Sampling of megabenthos was conducted between 1997 and 1998 in Ise Bay, central Japan to examine the food habits of three dominant species of crabs: Charybdis bimaculata, Paradorippe granulata, Cancer gibbosulus. The degree of gut fullness as well as the frequencies of occurrence and the relative volumes of food items were investigated. In the summer when the oxygen-poor water conditions developed, frequencies of occurrence of food items were low and the degree of the guts fullness in most of crabs collected was <50%; in contrast, these values were high from the winter to spring of the following year, when the oxygen-poor water conditions did not develop. A comparison between frequencies of occurrence and relative volumes of food items in the gut showed that crustaceans, polychaetes, gastropods and bivalves were important foods for the crabs. The main components of the gut contents were different between the three species, though this is probably due to differences in spatial distribution patterns of crabs and their prey items.

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The feeding habits and growth of the marine isopod Idotea ochotensis collected from Akkeshi Bay on the Pacific coast of Hokkaido, northern Japan, were studied. Individuals of I. ochotensis were maintained in the laboratory and provided with different kinds of plant material as food. The plants used were two species of seagrass (Phyllospadix iwatensis and Zostera asiatica) and two species of seaweed (Fucus distichus subsp. evanescens and Laminaria longissima). Fresh leaves of two species of seaweed and fresh and dead leaves of the two seagrass species were provided separately, for a total of six types of food. Food preferences of mature adults were examined through pairwise feeding tests in which individual animals were offered two types of food materials simultaneously and allowed to choose between them for 24 hours. Adults consistently preferred seaweeds to seagrasses, but no clear preference was detected between the two seaweed species or between the two seagrass species. For juveniles of three size classes and mature adults, one-day rates of food consumption were compared between the different food item. Small and middle-sized juveniles ate a greater amount of Zostera asiatica and dead leaves of Laminaria longissima than other food items, but for large juveniles and mature adults, Laminaria longissima was the most eaten food. Juveniles of all size classes ate significantly greater amounts of dead leaves of Zostera asiatica than fresh leaves of that species, but no such difference was seen in mature adults. Finally, starting within 3 days after hatching, small juveniles of I. ochotensis were reared for 4 weeks on the above six different types of food. Although not statistically significant at the 0.05 level, there was a weak trend for juveniles provided with their preferred food grow larger than those provided with a less preferred one. Inspection of the gut contents of field-collected individuals revealed that I. ochotensis in Akkeshi Bay feeds mainly on seaweeds and takes only a small amount of seagrasses, as expected from the results of the laboratory studies.

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The distribution and behavior of the marine luminescent ostracod, Vargula hilgendorfii, were investigated on the shoreline of Kumamoto Prefecture and in a laboratory setting. Vargula hilgendorfii mainly inhabit sand/mud bottoms near the shore and are completely nocturnal. From March 1999 to January 2000, baited traps enclosing cock liver, were set one to three times every month (except for August) at the Nishime site to collect V. hilgendorfii. Ovigerous females were collected from April to October and small individuals from June to December, indicating that V. hilgendorfii winter in the adult form. In the summer of 1999, baited traps were set for 24 hours at varying depths under a floating pier at the Ryugu site and the numbers of V. hilgendorfii in each trap were counted every three hours. All the V. hilgendorfii were collected within 1 m of the sea bottom between 6 p.m. and 6 a.m. Vargula hilgendorfii luminescence was observed both at the Nishime site and in the laboratory, from July to October in 1999 and 2001. On the shoreline, V. hilgendorfii luminesced for just a few seconds, mainly when predators such as Portunus pelagicus, R sanguinolentus, Takifugu niphobles, Plotosus lineatus attacked, although the predators still often ate them. In the laboratory, males luminesced more often than juveniles and females, and more frequently when in a group of males, than when in a group with females. Most males appear to luminesce to threaten predators or other males, rather than for courting females.

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The influence of reclamation on benthic animals in Hakata Bay, Fukuoka, Japan, was reviewed. In Hakata Bay, large-scale reclamation has been ongoing since the 1970s. The east side of Hakata Bay has been almost closed from the sea by the construction of two huge reclaimed lands, Kashii Park Port (126 ha) and Island City (artificial island, 401 ha). Temporal changes in species composition and the density of benthic animals including polychaetes, molluscs, crustaceans, were researched at Wajiro Flat and Kashii Beach. The density greatly decreased at both Wajiro (2, 819 ind./m² in 1982 to 1, 582ind./m² in 1993) and Kashii (3, 524 ind./m² in 1982 to 1, 326 ind./m² in 1998). The decrease in polychaetes was conspicuous at both Wajiro (1, 600ind./m² in 1982 to 341ind/m² in 1993) and Kashii (2, 081 ind./m² in 1982 to 457 id/m² in 1998). The changes may be due to the stagnation of seawater due to coastal reclamation and to increased sewage discharge.

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The endangered tidal flat snail, Cerithidea djadjariensis, was considered to extinct locally in Tokyo Bay, central Japan, before 1995. However, our investigation in September 2001 has shown that 16 individuals of this species were found on a muddy tidal flat in artificially constructed Shinhama Lagoon at the innermost part of the Bay, which was constructed in 1977. The size of these individuals ranged between 21.5 mm and 44.7 mm in shell length, and such wide size range suggests the continuous recruitment occurring in several years after the colonization. Spatial distribution of the snail was restricted to a small area (10 m along the shore with 3 m wide) at the edge of reed community, Phragmites australis, although such habitats were widely seen along the lagoon shore. Although origin of the place that planktonic larvae of these individuals came from is unknown, one possible route is the drainage canal about 2 km apart from the lagoon: a population of this species was also present on the tidal flat in the canal before 1990. The present case study suggests that scattering arrange of small habitats is essential to protect endangered tidal flat snails.

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Although the fiddler crab Uca arcuata (De Haan, 1833) has been widely recorded from Kii Peninsula, central Japan to Okinawa Inland, the local populations of this species have seriously declined recently. In the Red Data Book of Japanese marine and freshwater benthic animals in 1995, this species was listed in “RARE” category, which means the species with small populations and at risk of extinction. Our recent investigations (2000-2001) recorded small populations of this species in the estuaries of the Koya and Imazu rivers (Yamaguchi Prefecture) and the Nagae River (Okayama Prefecture) in Chûgoku District. In the Koya River, the average density was 3.7 individuals per m², and the estimated number of individuals inhabited was 65100. These numbers indicate that the population size in this locality was as large as that of the Hikawa River in the Ariake Inland Sea, Kyûshû, western Japan.

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Horizontal and vertical distributions of algal species and occurrence of phytal animals on the algae were investigated in the kelp bed, Laminaria religiosa, at Iwanai on the southwestern coast of Hokkaido, Japan, in June, 2001. A belt transect (1 m wide) was put on the sea floor from 0 to 250 m out from the shore line, and algae found along the transect were identified and counted. Laminaria religiosa was mostly abundant between 2 m and 140 m from the shore line, coverage recorded there being over 50%. Phytal animals on L. religiosa and other 13 algal species were collected by net (1 mm mesh) at a distance of 20 m from the shore line and a depth of 1.5 m. Forty-seven species were collected on 14 algal species, including 6 species of gastropods, 3 bivalves, 10 polychaets, and 25 crustaceans. The lacunid gastropod Temanella turrita was the dominat species on most of the algae. Species diversity of phytal animals was highest for Dictyota dichotoma (number of species occurring=22), followed in order by Symphyocladia latiuscula (21), Polysiphonia morrowii (17), and Sargassum confusum (17). The poorest phytal species diversity was recorded on L. religiosa. The degree of similarity in phytal animal species composition between the algal species was calculated by Kimoto's C₁₁ index, and cluster analysis was done by UPGMA method. Three clusters were recognized, L. religiosa alone, a cluster formed of 11 algal species with a phytal fauna dominated by Temanella turrita, and a cluster formed of 2 algal species with a phytal fauna dominated by crustaceans.

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Japanese crab fisheries for king crabs (Paralithodes spp.), snow crab (Chionoecetets opilio), and hair crab (Erimacrus isenbeckii), have declined in recent years. Various fishing regulations have been adopted to increase and maintain the stocks for these fisheries, however they have been ineffective. Here I propose behavioral ecological approaches that might help increase crab stocks through aquaculture and resource management, particularly through seed production at hatcheries and resource management of wild stocks. I present three case examples: 1) the operational sex ratio at crab hatcheries, 2) determination of male maturity size in the wild, and 3) resource management of wild crab stocks. Male crabs guard ripe females for several days before mating. This guarding behavior results in a great physiological cost for males. In such mating systems, a female-biased operational sex ratio, which is often observed in space-limited hatcheries, may reduce the potential reproductive ability of females. A more even operational sex ratio should be maintained to improve reproductive success in crab hatcheries. In the wild, male maturity should be determined based on the functional (behavioral) maturation size (defined as the size at which copulation begins), and size limits for fishing should be based upon on functional maturity. Selective fishing for larger male crabs, which is a common practice in these fisheries, may lead to female biased stocks on the fishing grounds. Removing the functionally mature males may lead to a serious depression in crab productivity by reducing the number of viable males available for copulation with recently molted receptive females. Furthermore, smaller male crabs usually have a lower fertilizing ability due to their small amounts of sperm (sperm limitation). Therefore, fishing regulations in large decapod fisheries should be enacted that ensure effective reproduction in order to realize the potential spawning capacity of unfished females. These examples illustrate that behavior-ecology approaches can be effective in the conservation of crab populations, and should be more frequently applied to crab aquaculture and fisheries.

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Commonly, parasites have no ways to migrate independently. Stream-dwelling organisms are often transported downstream in the water column. How do parasites migrate upstream and colonize other water systems in a mountain stream? The fish parasitic nematodes Rhabdochona denudata, R. coronacauda, and Cystidicoloides ephemeridarum were found in the upper reaches of streams in western Japan. They need two hosts to complete their life cycle: an ephemeropteran species as an intermediate host, and a freshwater fish as the definitive host. Interesting questions relating to their life history is why these nematodes need an intermediate host. One plausible ecological answer to the question is that the nematodes use the body of ephemeropteran nymph for nutrition, and the body of imago as a vehicle to migrate upstream. I studied the seasonal distribution of nematodes in mayfly nymphs to clarify the effect of upstream migration of imagoes from the viewpoint of maintaining riverine populations.

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The sand-bubbler crab Scopimera globosa is infected by the digenean trematode Gynaecotyla squatarolae in some tidal flats of Honshu and Kyushu Islands. The trematode utilizes three host taxa; potamidid snails (probably Batillaria cumingi) as the first intermediate host, S. globosa as the second, and some shorebirds as the third, definitive host. It may be hypothesized that the trematode occurs only in those habitats where all the hosts exist. In order to test the hypothesis, I examined the occurrence of crabs infected by the trematode in habi-tats with and without snails. The results demonstrated that infection of crabs by trematodes occurred only where snails were present. Therefore, trematode infection of S. globosa may imply high species richness in the local assemblages of the host species.

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The intertidal limpet Patelloida saccharina lanx from the coast of the Boso Peninsula, central Japan, is infected with an undescribed species of the copepod family Ventriculinidae or a related family. Large regional and seasonal variations in prevalence of the parasite were evident between the nine localities studied (up to 70.2%, mean=36.2%). Even within a small area, prevalence varied greatly. Since parasitic copepods may have negative effects on their growth and reproduction of the benthic hosts, a population study covering the whole life cycle of the limpet should be conducted to understand the effects of the parasite on the limpet.

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The reproductive strategy of the parasitic isopod Onisocryptus ovalis in the luminescent ostracod Vargula hilgendorfii was studied at a beach in Kumamoto Prefecture, Japan, and in the laboratory, mainly from April 2000 to May 2001. Onisocryptus ovalis is a protandrous hermaphroditic species and changes from a mobile male to an immobile female that loses most of its appendages and has a sack-like body. Male parasites O. ovalis invade female hosts V. hilgendorfii, eat the host eggs, change sex, produce eggs, and die after the eggs have hatched. Some parasites invade immature or male hosts, but move to female hosts immediately afterwards. The percentage of hosts parasitized by O. ovalis was low (0-18%) from summer to autumn, but increased greatly (28-91%) from winter to spring mainly because of a shortage of hosts. Usually, one or two male parasites were found in a host, and only one male of them changed sex. From winter to spring, however, up to 8 male parasites were found in a host and a maximum of five changed sex. The most adaptive parasitic strategy of O. ovalis seems to fertilize eggs as a male as many times as possible, and then to produce more eggs as the only female in a host, although these parameters are not available at present.

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The Bopyridae and Entoniscidae are two of six families that comprise the Epicaridea (Crustacea: Isopoda). The free-swimming epicaridean larva first infests a copepod and metamorphoses into a microniscus larva. The microniscus larva remains with the copepod until it metamorphoses into a free-swimming cryptoniscus larva that searches for the final host. Upon settling on the final host, the cryptoniscus metamorphoses into a female, or into a male if a newly metamorphosed female is already present. The parasitic status of Grapsicepon rotundum (Bopyridae) and Xanthion spadix (Entoniscidae) and their effects on their host crab, Leptodius exaratus, were studied on two pebble beaches (Tsujishima and Nogamajima) in Kumamoto, Japan. Crabs were collected monthly at Tsujishima from November 1999 to November 2000, and once at Nogamajima in February 2000. A similar study was examined of Entionella okayamaensis (Entoniscidae) in the Asahikawa River in Okayama from July to September 2001, which parasitizes Chiromantes dehaani. The prevalence of parasitic epicaridean isopods differed greatly with place and parasite species: G. rotundum (9.4-23.0%) and X spadix (3.9-13.1%) parasitizing L. exaratus at Tsujishima, G. rotundum (3.2%) and X spadix (0.5%) parasitizing L. exaratus at Nogamajima, E. okayamaensis (9.6-10.7%) parasitizing C. dehaani 8 km upstream from the mouth of the Asahikawa River, and E. okayamaensis (2.0-2.9%) parasitizing C. dehaani 3 km upstream from the river mouth. In most cases, one dwarf male accompanied a single mature female. In three crab species, no ovigerous host infested by the isopod was collected, so host reproduction may be greatly inhibited by the parasites.

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Life histories of gnathiid isopods are briefly reviewed, and the mode of ectoparasitism is discussed in relation to the risks involved in the parasitic behaviour, particularly predation by cleaning fishes. Gnathiids are fish-parasitic isopod crustaceans, and their larvae are generally known to develop to non-feeding adults after shuttling between host fishes and benthic habitats a number of times. During the locomotion between the host and habitat, gnathiid larvae may be exposed to risks, such as predation and loss of host/habitat. Recent studies indicate that gnathiids are one of the main food items of cleaning fishes. Gnathiids need to parasitize once for each molt, and each parasitism is considered to involve risks. Thus, built-in risks in the parasitism might affect the life history traits of gnathiids, such as the number of parasitisims. In all gnathiid species whose life cycles have previously been reported, larvae mature to adults after three parasitisms. Periodic surveys of the host-parasite relationship between the gnathiid, Elaphognathia cornigera, and the host goby, Chasmichthys dolichognathus, were conducted on an intertidal rocky shore of Oura Bay, Izu Peninsula, southern Japan. Fortnightly samplings of C. dolichognathus showed seasonal occurrence of the goby. Infestation of E. cornigera to C. dolichognathus was observed from spring to autumn, when the goby was relatively abundant. Gut content analysis of C. dolichognathus suggested that E. cornigera larvae were often eaten by the goby. In mid-August 1996, a total of 22 and 5 gnathiid individuals found on the skin and in the guts of 68 gobies, respectively. The predatory effects of C. dolichognathus seem to be strong and may be an important source of mortality for the parasitic isopod.

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Large numbers of seabirds were killed or injured in the “Nadhotka” oil spill in the Sea of Japan on 2 January 1997. More than one thousand dead or living seabirds were collected for rescue and scientific research along the oil-fouled beaches. Many of the dead birds were examined for parasitic helminths. Two trematode, one cestode, and six nematode species were obtained. An echinostomatid trematode, Aporchis sp., was found in the intestine of 10 of 166 ancient murrelets (Synthliboramphus antiquus (Gmelin, 1789)) and 1 of 37 rhinoceros auklets (Cerorhinca monocerata (Pallas, 1811)). A renicolid trematode, Renicola sp., was found in the kidney of 21 of 50 ancient murrelets and 15 of 50 rhinoceros auklets. An anisakid nematode, Contracaecum rudolphii Hartwich, 1964, was found in the intestine of 73 of 166 ancient murrelets, 2 of 3 marbled murrelets (Brachyramphus marmoratus (Gmelin, 1789)), 9 of 37 rhinoceros auklets, 6 of 12 Pacific loons (Gavia pac fica (Lawrence, 1858)), and 2 of 14 red-necked grebes (Podiceps grisegena (Boddaert, 1783)). Three species of acuariid nematodes were found in the gizzards: Paracuaria adunca (Creplin, 1846) in 5 of 43 ancient murrelets; Stegophorus stercorarii Leonov, Sergeeva & Tsimbalyuk, 1966 in 9 of 43 ancient murrelets; and Streptocara crassicauda (Creplin, 1829) in 1 of 19 rhinoceros auklets and 1 of 12 Pacific loons. Other helminths found were: Cestoda fam. gen. sp. in the intestine of 1 of 37 rhinoceros auklets; a syngamid nematode, Syngamus sp., in the intestine of 1 of 37 rhinoceros auklets; and a trichurid nematode, Capillariinae gen. sp., in the gizzard of 1 of 12 pacific loons. The parasitic helminth community was simpler in the rednecked grebe than in the other four examined birds. This may be attributable to the short history of adaptation to the marine environment of this host bird.

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Although populations of the lancelet Branchiostoma beicheri are said to have been declining in Japan, many specimens of the lancelet were collected from shallow marine, relatively mud-free, sand-to gravel-grade clastic bottoms, off Naka island, western part of the Seto Inland Sea. Tolerance of the lancelet to rapid burial with coarse-(1.0-2.0mm in diameter) and fine-grained (less than 0.125mm in diameter) clastics was determined through laboratory experiments. All the individuals were killed by only 3cm thick burial with the “exotic” fine-grained clastics, but some could tolerate up to 19cm thick burial with the “native” coarse-grained ones. This suggests that the lancelet has low tolerance to a rapid burial with fine-grained clastics. Coastal development such as offshore mining and dredging, which agitated the seafloor and induced mud-draping around there, is one of possible causal factors for population decline of the lancelet.

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Faunal response of bivalves and gastropods after the construction of the dyke for reclamation was compared among the case studies in Japan (Isahaya Bay) and Korea (Saemangeum and Sihwa). After the isolation of Isahaya Bay, aspects of water quality suddenly changed, and fauna of bivalves and gastropods was drastically replaced. In March 1997, before the dyke was completed, 15 species of marine bivalves and gastropods were collected in large numbers. These species were still alive in May 1997, but most of them had died off by August 1997. However, an introduced species, Potamocorbula sp. cf. P. laevis that was not found prior to the isolation of this bay, replaced the pre-isolation bivalve community. This species also increased in Saemangeum and Sihwa, western Korea after the dyke construction. Therefore, this species can survive and multiply alone through large environmental changes such as isolation. Fossils of Potamocorbula sp. were also found from several horizons in Pleistocene and Holocene deposits in Japan and China. These fossils have common features; 1) most specimens are less than 10 mm in shell length, 2) only this species densely occur in a shell bed, and 3) some marine bivalves occur in lower horizons of the shell bed. We observed that Potamocorbula sp. could survive and multiply alone after isolation, and we therefore suggest that aggregations of Potamocorbula shells in Pleistocene and Holocene deposits represent similar isolation events in the past.

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Major changes occurred in the benthic molluscan fauna of the Sea of Japan continental shelf during the transition from Pleistocene glacial to interglacial stages, owing to rapid warming associated with the inflow of the warm Tsushima Current. Molluscan associations representing this transition occur in the lower Pleistocene Omma Formation in central Japan and suggest that there were two patterns of faunal change. The first was when warm-water species migrated into the Sea of Japan and lived along with cold-water species, accompanied by a northward shift in species ranges. The second pattern involved the migration of warm-water mollusks shortly after the local extinction of cold-water species. In the latter, it is possible that benthic molluscan communities with very low diversity and density extended a few km laterally and a few tens of meters vertically on the southern Sea of Japan inner-to outer-shelf. Such environment may have temporarily prevailed at least three times during early Pleistocene deglacial periods. These deglaciations corresponded to the three highest peaks of July solar insolation at 65°N during oxygen isotope stages from 50 to 28. This anomalously high seasonality forced by orbital insolation cycles is likely to have played an important role in establishing non-analogous benthic mollusk communities with very low density and diversity in the early Pleistocene Sea of Japan.

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The clam Ruditapes philippinarum is common in estuarine tidal flats in Japan, and it is an important resource for the coastal fisheries. This paper reviews the general biology of this clam. It is widely distributed from Hokkaido to Kyushu, and the local reproductive season is highly variable. It is a gonochoristic species, and the adults release eggs or sperm into seawater. Size-age structure and size at maturity are also variable among local populations. After fertilization, planktonic larvae develop in shallow water, and after a while settle on the seabed. Settled spat grow in sandy sediment. High density populations occur in intertidal and subtidal zones, in particular on tidal flats. Catches of this clam in Japan have seriously declined since 1987. We have only poor knowledge about the biology of this species, and the factors responsible for this depletion have not been fully studied. To restore the clam population, we must make further efforts to study several aspects of its ecology and biology.

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There are many tidal flats in coastal areas of Ariake Sound, where a fishery for the clam Ruditapes philippinarum is well developed. However, a serious reduction in the clam catch has occurred in recent years, especially, in the coastal areas of Kumamoto, where the annual catch of 65, 732t in 1977 has decreased to 2, 000 t or less. Various causes for this decrease have been suggested, including environmental deterioration, regional topography, food habit of local people and predation. The clam populations in Kumamoto have two reproductive peaks, from spring to early summer and in autumn. However, in recent years, the populations have been maintained by recruits from the earlier of these seasonal peaks. Recently, the clam has come to be distributed higher in the tidal flats. In Kumamoto, this species of clam can reach commercial size within 1 year after settlement, and almost all individuals are fished during the period from spring to autumn of the next year. Predation, especially by the eagle rays, is another serious problem for maintenance of the clam population.

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The annual fisheries production of the manila clam Ruditapes philippinarum in Chiba Prefecture markedly declined to below 20, 000 metric tons (mt) in 1979, after reached 70, 000 mt at its maximum in the late 1960s. The decline was mainly due to the large-scale coastal development, e. g. the reclamation of the shallow tidal areas where the clam fisheries were intensively operated. However, clam fisheries production gradually decreased even after the cessation of a series of reclamations in 1979. In the Kisarazu area, clam production has been maintained between 5, 000 and 6, 000 mt since the late 1970s in spite of a de rease in the occurrence of wild clam seed. This is most probably due to 2, 000 to 3, 000 mt of seed clam transplanted from other areas and/or prefectures. On the contrary, in the northern Chiba area, where clam production relies only upon wild clam stocks without any transplantation, the clam population has been frequently affected by “Aoshio”, a bacterial bloom caused by upwelling of oxygen-deficient seawater, as well as by river floods of muddy freshwater and winter mortality. Despite these problems, the clam fishery in the northern region has continued during recent decades, because the clam stock has recovered rapidly after mass mortality in many cases. However, clam production sharply decreased from ca. 10, 000 mt in the late 1970s to 800 mt in 1999 due to the poor occurrence of wild juvenile clams. Although the causes of accidental mass mortality have been almost fully revealed, the reason for the decrease in wild juvenile clams is still not known.

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In 1987, the annual catch yields of the clam Ruditapes philippinarum in Japan reached their peak, ca. 140-160 thousand tons. The yields have drastically decreased since then, down to 25% of the peak after that period. This decrease was mainly due to decrease of the yields from Ariake Sound in Kumamoto Prefecture, southern Japan, which reached a peak (ca. 65 thousand tons) in 1977 and then drastically decreased, down to ca. 1% of the peak recently. In order to reveal the factors causing the catch depletion from 1977 onward in Kumamoto and from 1987 onward in Japan, we have studied patterns of larval recruitment of this species on tidal flats in Kumamoto. Frequent sampling was done for the different life stages of the clam (planktonic larvae, new settlers, juveniles, and adults), and cohort analyses were made using size frequency distributions. Our results indicate that larval supply may play critical role in structuring the age distribution and in either maintaining or inducing fluctuations in the benthic populations of the clam in Ariake Sound. This differs greatly from the our another study on tidal flats in Ise Bay along the Pacific coast of central Japan. Our studies in Ariake Sound strongly suggest that the drastic decrease in yields in the Sound may have been caused by a shortage of larval supply in addition to a decrease in size of the spawning stock and an increase in larval mortality resulting from environmental deterioration.

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Physical conditions of sea water strongly influence growth and survival of the clam Ruditapes philippinarum; however, quantitative analysis of this influence has not been fully made. In this paper, effects of some physical condition (vorticity, tidal current, and waves) on population characteristics of the clam (recruitment, growth, and passive emigration) on the tidal flat and shoal of Banzu, Tokyo Bay, are reported. The vorticity (an indicator of shear, such as that caused by tidal fronts or vortices) tended to be larger offshore. Abundant recruits (<1mm in shell length) were observed on offshore shoals. A positive correlation was observed between vorticity during the spring ebb tide and density of clam recruits. This suggests that planktonic larvae of the clam were transported onto the tidal flat during the flood tide, then settled on shoal bottoms where a large shear was induced during the ebb tide. A higher condition factor value and lower sharpness index were observed in clams on bottoms where the tidal current velocity was higher, suggesting that current velocity positively affects growth of the clam. During winter, most monitored clams seeded on the bottom (about 28 mm in shell length) were found to have emigrated under the strongest wave condition, which was associated with sediment erosion. Hydraulic model test indicates that the presence of layer culture nets decreases wave action. During the last two decades, the number of rows of layer culture structures on the lowest part of the tidal flat has decreased. This may lead to a decline in clam population during winter, due to a decreased capacity for wave buffering.

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Infection of a protozoan Perkinsus (Apicomplexa) on the clam Ruditapes philippinarum has been reported from various localities of Japan since 1998. However, no ecological data are available heretofore on this parasitic organism. This is the first report on the spatial distribution of the prevalence of Perkinsus sp. in Japanese waters. A Perkinsus sp. specific polymerase chain reaction (PCR) method and a Ray's fluid thioglycollate medium technique were used for detection of Perkinsus sp. Clams infected with Perkinsus sp. were found from 63 of 74 study sites all over Japan except for the eastern coast of Hokkaido, prevalence ranging from 3.1 to 100%. Moreover, all the clams imported from neighbor countries are noted to be infected with Perkinsus sp. The prevalence and intensity of infection were greater in clams inhabiting commercial digging places and fishinggrounds than in undisturbed or non-fishing grounds. In contrast, the prevalence was relatively low in clams occurring in estuaries and in clams smaller than 15 mm in shell length. Perkinsus infection has also been detected in other bivalves, such as Ruditapes variegata and Protothaca jedoensis, collected from areas near to shell digging beds of R. philippinarum.

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During 1970s, 40, 000 to 65, 000 tons of a clam, Ruditapes philippinarum, were harvested each year on the tidal flats in Kumamoto Ariake Region. However, in the past two decades, the tonnage harvested drastically declined due to the collapse of the clam-population in the whole area of the tidal flats. Fortunately, one of the measures to recover the clam population on the tidal flats, which is creating a sand cover on the tidal flats with clean sands collected from the sea floor in the offshore areas, succeeded to re-establish the dense patches ofthe clam with a density between 1, 000 to 6, 000ind. /m². We have monitored the population dynamics of the clam since a new sand cover was set on the tidal flats in May 1997. On the sand cover, the planknotonic larvae of the clam recruited four times in a year. The newly recruited individuals grew to 30 mm in shell length in less than ten months. The production of the clam patches was estimated to 226.7 gDW/m²/year (only dried flesh weight) and 7, 832.0gWW/m²/year (wet weight including shell). To support this scale of secondary production of the clam, we are able to predict that a large amount of the primary production of more than 470 gC/m²/year is required. Although we were not able to confirm the presence of the primary producers on the tidal flats, the results of the population study of the clam indicate that the large potential of primary production was still preserved on the tidal flats in Kumamoto Ariake Region.

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