Hiromori Shimabukuro

Effects of wave energy on the residence times of a fluorescent tracer in the canopy of the intertidal marine macroalgae, Sargassum fusiforme (Phaeophyceae)

We determined the relationship between the residence times of water within the canopy of the intertidal macroalgae, Sargassum fusiforme (Harvey) Setchell to the energy caused by hydrodynamic mixing. We measured the residence times (t) of fluorescein dye injected into the canopy (31 ± 9 ind/quadrat; canopy plan form area 6 × 1 m2) to estimate the length of time gametes persist within the canopy. The total kinetic energy (TKE) and wave energy (WE) was measured during dye dispersal, which ranged from 0.002 to 0.009 m2/s2 and 0.001 to 0.016 m2/s2, respectively. The experiments revealed that the canopy significantly (P < 0.0001) increased t, which was 56 ± 35 s inside of the canopy compared with 14 ± 4 s outside. Moreover, the relationship between t and energy could be statistically modeled with a power function, and for the results inside of the canopy, t = 3.67 TKE−0.50 for turbulent kinetic energy and t = 1.83 WE−0.38 for wave energy. Outside of the canopy, t = 0.98 TKE−0.50 and t = 1.83 WE−0.38 Based on the values of t determined for within the canopy, we developed a dispersion model to explore how gametes dispersed within the canopy. The estimated dispersion coefficient (D) with respect to WE, could be modeled as D = 403 WE0.55 and ranged from 10 to 42 cm2/s for the WE examined in the study. Areal gamete densities modeled in the canopy increased in density for increasing WE at short (0.5 h) durations of gamete discharge; however, the relationship reversed above 2 h of discharge.

A new record of the Kumamoto oyster Crassostrea sikamea in the Seto Inland Sea, Japan

We collected 138 Kumamoto like oyster specimens in and around the Nakatsu tidal flat to monitor species diversity. The mitochondrial large subunit of ribosomal DNA (LSrDNA) and the nuclear ribosomal internally transcribed spacer-1 (ITS-1) region were amplified by polymerase chain reaction and restriction fragment length polymorphism analysis, leading to the discovery of the Kumamoto oyster Crassostrea sikamea in the Nakatsu tidal flat. A more accurate species identification using the nucleotide sequences for cytochrome c oxidase subunit I and ITS-1 confirmed the presence of the oysters. These results provide the first evidence of C. sikamea in the Seto Inland Sea of Japan.

New record of Sargassum denticarpum Ajisaka (Fucales, Phaeophyceae) from Iriomote Island (Ryukyu Archipelago, Japan)

Abstract The brown macroalgal genus Sargassum forms beds around the Japanese coast; these beds are important for marine ecosystem functioning and fisheries. In recent years, a northward shift in the distribution patterns of southern Sargassum spp. has been postulated as a result of warming in the western Pacific. To examine this theory, we surveyed species richness and diversity of genus Sargassum algae around the Ryukyu Islands, Japan, which is the northern limit of selected Sargassum species native to tropical and subtropical regions. We collected from Iriomote Island (Ryukyu Archipelago) specimens of a species resembling Sargassum denticarpum, which is endemic to Vietnam. The specimens had the following morphological characteristics in common with S. denticarpum: (1) terete or slightly compressed primary branches, (2) simple lanceolate primary and lateral leaves with dentate margins in all stages, (3) spherical-to-elliptical vesicles with or without spines and wings, (4) monoecious, and (5) compressed receptacles with spines on the margins. In addition, internal transcribed spacer 2 (ITS2) sequences of the Ryukyu specimens and of S. denticarpum collected in Vietnam clearly showed that the two are closely related. On the basis of these morphological and molecular characteristics, we present the first report of S. denticarpum in Japan.

Taxonomic study of two Sargassum species (Fucales, Phaeophyceae) from the Ryukyu Islands, southern Japan: Sargassum ryukyuense sp. nov. and Sargassum pinnatifidum Harvey

Abstract Species of the genus Sargassum (Fucales, Phaeophyceae) belonging to the Schizophycus group (in subgenus Sargassum sensu lato) from the Ryukyu Islands, southern Japan, were morphologically reinvestigated, and two entities were recognized: Sargassum ryukyuense sp. nov. and Sargassum pinnatifidum. The new species is characterized by: 1) compressed primary branches; 2) simple lanceolate primary and lateral leaves with an undulate and dentate margin in all stages; 3) lateral branches issuing alternate-distichously on the main branches in one plane; 4) long and flattened receptacles arranged alternate-distichously on terminal branches; 5) main and lateral branches on the stem issuing as foliar expansions in the early stage of development; and 6) vesicles with a simple coronal leaf. This new species resembles S. pinnatifidum in features 1), 3) and 5). However, it differs from S. pinnatifidum in features 2), 4) and 6). Furthermore, this new species can be distinguished from all previously described species based on the complete set of diagnostic features above and by the presence of long lingulate receptacles with short stalks. The new species is known to be endemic in the Ryukyu Islands and adjacent waters of southern Japan.

Seasonal population dynamics of Sargassum fusiforme (Fucales, Phaeophyta), Suo-Oshima Is., Seto Inland Sea, Japan—development processes of a stand characterized by high density and productivity

Seasonal population dynamics of Sargassum fusiforme, one of the most important edible macroalgae in Japan, were studied. Recruits were mainly generated by vegetative reproduction at the margins of filamentous holdfasts. They first appeared in late spring and peaked in summer as upright thalli of the previous generation withered. After producing recruits, holdfasts withered indicating that holdfasts were also annual, the same as upright thalli. All recruits produced main branches and became new upright thalli in early autumn. During this transitional period, the thallus density decreased due to the crowded conditions induced by simultaneous growth initiation. After this early mortality, however, thallus density remained almost constant over much of the growth season. Thallus growth continued during winter and the stand biomass peaked in spring. During this biomass accumulation, development of a thallus size hierarchy was moderate and no size-dependent mortality was observed. Main branch number per thallus was also constant until spring, indicating the main branches also persisted after being produced in early autumn. This lack of severe intraspecific competition both at a thallus and main branch level is supported by the ambient wave condition of the habitat which gives moderate undulation and enables light and nutrients to be supplied to each thallus, and allowed the S. fusiforme stand to maintain its densely-packed feature with a high productivity.

Occurrences of the Indo-West Pacific rock oyster Saccostrea cucullata in mainland Japan

We surveyed 110 Saccostrea oysters sampled from southern Kagoshima Bay and Wakayama Prefecture to monitor the species diversity of marine benthos using morphological features and DNA barcoding. Prior to the survey, we re-identified 55 Saccostrea oysters sampled from the Yaeyama Islands, a hot spot for Saccostrea species in Japan. Analysis of partial nucleotide sequences from the mitochondrial large subunit of ribosomal DNA (LSrDNA) led to the discovery of the Indo-West Pacific rock oyster S. cucullata -F in Kagoshima Bay, and S. cucullata -C and S. cucullata -F in Wakayama Prefecture. This is the first record of the Indo-West Pacific rock oyster in mainland Japan (Hokkaido, Honshu, Shikoku and Kyushu Islands). We postulate that there may have been a northward shift in the distribution patterns of the species due to global warming, these Pacific rock oysters are relic species from the Middle or Late Pleistocene, or the non-indigenous species were introduced by oil tanker or other industrial activities.

DNA barcoding of flat oyster species reveals the presence of Ostrea stentina Payraudeau, 1826 (Bivalvia: Ostreidae) in Japan

BackgroundDNA barcoding is an effective method of accurately identifying morphologically similar oyster species. However, for some of Japan’s Ostrea species there are no molecular data in the international DNA databases.MethodsWe sequenced the mitochondrial large subunit ribosomal DNA (LSrDNA) and cytochrome c oxidase subunit I (COI) gene of five known and two unidentified Ostrea species. Phylogenetic comparison with known Ostrea species permitted accurate species identification by DNA barcoding.ResultsThe molecular data, which were deposited in an international DNA database, allowed for a clear distinction among native Ostrea species in Japan. Moreover, the nucleotide sequence data confirmed that O. stentina (Atsuhime-gaki) inhabits Kemi and Ibusuki, Japan.ConclusionsThis is the first record of O. stentina in Japan. These results provided for accurate species identification by DNA barcoding of the taxonomically problematic species O. futamiensis, O. fluctigera, O. setoensis and O. stentina in Japan.

Quantifying the Fate of Captured Carbon: From Seagrass Meadows to the Deep Sea

To help evaluate the sequestration and carbon dioxide storage function of seagrass meadows, we describe the processes by which carbon is sequestered in eelgrass beds and transported from shallow coastal waters to the deep sea. A part of the carbon taken up by eelgrass is decomposed and returned to biological production or the water column’s dissolved inorganic carbon pool, some is accumulated and stored in the shallow sea bottom, and the rest flows out into the deep sea. Here, we describe the growth of eelgrasses and the processes of decomposition, sedimentation, and transportation of eelgrass-derived organic carbon using the Seto Inland Sea as a model site. We estimated the amount of carbon sequestered and stored in eelgrass beds, the fate of eelgrass-derived organic carbon, and the amounts accumulated in the shallow coastal water and transported to the deep sea. According to our estimates based on calculations from tracking carbon over a 1-year period, of the 73,000 tons of carbon sequestered by eelgrass annually in the Seto Inland Sea, 40.9% is accumulated in the Seto Inland Sea and 8.3% flows out to the deep sea. In other words, the eelgrass beds in the Seto Inland Sea have an annual potential capacity of 36,000 tons of carbon storage. In addition, most of the organic carbon was accumulated in the shallow coastal waters rather than in the deep sea.

Carbon Sequestration by Seagrass and Macroalgae in Japan: Estimates and Future Needs

In this chapter, we estimated carbon sequestration by seagrass and macroalgal beds, defined as the integration of annual plant tissue (organic carbon) production, in Japan. Each of the main four beds, eelgrass beds (Amamo-ba), Sargassum beds (Garamo-ba), warm-temperate kelp beds (Arame-ba), and cold-temperate kelp beds (Kombu-ba), exhibited a distinctive geographic distributional pattern along Japan’s coasts that depended on regional climate and topographic characteristics. The total area of the four beds was approximately 230,000 ha nationwide, based on an analysis of the latest satellite images and information on past distributions of the beds. Carbon sequestration of each seagrass or macroalgal bed was evaluated as integrated annual plant tissue production converted to organic carbon, which was defined by subtracting dissolved organic matter production from net primary production. Plant tissue production of the main constituent macrophyte of the seagrass and macroalgal beds was directly measured in each coastal region, and production values from past reports were also collected and utilized. Annual carbon sequestration by seagrass and macroalgal beds in Japan, expressed in a CO2-converted base, was about 4.7 million tons per year, which is comparable to the CO2 emissions of the industrial sectors of agriculture and fisheries.

Production of Zostera marina with different shoot size and stand structures in the Seto Inland Sea, Japan : production in the luxuriant season in 2012

2012年６月に瀬戸内海の３つのアマモ場の，それぞれ異なる株サイズと群落構造を示す計7地点のアマモ群落において生産量を測定，比較した。株あたりの生産量は株サイズに依存し，大きい株が相対的に低い密度で生育する生野島（広島県；安芸灘）のアマモ場で50.0-73.2 mg DW shoot -1 d-1であり，小さい株が密生する平郡島（山口県；伊予灘）の株あたり生産量（7.7-27.4 mg DW shoot -1 d-1）より大きかった。面積あたり生産量において，株あたり生産量の差は株密度により相殺される傾向もみられたが，生野島のアマモ場の生産量（2.89-5.38 g DW m-2 d-1）の方が平郡島のアマモ場の生産量(1.63-2.56 g DW m-2 d-1) よりも大きかった。これら２つのアマモ場では底質に大きな相違がみられ，アマモの群落構造や生産量に影響を与えていると考えられた。すなわち，生野島の底質はほとんど泥により構成され有機物含量も高い一方で，平郡島の底質は中砂・細砂を中心に構成されより厳しい波浪環境を反映していた。 2011年秋季のアイゴの食害による消失から回復途上にある阿波島（広島県；安芸灘）のアマモ場では新たに発芽した実生由来の株がパッチ状の群落を作り，株密度も現存量も低かった。調査した群落の中で生産量は最も低かったが(0.60 g DW m-2 d-1)，その現存量回転率（6.5％ d-1）は，他群落（1.7-3.3％ d-1）のそれよりも大きかった。