In this study, I compared faunal change of bivalves and gastropods after the construction of the reclamation dike in case studies in Japan (Isahaya Bay) and South Korea (Saemangeum). In April 1997, the inner part of Isahaya Bay was isolated from the rest of Ariake Bay by a reclamation dike. In the inner part of this bay, 15 species of marine bivalves and gastropods were collected in large numbers before the dike was completed. These species were still alive in May 1997, but most of them had died by August 1997. However, a brackish bivalve species, Potamocorbula sp. that was not found prior to isolation of this bay, replaced the pre-isolation bivalve community from August 1997. In the outer part of Isahaya Bay, hypoxic water masses appeared around the mouth of the bay in June 1997, and most of the bivalve species distributed near the hypoxic regions decreased rapidly from 1997 to 1999. Thereafter, only a few bivalve species such as Modiolus (M.) comptus increased rapidly in abundance from June 2002. These faunal changes were strongly influenced by environmental changes, such as occurrence of hypoxic water and decrease of grain size of bottom sediments, and those are caused by isolation of Isahaya Bay. In Saemangeum area, Potamocorbula sp. and a few other species of bivalves and gastropods also increased temporarily after the dike construction. These results suggest that drastic changes of bivalves and gastropods after isolation are very similar in the Yellow Sea and Ariake Bay.

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The genetic relationship between the toxic marine dinoflagellates Alexandrium tamarense and Alexandrium catenella, which share similar morphological characteristics and are clustered as a sister group on the phylogenetic tree based on 28S rDNA analysis was examined by DNA–DNA hybridization. This method has contributed to the information on relative comparisons of total sequences in the field of microbiology; however limited DNA–DNA hybridization studies have been carried out on microalgae, including dinoflagellates. The intragroup homology of DNA–DNA hybridization within the A. tamarense and A. catenella groups, that was identified on the basis of morphotypes, was 85.0%–99.8% in both cases, whereas the intergroup homology between these two groups was 62.5%–70.3%, which was lower than the intragroup homology. These percentages revealed that not only the rDNA sequences but also the total sequence reflects slight morphological differences between A. tamarense and A. catenella, indicating the possibility that strains identified as these two species could be differentiated as distinct groups based on the percentage obtained using DNA–DNA hybridization. The homology of A. tamarense with other Alexandrium species was low—37.1%–45.2% with A. tamiyavanichii, 41.0%–45.5% with A. affine, and 24.8%–35.8% with A. ostenfeldii. The relatedness between A. catenella and other Alexandrium strains tested (e.g. between A. catenella and A. tamiyavanichii was 39.0%–51.2%) was also lower than that between A. tamarense and A. catenella. Further, these percentages were consistent with the rDNA phylogenetic analysis, indicating that DNA–DNA hybridization might be a useful tool to understand the relatedness among Alexandrium species.

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Sediment samples collected at 33 stations in Tokyo Bay during the autumn of 1999 were examined to determine the distribution and abundance of resting cysts of Alexandrium tamarense and/or A. catenella. Extremely low concentrations of cysts were observed in Tokyo Bay compared with previously investigated sites in the Seto Inland Sea and Lake Hamana which have almost the same marine environmental conditions as found in Tokyo Bay. Cysts were only found in the upper 2.0–3.0 cm of sediment at three stations along the northwestern coast of the bay, at concentrations ranging from 0.8–2.1 cysts cm⁻³ of wet sediment. All of the observed cysts consisted of the outer thecal integument with no vegetative contents and are therefore concluded to be unable to play a role in the seeding of blooms. Thus, currently the possibility of paralytic shellfish poisoning, caused by a bloom of A. tamarense and/or A. catenella initiated from benthic cysts, is considered to be low in Tokyo Bay. However, commercial shellfish fisheries and recreational shellfish gathering are expected to remain important activities in this bay. Careful monitoring of toxic dinoflagellates and shellfish toxicity is necessary, because invasion by toxic dinoflagellates into Tokyo Bay via ballast water or oceanic water are potential events in the future.

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There are many sand banks in Seto Inland Sea, making patchy shallow zones less than 10 m deep. Due to the shallow environment, the surface sediment on the sand banks in the Hojo area, in the southern part of Aki Nada, Shikoku Island, Japan, often has a larger amount of benthic microalgae than other areas. We hypothesized that benthic microalgae contributed to the secondary production of coastal waters around sand bank areas, and investigated the food-web structure of the Hojo area using stable isotopes in early summer, mid summer and fall. Mean carbon isotopic signatures of several consumers in early summer (−16.9 to −15.1‰ for polychaeta; −17.3 to 13.9‰ for brachyuran crabs; −17.2 to −15.3 for fish) and fall (−16.2 to −14.3‰ for shrimps; −14.0 to −11.2‰ for brachyuran crabs) were more enriched than that of the particulate organic materials of the surface water around sand banks (mainly phytoplankton) (−20.5±0.0‰ in early summer and −18.3±0.1‰ in fall). Organic materials attached to cobbles, representative of benthic microalgae, showed similar or more enriched signatures than consumers (−14.7±0.0‰ in early summer and −10.3±0.0‰ in fall). These results suggest that benthic microalgae on the sand bank contribute greatly to the secondary or higher production of coastal waters in Hojo area.

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