The Sea of Japan is a marginal sea surrounded by narrow, shallow straits which became isolated from neighboring waters when the sea level fell during the Pleistocene glacial periods. In order to understand the influences of such paleoenvironmental changes on the formation of populations of marine organisms in the Sea of Japan, deep-sea whelks (genera Buccinum and Neptunea (Buccinidae)) are appropriate materials considering their low dispersal abilities through direct development. Deep-sea whelks are also important marine species for fisheries in the Sea of Japan. However, several taxonomical problems due to morphological variations and lack of knowledge about gene flows among local populations in deep-sea whelks may lead to inappropriate fisheries management. We and my collaborators examined the genetic inter- and intraspecific relationships of deep-sea Buccinum and Neptunea species by means of molecular markers with samples from various stations around Japan (mainly from the Sea of Japan). We found some intriguing genetic structures closely related to their characteristic reproductive mode and paleoenvironmental changes in the Sea of Japan. In addition, by comparing inter- and intraspecies relationships of some Buccinum species, I clarified some novel taxonomical aspects of these species. Here, I review these genetic structures of deep-sea whelks and describe the future direction of phylogeographical approaches to deep-sea organisms in the Sea of Japan.

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The small gastropod, Lacuna decorata Adams, living on macrophytobenthos or surface sediment, is one of the most dominant species of macrozoobenthos in Hichirippu lagoon covered with seagrass and macroalgae, eastern Hokkaido, Japan. We measured the standing stocks of primary producers and macrozoobenthos, and determined the stable carbon and nitrogen isotope ratios of the primary producers and L. decorata. With these results, we identify the main food items for L. decorata and discuss the feeding strategy of the small gastropod. This gastropod occupied about 64% in density and about 25% in biomass of the macrozoobenthos at all six sampling stations in the lagoon. It occurred densely on the surface of the sediment with dense patches of benthic microalgae (BMA), which contained extremely high levels of Chl.-a between 84 to 226 mg m⁻² throughout the period of this study. Nevertheless, the stable isotope signatures of carbon and nitrogen of this gastropod clearly show the direct utilization of organic matter derived from seagrass, Zostera japonica, in the areas where the seagrass luxuriated. However, it shows also a flexible feeding strategy in food preference. It fed green algae such as Ulva pertusa and Urospora wormskioldii in the areas where the seagrass grew scarcely.

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The polyp stage is an important part of the Aurelia aurita s.l. life cycle, because polyp clones are able to increase their population size by budding, providing the opportunity to increase the medusa population by the production of many ephyrae through strobilation. We investigated asexual reproduction of A. aurita polyps on an individual basis at 4 different temperatures (i.e. 18, 22, 26 and 28°C) and at 5 different food levels (i.e. 1.7, 3.3, 6.6, 10 and 13.3 μg C polyp⁻¹ d⁻¹) in the laboratory. Three types of asexual reproduction were observed: polyps directly budded from the parent stalk (DBP), polyps budded from the parent pedal stolon (SBP), and podocysts (PC). DBP was the major reproductive method (94% of the total) and SBP and PC accounted for only 5 and 1%, respectively. PC were produced by the polyps kept under low food supply (≤3.3 μg C polyp⁻¹ d⁻¹) and high temperature (≥26°C). Production of new polyps by DBP and SBP significantly increased with increasing food and temperature. The somatic growth of parent polyps significantly increased with more food and cooler temperatures. We conclude that both increases in water temperature through global warming and increases in abundances of zooplankton prey because of eutrophication may be responsible for the prominent blooms of A. aurita medusae in East Asian coastal waters in recent times.

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Based on sampling of specimens and field observations on jellyfish fisheries in Malaysia, Indonesia, and Myanmar, six trade types of edible jellyfish were identified and the proper scientific names were applied to them with synopses of their taxonomy. An identification key for the Suborder Daktyliophorae, allowing identification of edible species, is provided. Jellyfish fishing activities at various locations in Southeast Asia are reported.

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Ephyrae of the scyphozoan jellyfish, Aurelia, were exposed to hypercapnic seawater (pCO₂ 5,000 to 50,000 μatm) for 96 h, to study the impacts of potential CO₂ seepage from a geological storage site beneath the ocean floor. Geological CO₂ storage has been proposed as a mitigation measure against global warming but ecological consequences in the case of seepage are largely unknown. No mortality occurred within the pCO₂ range used in the present study. Swimming arm pulsation was significantly depressed in animals exposed to 5,000 μatm pCO₂ compared to control animals, and immediately ceased in animals exposed to ≥30,000 μatm. When returned to normocapnic seawater (pCO₂ 380 μatm) after 96 h exposure to 50,000 μatm pCO₂, some ephyrae showed strong arm inversion. These results indicate that even though Aurelia is able to survive short-term exposure to pCO₂ of up to 50,000 μatm, the strong inhibition of swimming activities under these conditions would reduce the environmental fitness of affected animals.

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