Shiraho Reef Research Articles

SEASONAL VARIATION IN WATER CIRCULATIONS AND ITS IMPACT ON THERMAL ENVIRONMENT IN A CORAL REEF AREA OF SOUTHWEST JAPAN

Summer and wintertime observations were conducted through deploying horizontal oceanographic time series sensors in a coral reef region of Southwest Japan to assess the existing hydrodynamic and thermal environments in relation to seasonal changes. A significant variation in water circulation and temperature distribution pattern was recorded in Shiraho reef area between two different seasons. During winter, a strong inflow of oceanic water was recorded inside the reef on a semidiurnal pattern that led to significant water temperature rising while in summer, considerable amount of sea water entered the reef area with a diurnal frequency causing water temperature slightly lower. Outside the reef, current was found to flow nearly parallel in direction to the north-south stretch of the reef.

Spatial Autocorrelation Analysis Using MIG-seq Data Indirectly Estimated the Gamete and Larval Dispersal Range of the Blue Coral, Heliopora coerulea, Within Reefs

Spatial autocorrelation analysis is a well-established technique for detecting spatial structures and patterns in ecology. However, compared to inter-population genetic structure, much less studies examined spatial genetic structure (SGS) within a population by means of spatial autocorrelation analysis. More SGS analysis that compares the robustness of genome-wide single nucleotide polymorphisms (SNPs) and traditional population genetic markers in detecting SGS, and direct comparison between the estimated dispersal range based on SGS and the larval dispersal range of corals directly surveyed in the field would be important. In this study, we examined the SGS of a reef-building coral species, Heliopora coerulea, in two different reefs (Shiraho and Akaishi) using genome-wide SNPs derived from Multiplexed inter-simple sequence repeat (ISSR) genotyping by sequencing (MIG-seq) analysis and nine microsatellite loci for comparison. Microsatellite data failed to reveal significant spatial patterns when using the same number of samples as MIG-seq, whereas MIG-seq analysis revealed significant spatial autocorrelation patterns up to 750 m in both Shiraho and Akaishi reefs based on the maximum significant distance method. However, detailed spatial genetic analysis using fine-scale distance classes (25–200 m) found an x-intercept of 255–392 m in Shiraho and that of 258–330 m in Akaishi reef. The latter results agreed well with a previously reported direct field observation of larval dispersal, indicating that the larvae of H. coerulea settled within a 350 m range in Shiraho reef within one generation. Overall, our results empirically demonstrate that the x-intercept of the spatial correlogram agrees well with the larval dispersal distance that is most frequently found in field observations, and they would be useful for deciding effective conservation management units for maintenance and/or recovery within an ecological time scale.

Aerial images can detect 3D small patch reefs that are potential habitats for anemonefish Amphiprion frenatus

Patchy habitats often enhance species coexistence and, consequently, abundance of each species. The present study examined two indicators of potential habitats for anemonefish Amphiprion frenatus: total area of dark-colored patch reefs that are detectable on an aerial image with image analysis software, and total area of tall patch reefs (>1.5 m in height) that are detectable on stereoscopic aerial images with a stereoscope. Relationships between patch reef area and anemonefish abundance, as estimated by number of host anemone Entacmaea quadricolor, were investigated at Shiraho Reef, Ishigaki Island, Japan. Total number of anemones was not correlated with total area of tall patch reefs but was highly correlated with total area of dark-colored patch reefs. Underwater observations confirmed that upper surface of tall patch reefs often involved bare substratum, whereas dark-colored patch reefs did not include this substratum. Because reef area approximately reflected reef volume within shallow back reefs, total area of dark-colored patch reefs that are detectable by aerial photography will reflect total volume of reef fish habitat. The present study suggests that aerial photography with a drone can provide a useful predictor of reef fish abundance.

Incorporating benthic community changes into hydrochemical-based projections of coral reef calcium carbonate production under ocean acidification

The existence of coral reefs is dependent on the production and maintenance of calcium carbonate (CaCO3) framework that is produced through calcification. The net production of CaCO3 will likely decline in the future, from both declining net calcification rates (decreasing calcification and increasing dissolution) and shifts in benthic community composition from calcifying organisms to non-calcifying organisms. Here, we present a framework for hydrochemical studies that allows both declining net calcification rates and changes in benthic community composition to be incorporated into projections of coral reef CaCO3 production. The framework involves upscaling net calcification rates for each benthic community type using mapped proportional cover of the benthic communities. This upscaling process was applied to the reef flats at One Tree and Lady Elliot reefs (Great Barrier Reef) and Shiraho Reef (Okinawa), and compared to existing data. Future CaCO3 budgets were projected for Lady Elliot Reef, predicting a decline of 53 % from the present value by end-century (800 ppm CO2) without any changes to benthic community composition. A further 5.7 % decline in net CaCO3 production is expected for each 10 % decline in calcifier cover, and net dissolution is predicted by end-century if calcifier cover drops below 18 % of the present extent. These results show the combined negative effect of both declining net calcification rates and changing benthic community composition on reefs and the importance of considering both processes for determining future reef CaCO3 production.

Impacts of multiple disturbances on coral communities at Ishigaki Island, Okinawa, Japan, during a 15 year survey

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 509:171-180 (2014) - DOI: https://doi.org/10.3354/meps10890 Impacts of multiple disturbances on coral communities at Ishigaki Island, Okinawa, Japan, during a 15 year survey Saki Harii1,*, Chuki Hongo2, Mitsunori Ishihara3,6,**, Yoichi Ide4, Hajime Kayanne5 1Sesoko Station, Tropical Biosphere Research Center, University of the Ryukyus, 3422 Sesoko, Motobu, Okinawa 905-0227, Japan 2Department of Physics and Earth Sciences, University of the Ryukyus, 1 Senbaru, Nishihara, Okinawa 903-0213, Japan 3National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba, Ibaraki 305-8506, Japan4Oceanic Planning Corporation, 6015-7 Imazu, Fukuoka 819-015, Japan 5Department of Earth and Planetary Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan6Present address: National Institute for Agro-Environmental Sciences, 3-1-3 Kannondai, Tsukuba 305-8604, Japan *Corresponding author: sharii@lab.u-ryukyu.ac.jp**Addresses amended after publication ABSTRACT: Coral reefs have been threatened by various human and natural environmental disturbances, especially by widespread water temperature increases in 1997/1998. To understand the recovery of coral communities and shifts in their species compositions, long-term monitoring at the same location is important. Previously, we reported changes in the dominant taxa in a coral community before and after bleaching in 1998 at Shiraho Reef in the southern Ryukyus, Japan. In the present study, we continued monitoring the site for 15 yr to quantify how the coral community changed temporally and spatially. We used transect surveys and time-series aerial photographs and analysed the data with reference to seawater temperature and typhoon records. Net coral area along the transect lines increased from 1998 to 2003, but then decreased by 2008 mostly due to a decline in branching Montipora spp. The resulting net coral area was lower in 2012 than it was before the 1998 bleaching event. Aerial photographs also showed that the coral area at Shiraho Reef was similar between 1995 and 2000, but declined afterward. This decrease resulted from multiple disturbances, including bleaching events in 1998 and 2007, physical damage by 5 consecutive strong typhoons and likely inputs of sediments from heavy rain. Coral taxa reacted differently to the environmental stresses. The main change observed was a shift in the dominant taxa from branching Montipora and Acropora to Heliopora coerulea and massive and branching Porites. Those species have persisted due to high recruitment rates in H. coerulea and/or their tolerance to disturbances such as high thermal stresses, sedimentation and physical damage by typhoons. KEY WORDS: Coral reef · Disturbances · Long-term changes Full text in pdf format Supplementary material PreviousNextCite this article as: Harii S, Hongo C, Ishihara M, Ide Y, Kayanne H (2014) Impacts of multiple disturbances on coral communities at Ishigaki Island, Okinawa, Japan, during a 15 year survey. Mar Ecol Prog Ser 509:171-180. https://doi.org/10.3354/meps10890 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 509. Online publication date: August 27, 2014 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2014 Inter-Research.

Influence of Land Development on Holocene Porites Coral Calcification at Nagura Bay, Ishigaki Island, Japan

To evaluate the relationships between coral calcification, thermal stress, and sedimentation and eutrophication linked to human impact (hereafter referred to as “land development”) by river discharge, we analyzed growth characteristics in the context of a paleoenvironment that was reconstructed from geochemical signals in modern and fossil (1.2 cal kyr BP and 3.5 cal kyr BP, respectively) massive Porites corals from Nagura Bay (“Nagura”) and from modern Porites corals from the estuary of the Todoroki River, Shiraho Reef (“Todoroki”). Both sites are on Ishigaki Island, Japan, and Nagura is located approximately 12 km west of Todoroki. At Nagura, the individual corals provide time windows of 13 (modern), 10 (1.2 cal kyr BP), and 38 yr in length (3.5 cal kyr BP). Here, we present the coral annual calcification for Nagura and Todoroki, and (bi) monthly resolved records of Sr/Ca (a proxy of sea surface temperature (SST)) and Ba/Ca (a proxy of sedimentation and nutrients related to land development) for Nagura. At Nagura, the winter SST was cooler by 2.8°C in the 1.2 cal kyr BP, and the annual and winter SSTs in the 3.5 cal kyr BP were cooler by 2.6°C and 4.6°C, respectively. The annual periodicity of Ba/Ca in modern coral is linked to river discharge and is associated with land development including sugar cane cultivation. Modern coral calcification also has declined with SST warming and increasing Ba/Ca peaks in winter. However, calcification of fossil corals does not appear to have been influenced by variations in Sr/Ca and Ba/Ca. Modern coral growth characteristics at Nagura and Todoroki indicate that coral growth is both spatially and temporally influenced by river discharge and land development. At Nagura, our findings suggest that land development induces negative thermal sensitivity for calcification in winter due to sugar cane harvest, which is a specifically modern phenomenon.

Large-scale mono-clonal structure in the north peripheral population of blue coral, Heliopora coerulea

We examined the genotypic diversity of the large population of Heliopora coerulea, discovered recently in Ooura Bay, northern Okinawa Main Island Japan, together with another large population in Shiraho Reef, also in southwest Japan, using 9 polymorphic microsatellite markers. From each population, 40 samples were systematically collected along 2 transect lines with 4-m intervals. Surprisingly, all 40 samples from Ooura Bay were mono-genotypic, implying that the huge coral structure (30m×80m) originated from a single larva. Conversely, the 40 samples collected from the Shiraho Reef site all had different genotypes; measurements of genetic diversity, HE and HO, were 0.075–0.975 and 0.064–0.655, respectively. At least four factors are considered to make such a huge H. coerulea population with a single genotype in Ooura Bay, (1) origin of single larva or fragment (2) a genetic bottleneck, (3) post settlement selection, and (4) frequent asexual propagation.

Simultaneous estimation of benthic fractional cover and shallow water bathymetry in coral reef areas from high-resolution satellite images

This article describes the development of a technique to estimate shallow water benthic cover and depth simultaneously from high-resolution satellite images of reef areas, specifically from the high-resolution sensor onboard IKONOS. The technique to derive the estimates of five bottom benthic cover types (sand, coral, seagrass, macroalgae and pavement) and depth from the four-band images uses a coupling of radiative transfer (RT) theory and spectral unmixing implemented in an iterative manner. To resolve the cover types for the unmixing, the method employed a combinatorial approach to select benthic cover composition. The estimation technique was applied to two reef areas around the coast of the Ishigaki in southern Ryukyus, namely, the Fukido River mouth area and the Shiraho Reef. The IKONOS images of Fukido River mouth area and Shiraho Reef were acquired in 2003 and 2002, respectively. The accuracy of the fractional cover and the depth estimates from the satellite images are then presented and compared with sea truth data and depth measurements. The results indicate good correspondence between estimated and measured depths, while the estimates for the benthic cover were at reasonable levels of accuracy.

Estimation of nearshore groundwater discharge and its potential effects on a fringing coral reef

Radon (222Rn) measurements were conducted in Shiraho Reef (Okinawa, Japan) to investigate nearshore submarine groundwater discharge (SGDnearshore) dynamics. Estimated average groundwater flux was 2–3cm/h (maximum 7–8cm/h). End-member radon concentration and gas transfer coefficient were identified as major factors influencing flux estimation accuracy. For the 7-km long reef, SGDnearshore was 0.39–0.58m3/s, less than 30% of Todoroki River’s baseflow discharge. SGDnearshore was spatially and temporally variable, reflecting the strong influence of subsurface geology, tidal pumping, groundwater recharge, and hydraulic gradient. SGDnearshore elevated nearshore nitrate concentrations (0.8–2.2mg/l) to half of Todoroki River’s baseflow NO3-–N (2–4mg/L). This increased nearshore Chl-a from 0.5–2μg/l compared to the typically low Chl-a (<0.1–0.4μg/l) in the moat. Diatoms and cyanobacteria concentrations exhibited an increasing trend. However, the percentage contributions of diatoms and cyanobacteria significantly decreased and increased, respectively. SGD may significantly induce the proliferation of cyanobacteria in nearshore reef areas.

The effect of patch reef size on fish species richness in a shallow coral reef shore zone where territorial herbivores are abundant

AbstractSmall patch reefs can harbor many reef fishes because most fishes have a drifting larval phase to randomly disperse over patchy habitats. We examined the species–area relationship (SAR) of damselfish (Pomacentridae) assemblages over 84 small patch reefs (0.05–45.4 m2) using an enlarged section of a high‐resolution color aerial photograph as a field map (1/2500) in a shallow coral reef shore zone (&lt;2 m deep, 3.6 ha, Shiraho Reef, Ishigaki Island, Japan). This study confirmed that the logarithmic function is better than other functions (including the power function) to explain the SAR in this scale. Actual species richness (24) over the entire study site was much higher than the species richness (15.4) extrapolated from the regression line in semi‐log space. Better estimates were obtained using random placement models and computer simulations. These results suggest that several small patch reefs are likely to have higher species richness than a single large reef of equivalent area at the study site. The total number of individuals of the four most abundant territorial herbivores increased almost linearly with patch reef area, but that of other species roughly increased with the square root of the area. While no territorial species were found in the smallest reefs, the large territorial herbivore, Hemiglyphidodon plagiometopon, was abundant and had negative effects on species richness in large reefs. Although the well‐known single‐large‐or‐several‐small (SLOSS) debate has largely been settled, this dichotomy can be important in places where territorial herbivores do not occupy the smallest reefs.

Estimation of photosynthesis and calcification rates at a fringing reef by accounting for diurnal variations and the zonation of coral reef communities on reef flat and slope: a case study for the Shiraho reef, Ishigaki Island, southwest Japan

Seven coral reef communities were defined on Shiraho fringing reef, Ishigaki Island, Japan. Net photosynthesis and calcification rates were measured by in situ incubations at 10 sites that included six of the defined communities, and which occupied most of the area on the reef flat and slope. Net photosynthesis on the reef flat was positive overall, but the reef flat acts as a source for atmospheric CO2, because the measured calcification/photosynthesis ratio of 2.5 is greater than the critical ratio of 1.67. Net photosynthesis on the reef slope was negative. Almost all excess organic production from the reef flat is expected to be effused to the outer reef and consumed by the communities there. Therefore, the total net organic production of the whole reef system is probably almost zero and the whole reef system also acts as a source for atmospheric CO2. Net calcification rates of the reef slope corals were much lower than those of the branching corals. The accumulation rate of the former was approximately 0.5 m kyr−1 and of the latter was ~0.7–5 m kyr−1. Consequently, reef slope corals could not grow fast enough to keep up with or catch up to rising sea levels during the Holocene. On the other hand, the branching corals grow fast enough to keep up with this rising sea level. Therefore, a transition between early Holocene and present-day reef communities is expected. Branching coral communities would have dominated while reef growth kept pace with sea level rise, and the reef was constructed with a branching coral framework. Then, the outside of this framework was covered and built up by reef slope corals and present-day reefs were constructed.

Incorporating fine‐scale seascape composition in an assessment of habitat quality for the giant sea anemone Stichodactyla gigantea in a coral reef shore zone

AbstractHabitat loss due to land reclamation often occurs in sandy coral reef shore zones. The giant sea anemone Stichodactyla gigantea, which harbors the false clown anemonefish Amphiprion ocellaris, both of which are potentially flagship species, inhabit these places. To assess habitat quality for S. gigantea, we examined correlative associations between the number and the body size of S. gigantea and the amount of habitat types in fine‐scale seascape composition quantified from an enlarged section of a high‐resolution (1/2,500) color aerial photograph of the shallow shore zone of Shiraho Reef, Ishigaki Island, Japan. This study confirmed that anemones were most abundant at the edges of dense seagrass beds characterized by shallow sandy bottoms, rock beds, and sparse seagrass beds, while they were less abundant in coral patch reefs. However, anemones inhabiting coral patch reefs were significantly larger and their rate of disappearance over 3 years was lower than those inhabiting other habitats. This suggests that coral patch reefs may be more suitable habitats supporting larger animals and greater persistence of S. gigantea. The visual census techniques applied here, combined with aerial photography and image‐analysis software, may be useful as a simple analytical tool for local assessment of suitable habitats for relatively small‐bodied marine fauna in shallow‐water seascapes.

Effects of Local Topography and Meteorological Conditions on Spatio-temporal Variations of Thermal Stress Causing Coral Bleaching in a Fringing Reef

Significant coral bleaching was observed in summer in Shiraho reef, a fringing reef in the east coast of Ishigaki Island, Okinawa, Japan. To find relationship between coral bleaching and thermal stress in the reef and its governing factors, we have examined data obtained by field surveys in July 2003 and 2007 at Shiraho reef and the corresponding numerical simulation results for spatio-temporal variations of water temperature. The results of analysis showed that the spatial distribution of the degree of coral bleaching in the reef is closely related with that of the mean water temperature, which is strongly governed not only by local topography but also by meteorological conditions, especially latent heat flux.

A Photogrammetric Correction Procedure for Light Refraction Effects at a Two-Medium Boundary

We report on a correction procedure for light refraction effects at a two-medium boundary, based on the stereo view of underwater objects, to estimate underwater topography using photogrammetry. Because theoretically, no solution exists for photogrammetrically observed positions when the incident angles of light rays from an underwater object of interest to two cameras are different; approximation in solving the positions is needed. We show the feasibility of the approximation theoretically by examining the horizontal differences between the observed and true positions when objects are in line along an airplane track or when the incident angles are identical. We applied the procedure to bathymetric mapping of Shiraho Reef, southwest Japan, using a stereo-pair of aerial photographs. Comparison of the corrected depths with measured depths at 658 points showed a mean error and standard deviation of 20.06 m and 0.36 m, respectively, for measured depth range of 23.4 m to 20.2 m.

Configuration of small patch reefs and population abundance of a resident reef fish in a complex coral reef landscape

AbstractHabitat use by the resident coral reef anemonefish, Amphiprion frenatus, was examined in the complex coral reef landscape of Shiraho Reef, Ishigaki Island, Okinawa, Japan, using an enlarged color aerial photograph processed using image analysis software as an accurate field map. The anemonefish inhabit assemblages of the host sea anemone, Entacmaea quadricolor (clonal type), which inhabit various patch reefs in the back reef moat. We located all patch reefs inhabited by the host in the map based on snorkel observations: 297 anemonefish were found in 93 host assemblages in the study site of 2.9 ha. These patch reefs could be recognized by the reef colors, including the shadows (blackish color) in the photograph. Using image analysis software, the colors of the patch reefs were chosen and pixels with the same color values were regarded as potential habitat patches for the fish (PHPs). PHPs were 3D patch reefs (&gt;0.5 m in height). Total areas (TA) and total perimeters (TP) of PHPs were measured in nine quadrats in the digitized aerial photograph. Host abundance and anemonefish abundance in a quadrat showed stronger correlations with the product of TA and TP of PHPs than TA alone. A site with numerous large 3D patch reefs (≥0.75 m2 in situ) can be a better habitat for the fish than other sites consisting of several huge 3D patch reefs of the same total area. The methodology applied here may be useful for assessing the quality of habitats for small resident animals in shallow subtidal reefs.

Measurement of air–water CO 2 transfer at four coastal sites using a chamber method

We measured the air–water CO 2 flux in four coastal regions (two coral reefs, one estuary, and one coastal brackish lake) using a chamber method, which has the highest spatial resolution of the methods available for measuring coastal air–water gas flux. Some of the measurements were considerably higher than expected from reported wind-dependent relationships. The average k 600 values for Shiraho Reef, Fukido Reef, Fukido River, and Lake Nakaumi were 1.5 ± 0.6, 3.2 ± 0.3, 0.69 ± 0.26, and 2.2 ± 0.9 (mean ± S.D.) times larger than the wind-dependent relationships. Results were compared with current-dependent relationships and vertical turbulence intensity (VTI). VTI is an index of water-surface stirring and is calculated from near-surface vertical velocity. Although some measurements from the reefs and river closely matched those expected from wind-dependent relationships, others were considerably higher. All data were correlated with VTI and were qualitatively explained by bottom macro-roughness enhancement. In Lake Nakaumi, results tended to differ from the wind-dependent relationships, and the difference between the measured and expected gas-transfer velocity was correlated with biological DO changes and/or the intensity of density stratification. We found these factors to have important effects on coastal gas flux. In addition, the chamber method was an effective tool for evaluating coastal gas flux.

HYDRODYNAMIC AND THERMAL ENVIRONMENTS IN AND OUTSIDE THE SHIRAHO CORAL REEF AREAS OF ISHIGAKI ISLAND, SOUTHWEST JAPAN

Study results show the propagation of near bottom cold onshore currents that followed the internal waves in an area where bottom topography is characterized by gentle slopes. The cold water currents finally penetrated into Shiraho reef area and were found to affect the water circulation pattern and thermal environment significantly around Tooru-guchi region where a big reef gap is located. After cold westward current penetration, a significant temperature fall was observed in the reef area. As cold westward current flowed in the bottom layer, warm currents headed offshore through the surface layer. The characteristic currents and thermal environment of the region are discussed in detail.

Larval dispersal, recruitment, and adult distribution of the brooding stony octocoral Heliopora coerulea on Ishigaki Island, southwest Japan

Larval dispersal and recruitment are important factors that determine the distribution of adult corals. The relationships between larval dispersal, recruitment, and the adult distribution of the blue octocoral, Heliopora coerulea, were investigated on Shiraho Reef, Ishigaki Island, southwest Japan. Heliopora coerulea is a surface brooder that releases planulae in June or July on Shiraho Reef. We observed planulae between 1998 and 2000 and found that they did not swim actively; instead, they crawled into their settlement positions after becoming grounded on the substratum. Planulae occurred throughout the water column and were dispersed by tidal and wind-driven currents around the parent population on the reef flat. Recruitment was observed only within 350 m of the parent populations, including areas between the branches of the adult colony. The planulae of H. coerulea had a narrow dispersal range as a result of their mostly benthic, shorter larval duration, and the influence of weaker currents. Thus, the dispersal distance of larvae is determined by their position in the water column, the currents that deliver the larvae, and the competency period of the larvae. The narrow dispersal range of H. coerulea was consistent with recruitment of sexually derived larvae onto their natal reef.

Significance of groundwater nitrogen discharge into coral reefs at Ishigaki Island, southwest of Japan

Groundwater discharge from adjacent terrestrial areas can be a potentially important nutrient source to coastal coral reefs, since adjacent lands are often overlaid with permeable bedrock such as limestone. The quantity of groundwater nitrogen discharged into the Shiraho and Kabira coral reefs from their namesake watersheds on Ishigaki Island southwest of the Ryukyu Islands, Japan (24°19′–37′N, 124°4′–21′E) was monitored. These watersheds were subject to different types of nitrogen loading. The groundwater nitrogen discharge was compared by two independent methods, one based on measuring dissolved inorganic nitrogen (DIN) concentrations in the groundwater near the coastline, the other by estimating nitrogen loading from various land uses within the watershed. For a common watershed, the two methods agreed within a factor of two. The Shiraho reef received 4- or 5.5-fold more nitrogen than the Kabira reef. Groundwater discharge contributes significantly to the reef nitrogen budget, and is potentially a key factor controlling the biomass and succession of aquatic vegetation of the reefs.

Larval Settlement of Corals in Flowing Water using a Racetrack Flume

Although the larvae of marine invertebrates settle in currents, few studies have examined settlement under flow. We studied the settlement of two brooding coral larvae with different behavior in a flow using a small racetrack flume made of polycarbonate and acrylic (58 cm long). Flow was generated using a digitally controlled gear-motor-driven paddle wheel. The settlement rates of the larvae, Heliopora coerulea and Pocillopora damicornis, were observed at three flow speeds: 1.6, 4.4, and 9.8 cm/sec, which correspond to the currents at low, mean, and ebb or flood tides, respectively, at Shiraho Reef flat, Ishigaki Island, Japan. The settlement rate of H. coerulea larvae decreased with increasing flow speed (20% at 1.6 cm/sec, 2% at 9.8 cm/sec). In contrast, the settlement rate of P. damicornis larvae was high at all flow speeds (&gt; 50% at all flow speeds). This difference in the larval settlement rates of the two species is in accordance with the current conditions in which the adult corals are distributed.