The Seto Inland Sea is the largest enclosed coastal sea in Japan and is also a major fishing ground including aquacultures of fish, bivalves and seaweeds. The incidents of red tides dramatically increased in frequency and scale in the Seto Inland Sea along with serious eutrophication in the 1960s and 1970s. The maximum incident of 299 was recorded in 1976, but the incident has since shown a clear decreasing trend, reaching about 100 per year in the late 1980s by virtue of regulation by law, and this level has been maintained thereafter with the level of nutrients supporting red tide occurrences. The “Law Concerning Special Measures for Conservation of the Environment of the Seto Inland Sea” was legislated in 1973 and industrial loading was decreased to half the level of 1972. The important red tide organisms causing huge fishery damages by fish-kill are Chattonella antiqua, C. marina, C. ovata and Heterosigma akashiwo (Raphidophyceae), and Karenia mikimotoi and Cochlodinium polykrikoides (Dinophyceae). The maximum fishery damage (death of 14.2 million yellowtails) was 7.1 billion yen (about US $60 million) caused by C. antiqua in Harima-Nada in 1972. In 1988, a novel red-tide dinoflagellate species Heterocapsa circularisquama appeared for the first time, and has repeatedly killed both natural and aquacultured bivalves, with the highest damage of 3.9 billion yen to cultured oysters in Hiroshima Bay in 1998. Among the important red-tide organisms, C. antiqua, H. circularisquama and C. polykrikoides are rated as extremely harmful species that can easily reach the warning level of fishery damage by consuming only small amounts of nutrients. In toxic blooms, the dinoflagellate Alexandrium tamarense has become dominant in the Seto Inland Sea in the spring season, causing toxicity in short-necked clams and cultured oysters almost every year. Many countermeasures have been applied for harmful algal blooms in Japan. Laws for the regulation of water quality have been most effective in decreasing red-tide occurrences. No physical and chemical controls have been successful except for clay treatments. Clay spraying has been investigated and implemented in Kyushu and Korea for the removal of C. polykrikoides red tides. As environment-friendly mitigation strategies for red tides, biological controls using algicidal bacteria and viruses are proposed. A new finding of the abundant existence of algicidal bacteria on the surface of seaweeds suggests that co-culturing fish and seaweed is a prevention strategy for harmful algal blooms by virtue of the continuous release of many algicidal bacteria to the surrounding seawater. The artificial development of seaweed beds would also be effective as a prevention strategy for red tides.

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Lipid content and composition of some dominant copepods inhabiting the Oyashio waters were analyzed by the thin layer chromatography flame ionization detection (TLC/FID) method. Copepod samples were collected during the RV Shoyo-maru cruise from September 18 to October 15 in 1988. A new three-step development: the first in hexane and diethyl ether (66 : 4, v/v), the second in hexane, diethyl ether and formic-acid (55 : 15 : 0.5) and the last in hexane and benzene (1 : 1), single scan method by TLC/FID was found to be useful for separation and analysis of lipids in boreal copepods. As a result, the eight species of calanoid copepods analyzed in this study could be divided into two groups according to their lipid class proportions. One group consisted of copepods containing many wax esters and includes Neocalanus flemingeri, N. cristatus, Pseudocalanus minutus, Metridia pacifica and M. okhotensis. The other group containing many triglycerides includes Eucalanus bungii, Pleuromamma abdominalis and Acartia omorii. I have deduced that copepods storing wax esters could have long periods of dormancy in winter and consume lipids mainly for reproduction, while copepods storing triglycerides could overwinter without a period of dormancy and be opportunistic feeders.

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Analyses of monthly depth-stratified (0–2000 m) sampling data in the Oyashio region have revealed that Aglantha digitale is distributed largely in the upper 250 m throughout the year. Population biomass ranged from <30 mg DW m⁻² (January) to 1575 mg DW m⁻² (August), with an annual mean of 186 mg DW m⁻². Judging from seasonal sequences of the population structure, the generation length of A. digitale is considered to be one year with their spawning season in June-September, which is consistent with that observed at Station P in the eastern subarctic Pacific.

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Photosynthesis and respiration of a heterotrophic dinoflagellate Noctiluca scintillans that contained Pedinomonas noctilucae as an endosymbiont, were examined on cultures and natural populations in Manila Bay, Philippines, using a Clark-type oxygen electrode. The cultures isolated from the inner Gulf of Thailand were of two types: one required external supply of Dunaliella tertiolecta as food (feeding strains) and the other did not (non-feeding strains). The non-feeding strains grew photoautotrophically for generations, but they also fed on D. tertiolecta, indicating phagotrophy was facultative. Gross photosynthesis was at the same level in both types, but net photosynthesis was significantly higher in the non-feeding strains than the feeding ones. The difference was due to high respiration activity in the feeding strains. This was consistent with observations in the natural population of Manila Bay, where net photosynthesis was significantly higher in cells lacking food vacuoles than those with food vacuoles. The relationship of photosynthesis with irradiance was characterized by low intensity of light saturation and absence or weak photoinhibition, showing efficient utilization of a wide range of light intensities. P. noctilucae likely assures a supply of organic matter to the host, and facilitates survival of N. scintillans during shortages of food particles.

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The symbiotic poecilostomatoid copepod Anthessius graciliunguis was redescribed from plankton samples collected in Ariake Bay, Japan. This species was created based on a single female in a blue mussel from Japan; the present study is the second record of the species and the first record of the male. Specimens described as A. graciliunguis from scallops in Korea belong to a different species because of differences in the caudal setae and ornamentation of the male maxilliped. Copepods were not rare in plankton (9 copepods in 200-liter water) as compared with limited previous information of Anthessius species from plankton samples and they occurred in the same season of the two years studied. This suggests that the free-swimming animals are not straying ones but result from usual behavior. The possible advantages of the behavior are releasing nauplii from egg sacs and changing the host.

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We investigated the temporal changes of the environmental conditions of the sea floor and the abundance of marine nematode community in the sediment in the subtidal areas near a river mouth with tidal flats in Takamatsu, which faces Seto Inland Sea, Japan. We collected particulate organic matter sinking on the sea floor with a sediment trap and sediment samples with a core sampler bihourly from 11:00 on July 18 to 9:00 on July 19, 2001, and assessed the temporal changes of particulate organic carbon (POC) fluxes in the water column and the chemical characteristics of sediment. The Chl-a concentration and organic matter content of the sediment tended to increase in the surface layer of the sediment at both of the low tides during daytime and early morning, since the deposition of organic particulate containing Chl-a on the sea floor was concentrated at low tide. The organic particulates were produced and transported from the shore and tidal flats. The temporal increase of the density of the nematodes in the sediment up to 5 cm in depth just after the increase of the POC flux in the water column indicates the numerical response of the nematodes to utilize the organic matter deposited on the sea floor. It is likely that the nematodes rose to the surface layer of the sediment to feed the organic particulates and bacteria temporarily increased with them just after the intensive deposition of organic particulates at low tide.

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We applied the seawater soft-agar microcosm approach in order to reveal the effects of sodium sulfide on burrowing activity of Capitella sp. I and on bacterial respiratory activity on mucus-lined burrow. The presence of sodium sulfide up to 7 mM in the soft-agar was not detrimental to the burrowing activity and the survival of the worms, but seemed to enhance the secretion of mucus from the worms. Addition of INT into overlying water (0.01% v/v) of the soft-agar microcosms containing the same concentrations of sodium sulfide caused intensive deposition of INT-formazan in the water, on the mucus over soft-agar surface and on the burrow lining. Both image analyses and spectrophotometric determination of the red-color INT-formazan showed a marked increase in the presence of 3.5 mM sulfide. From these results, we concluded that sulfide can promote burrowing activity of Capitella sp. I and thus enhance the bacterial respiratory activity on the burrow lining. These findings will help clarify the possible interaction between Capitella worms and sulfur-oxidizing bacteria in organically-enriched, sulfide rich sediment in natural environments.

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