Kenji Okoshi

Development of novel microsatellite DNA markers from the Pacific oyster Crassostrea gigas.

We document the potential of novel microsatellites as a genetic tool in furthering our understanding of the Crassostreagigas genetic structure. From the microsatellite-enriched libraries we constructed, 123 repeat regions that had sufficient sequence information to design polymerase chain reaction primer sets were isolated. From these, 9 primer pairs were screened in a C. gigas population of 67 individuals to evaluate the genetic variability. All but 1 of the 9 loci showed allelic variation (number of alleles, 2–20; observed heterozygosity, 0.119–0.925; unbiased expected heterozygosity, 0.139–0.914). Considerable discrepancy of genotypic proportions from the Hardy-Weinberg equilibrium was observed at 1 locus with an apparent heterozygote deficiency. Several loci were successfully amplified in 3 other related species with the appropriate allele size: 6 loci in C. sikamea, 4 loci in C. ariakensis, and 5 loci in C. nippona.

Polydorid species (Polychaeta: Spionidae) in south-western Australian waters with special reference to Polydora uncinata and Boccardia knoxi

Eight species of polydorid polychaetes were found to inhabit mollusc shells from south-western Australian waters. Numerous individuals of Polydora uncinata were extracted for the first time from the shells of both land-based cultured abalone Haliotis laevigata and H. roei, as well as from natural subtidal H. roei and Chlamys australis. Shells of the oyster Saccostrea commercialis cultured in sea-based systems were infested by Boccardia knoxi which was first recorded in these waters. Polydora aura, Dipolydora giardi, D. armata, D. aciculata and Boccardia proboscidea were common among shells of various natural intertidal and subtidal molluscs. A small number of P. haswelli were extracted from their self-excavated burrows in shells of cultured oysters. Boccardia knoxi and D. aciculata were redescribed based on the newly collected materials. Polydora uncinata and B. knoxi exhibited similar larval development patterns (exolecithotrophy and adelphophagy), iteroparity and longer life span, suggesting a high reproductive potential. This study suggests that further monitoring of polydorid species is needed not only from the viewpoint of marine biology but also to survey the risk invasive species pose to commercially important molluscs in this region and worldwide.

Characteristics of shell microstructure and growth analysis of the Antarctic bivalve Laternula elliptica from Lützow-Holm Bay, Antarctica

Growth performance of the Antarctic bivalve Laternula elliptica was examined both by shell microstructural observation and by applying a fluorescent substance, tetracycline, as a shell growth marker. The shell was composed of two calcareous layers: the thick outer layer was homogeneous or granular in structure and the thin inner layer was nacreous. The architecture of Antarctic L. elliptica was different from that of temperate L. marilina, and the ratio of thickness between the outer and inner layers appeared to be different. The growth rate of the nacreous layer was analyzed to be very low. High correlations were found between the major axis of chondrophore and both shell length and shell dry weight, respectively. It is suggested that the chondrophore is an appropriate growth indicator, and combining the information of growth increments with the fluorescent method may be useful in estimating the bivalve growth performance in the Antarctic sea.

Polycyclic Aromatic Hydrocarbons in Sediments and Bivalves on the Pacific Coast of Japan: Influence of Tsunami and Fire

Surface sediments and at least one edible bivalve species (Ruditapes philippinarum, Mytilus galloprovincialis, and Crassostrea gigas) were collected from each of seven intertidal sites in Japan in 2013. The sites had experienced varying levels of tsunami and fire disturbance following the major earthquake of 2011. Eight polycyclic aromatic hydrocarbons (PAHs) were identified and analyzed by gas chromatography–mass spectrometry. Total sediment PAH concentration (CT), the sum of the average concentrations of the eight PAHs, was 21–1447 μg kg-1-dry. Relative to the average level of one type of PAH in sediments collected around Japan in 2002 (benzo[a]pyrene = 21 μg kg-1-dry), five of the seven sites showed concentrations significantly lower than this average in 2013. The CTs for the three bivalves (134–450 μg kg-1-dry) were within the range of the previous reports (2.2–5335 μg kg-1-dry). The data suggest that the natural disaster did not increase PAH concentrations or affect the distribution within sediment or bivalves in Tohoku district. Although PAH concentrations at the sites pose no risk to human health, the findings highlight that the observed PAH levels derive from pre- rather than post-quake processes.

Genetic Structures of Laguncula pulchella Metapopulations Along the Northeast Coast of Japan After the Tsunamis Caused by the Great East Japan Earthquake

The carnivorous snail (Laguncula pulchella) was once considered an endangered species in Japan because its habitat was limited to western Japan. However, alien L. pulchella populations were transported from China and the Korean Peninsula to the Pacific coast of northeast Japan, presumably via anthropogenic activities. This study examined the genetic structures of the invasive L. pulchella populations in various coastal estuaries of Sendai Bay and Sanriku Ria areas to determine how the metapopulations were affected by the tsunamis caused by the Great East Japan Earthquake. The genetic compositions of these carnivorous snails significantly varied between the Sendai Bay and Sanriku areas. The result suggests that local L. pulchella populations were relatively isolated, and their genetic structure was minimally affected by the tsunamis. In addition, their genetic compositions were locally fixed. These results suggest that difference in the genetic composition among L. pulchella populations reflects that of artificial transport source.

Swimming behavior of the spoon worm Urechis unicinctus (Annelida, Echiura).

Large numbers of swimming and stranding Urechis unicinctus were observed at night during low tide in Sasuhama, Miyagi Prefecture, northeastern Japan, during the periods from January to February in 2012 and 2013. Worms did not drift passively but swam actively, therefore hinting at a certain purpose for such behavior. As trochophore larvae of U. unicinctus were observed to occur simultaneously in the plankton, we infer the possibility that this is an event of reproductive swarming. Anatomical observations of both swimming and stranding U. unicinctus showed that none of the specimens had gametes, which may suggest that these were completely spent after spawning. Urechis unicinctus seemed to begin swimming after dusk and the observed swimming behavior occurred during the evening ebb tide throughout the night low tide during winter time. Stranding U. unicinctus have long been known in Japan and have been attributed to sea storms. The present study shows for the first time the possibility that U. unicinctus swims in order to reproduce at night and that this swimming behavior is closely linked to the stranding of U. unicinctus individuals.

Impact of Repeating Massive Earthquakes on Intertidal Mollusk Community in Japan

History repeats itself. In these 3,500 years, massive earthquake like the Pacific coast of Tohoku earthquake in 2011 has occurred at least seven times. Major earthquakes with big tsunami have occurred in 500-year cycles along the Pacific coast of northern part of Japan. The tsunami caused a lot of damage to the Tohoku region including marine life. During the earthquake, wavelike movement observed on the surface of the tidal flats. Liquefaction occurred just after the earthquake before tsunami. Bivalves distributed inside the sediment were dug out to surface with a jet of water. Not only the tsunami but also the liquefaction brought quick damage in the coastal marine life. The earthquake caused sudden land subsidence of about 1 m in maximum where intertidal zone became subtidal zone. It may take at least several 10 years to recover the coseismic land subsidence. We have to clarify not only short-term effect which may explain an initial recovery of intertidal and subtidal animals but also a long-term effect which may explain continuous changes in mollusk population with land subsidence. Construction of tsunami seawalls has begun without environmental assessment. Huge seawalls break the continuity of nature to land from the sea. We have to clarify the impact on the coastal ecosystem of artificial structures.

Harmful Shell Borers, Polydora Species (Polychaeta: Spionidae), from Commercially Important Mollusk Shells in East Asia and Australia

Shell-boring polydorids (Polychaeta: Spionidae) are economically and ecologically important species that must be monitored owing to the risk they pose to commercially important mollusk shells. Tracking polydorid species internationally requires accurate species identification, which is based on both morphological characteristics and nuclear 18S rRNA gene sequences. Four serious shell-boring Polydora in East Asian and Australian aquaculture, i.e., Polydora brevipalpa, P. uncinata, P. haswelli, and P. aura, are described here. The shell-associated polydorids that have been reported from these areas were summarized and reviewed at the same time.

The “tsunami break:” impact of the Great East Japan Earthquake and the accompanying tsunamis on the shell growth of the invasive clam-eating snail Laguncula pulchella

The Great East Japan Earthquake and accompanying tsunamis occurred on 11 March 2011, causing huge damage to marine organisms. The invasive naticid gastropod Laguncula pulchella (Euspira fortunei), which was introduced with the imported clam Ruditapes philippinarum from China and Korea, survived the earthquake. The “growth break line” observed on the shell surface in over 90% of the individuals collected in Matsushima Bay and Matsukawa-ura Lagoon after the tsunamis was examined by scanning electron microscopy. Each shell presented three layers before and three after the growth break. However, the fracture surface consisted of five layers—two prismatic and three crossed lamellar—around the growth break line. This suggests that shell formation temporarily ceased following the tsunamis and that the five-layered shell may have developed in response to the stress caused by the tsunamis. The newly formed middle layer became thinner after the “tsunami break,” which may be the result of a rapid change in the mineralization process, including rapid shell growth and/or repair. These results suggest that the damage to and forcible removal of habitats by the tsunamis was stressful for L. pulchella. A decrease in or the cessation of shell formation after a tsunami may be a common phenomenon in mollusks that inhabit tidal flats.

The Effects of Liquefaction, Tsunami, and Land Subsidence on Intertidal Mollusks Following the Great East Japan Earthquake

The Great East Japan Earthquake that occurred on 11 March 2011 caused a great deal of damage to the organisms that inhabit the coast of the Tohoku district of Japan. In this paper, I mainly focus on the earthquake’s impact on bivalves that inhabit the sandy intertidal zone, with specific emphasis on the short-term effects from liquefaction and the tsunamis and the long-term effects of land subsidence. As previous studies have not discussed the details of the separate physical events of this type of natural disaster, this study divided it into three main events: the initial earthquake including liquefaction, subsequent tsunamis, and land subsidence. During the liquefaction period, bivalves were brought to the surface with jets of water and were then swept away by the tsunamis. As such, both the magnitude of the tsunamis and length of the liquefaction period were factors critical to the survival of clams. The tsunamis have also contributed to dispersion of benthic organisms. For example, the redistribution of oysters across the mud bottom increased their range. In addition to these events, land subsidence resulted in marked changes to the vertical position of the intertidal zone. These large changes in the physical environment cannot recover in the short term, and the readjustment of tidal communities is expected to take several decades. However, land subsidence has also formed new intertidal zones, which will ultimately promote the colonization of various organisms. At some sites where the silt content of the bottom sediments increased due to land subsidence, the dominant species have changed from those that prefer a sandy bottom to those that prefer a muddy bottom. These disturbances to the ecosystem have destabilized these bivalve species resulting in fluctuations in abundance and community structure for at least 3 years following the earthquake. Finally, continual monitoring studies are needed to understand the ecological impacts resulting from the earthquake, and the artificial seawall and tidal flat that have been constructed since the quake, on benthic organisms inhabiting the coastline of the Tohoku district.