiency of runoff generation, which further affected the sediment transport. Four types of hysteresis were observed: clockwise (48%), counter–clockwise (16%), figure–eight (25%), and complex (11%). The results confirmed the complexity and heterogeneity of both the hydrological and sedimentological responses in the studied watershed. Even though the analyses revealed the efficiency of the soil erosion–reduction schemes that had been implemented in the studied watershed, the red soil outflow is still relatively high. Therefore, additional measures (e.g., slope gradient modification, grass strips, and cover crop planting prior to summer sugarcane planting) are necessary to reduce soil erosion and sediment discharge.

酸素・炭素安定同位体比・Mn/Ca比を用いた石垣島轟川河口付近のハマサンゴの壊死研究

酸素・炭素安定同位体比・Mn/Ca比を用いた石垣島轟川河口付近のハマサンゴの壊死研究

Morphological description of four species belonging to the genus Nigrobaetis (Ephemeroptera: Baetidae) from Japan

We associated imagoes and nymphs of four Nigrobaetis species from Japan and described their morphological characters. We found that N. sp. P comprised two Nigrobaetis species in Kanto Plain and in Ishigaki Island, which we described as N. apterus Fujitani n. sp. and N. ishigakiensis Fujitani n. sp., respectively. We also described N. sp. I as N. latus Fujitani n. sp. We recorded N. acinaciger (Kluge), the nymph of which had been confused with N. chocoratus (Gose), from Japan for the first time. We provide identification keys for eight Nigrobaetis species of Japan.

Role of Demersal Zooplankton as a Food Source for Higher Trophic Levels at Fukido Estuary, Ishigaki Island, Okinawa, Japan

Demersal zooplankton (DZ) appear in the water column at night, and are highly abundant in mangrove, seagrass, and coral reef habitats; however, few studies have discussed their role in aquatic food webs, considering different consumers and their preferences on different DZ’ size classes. This study elucidates the role of DZ as a food source for higher trophic levels in an estuarine area, particularly with respect to the food preference and size selection of their consumers. The study was conducted in the mangrove forest of Fukido Estuary and an adjacent reef lagoon (with seagrass-dominated and seagrass-coral mixture areas) on Ishigaki Island, Japan. The abundance of demersal zooplankton was 4.0, 5.4, and 11.3×10 4 ind.m -2 for seagrass, mangrove, and seagrass-coral mixture habitats, respectively. The lowest DZ biomass was recorded in mangroves and mainly dominated by smaller organisms, because their consumers in this habitat prefer large-sized prey. The δ 13 C and δ 15 N signatures showed that, in mangroves, demersal zooplankton constituted a higher proportion of the diet of fishes than in lagoon habitats; however, demersal zooplankton did not have a significant role in the diet of fishes and macroinvertebrates in the lagoon. Consistency among biomass, stomach contents, and the proportions of DZ of all size classes in the diet of mangrove fishes indicated that DZ serve as a major food source. In contrast, fishes in lagoon habitats consumed more crabs, shrimps and mollusks than DZ. In conclusion, our analytical approach allowed us to demonstrate that DZ of different body sizes serve as food sources for different consumers in different habitats of the estuarine ecosystem.

石垣島川平湾における有藻性イシサンゴ類の現状と近年の変化

石垣島川平湾における有藻性イシサンゴ類の現状と近年の変化

Assessment of proposed approaches for bathymetry calculations using multispectral satellite images in shallow coastal/lake areas: a comparison of five models

Bathymetric information for shallow coastal/lake areas is essential for hydrological engineering applications such as sedimentary processes and coastal studies. Remotely sensed imagery is considered a time-effective, low-cost, and wide-coverage solution for bathymetric measurements. This study assesses the performance of three proposed empirical models for bathymetry calculations in three different areas: Alexandria port, Egypt, as an example of a low-turbidity deep water area with silt-sand bottom cover and a depth range of 10.5 m; the Lake Nubia entrance zone, Sudan, which is a highly turbid, unstable, clay bottom area with water depths to 6 m; and Shiraho, Ishigaki Island, Japan, a coral reef area with varied depths ranging up to 14 m. The proposed models are the ensemble regression tree-fitting algorithm using bagging (BAG), ensemble regression tree-fitting algorithm of least squares boosting (LSB), and support vector regression algorithm (SVR). Data from Landsat 8 and Spot 6 satellite images were used to assess the performance of the proposed models. The three models were used to obtain bathymetric maps using the reflectance of green, red, blue/red, and green/red band ratios. The results were compared with corresponding results yielded by two conventional empirical methods, the neural network (NN) and the Lyzenga generalised linear model (GLM). Compared with echosounder data, BAG, LSB, and SVR results demonstrate higher accuracy ranges from 0.04 to 0.35 m more than Lyzenga GLM. The BAG algorithm, producing the most accurate results, proved to be the preferable algorithm for bathymetry calculation.

Toxicity and toxins of the greater blue-ringed octopus Hapalochlaena lunulata from Ishigaki Island, Okinawa Prefecture, Japan

Toxicity and toxins of the greater blue-ringed octopus Hapalochlaena lunulata from Ishigaki Island, Okinawa Prefecture, Japan

Dynamics of dissolved organic matter in the mangrove area of the Fukido River, Ishigaki Island

Dynamics of dissolved organic matter in the mangrove area of the Fukido River, Ishigaki Island

Growth characterization of non-photosynthetic diatoms, <i>Nitzschia</i> spp., inhabiting estuarine mangrove forests of Ishigaki Island, Japan

Growth characterization of non-photosynthetic diatoms, <i>Nitzschia</i> spp., inhabiting estuarine mangrove forests of Ishigaki Island, Japan

Formation of gas discharging from Taketomi submarine hot spring off Ishigaki Island in the southern Ryukyu Islands, Japan

Taketomi submarine hot spring lies off Ishigaki Island in the southern Ryukyu Islands and vents hot spring waters at temperatures up to ~50°C from the seafloor at a depth of 20m. We investigated the chemical and isotopic composition of gases discharging from Taketomi hot spring. The gases were composed mainly of methane, with secondary nitrogen at higher than atmospheric concentration. Carbon and hydrogen isotope data suggest that the methane in the discharging gases was derived mainly from thermal decomposition of organic matter. Helium isotopes were enriched in 3He relative to the atmosphere, suggesting a supply of mantle-derived helium to the discharging gases. The mantle-derived gases transfer the deep-originated thermal energy to the hot spring and thermogenesis of organic matter. The hydrocarbons in the venting gas could be sourced from sedimentary rocks of the Yaeyama or Shimajiri Groups, or Yaeyama metamorphic rocks, and added to the ascending gases as they pass through those source rocks on their way to the surface. Because the Pleistocene rocks of the Ryukyu Group beneath the hot spring have been altered by the spring activity, the Taketomi hot spring began venting after the Pleistocene.

Underwater three-dimensional imaging laser sensor with 120-deg wide-scanning angle using the combination of a dome lens and coaxial optics

We developed an underwater three-dimensional (3-D) imaging sensor using a 532-nm laser. The sensor system combines a dome lens with coaxial optics to realize a wide-scanning angle of 120 deg (horizontal)×30 deg (vertical) while having a compact size of 25-cm diameter and 60-cm length. A detector sensitivity time control circuit and a time-to-digital converter are used to detect a small signal and suppress the unwanted backscattered signals due to marine snow. 3-D imaging of the seafloor with 20-m width and 60-m length was demonstrated in the sea around Ishigaki Island, Japan.

Seasonal Growth Fluctuations of Four Species of Neritid Gastropods in an Upper Mangrove Estuary, Ishigaki Island, Japan

Populations of amphidromous neritid gastropods are prone to habitat degradation by anthropogenic disturbances. To accumulate information on the life history of these neritids, seasonal growth patterns of four neritid species, Neritina turrita, N. plumbea, N. auriculata, and Clithon brevispina, were examined in an upper mangrove estuary on Ishigaki Island. Individual neritids in a fixed sampling area were captured and mark-released bimonthly over a 1-yr period. Growth rates of the four species were estimated from linear regression models and Ford-Walford graphs using data from recaptured individuals. A model-selection procedure identified the common L max model as the best model for describing seasonal growth fluctuations of the four species. Growth of the four species was generally higher in the warm season (April–October) and lower in the cool season (October–April). In addition, N. plumbea stopped growing in midsummer (June–August). Temporal changes in the size-frequency histograms showed several recruitment events of the four species. Estimated growth curves, in some cases, did not fit well to temporal changes of the size cohorts derived from the size-frequency histograms, indicating that size-dependent migration or mortality may occur in the sampling area.