The Hitotsuba Lagoon is a small estuary environment on the east coast of Kyushu island with about 12 ha of water area, half of which may appear as a continuous tidal flat. After a series of line transect and quadrate surveys in the lagoon from 2006 to 2014, we estimated the total number of three potamidid gastropods and discussed on their life history and population size. The Hitotsuba population of Cerithidea rhizophorarum A. Adams, 1855 was rather stable and not exceeded over 20,000 individuals in total within the lagoon. Two other species, Cerithideopsilla djadjariensis (K. Martin, 1899) and Cerithidea cingulata (Gmelin, 1791), fluctuated in the population size. The last two species are thought to increase in number year by year within the lagoon. The seasonal changes in size distribution of these two potamidid species in the Hitotsuba Lagoon were studied based on the periodical samplings in the intertidal zone from April to December, 2006 and another series in the subtidal from April, 2007 to February, 2008. As a conclusion of these surveys, small size gastropods with less than 5 mm in shell length of these species may appear on the tidal flat until October, then grow throughout the next year and finally reach to the full grown size until the second spring after entering to the tidal flat. Stopping the growth in the shell length after the development of the retroflexed lip of shell mouth, the adult gastropods may start to spawn in early summer. In the lagoon, Cerithidea cingulata may have a longer spawning period than Cerithideopsilla djadjariensis and recorded their small subtidal specimens throughout the sampling year.

We investigated the spatial distribution of macrobenthos in salt marshes along the coast of Tokyo Bay, with environmental factors that influence their distribution. The field survey was conducted at 31 sites along the entire coast of the bay, in which presence or absence of the salt marsh animals and environmental factors (salinity, ORP, median grain size, elevation, vegetation area) of the habitats were examined. Most of common species of mollusks and crustaceans were distributed on entire coast of the bay, but some of endangered species showed restricted distribution in the coast. The restricted distribution of endangered salt marsh crabs were explained by several environmental factors; higher salinity and lower elevation of the habitat were suitable for Helicana japonica and Parasesarma affines, which were distributed in eastern coast in the bay, and Sesarmops intermedium, that was distributed in western coast and inner part, tended to inhabit higher elevation sites. Since local populations of these salt marsh species in the bay may be consisted of bay scale metapopulation that are formed by connectivity between small local populations through larval dispersal, distributional restriction of the salt marsh crabs in the bay may indicate that these endangered species are regarded as being difficult to persist in the bay. New local populations of the salt marsh benthos animals could be restored by creation of salt marsh habitats along the bay, to increase of the connectivity between local populations in the bay.

Interstitial free-living nematodes play an important role in nutrient cycles and biological production on tidal flats. However, to dates, we have little information on these nematodes on Japanese tidal flats, e.g., on the characteristics of quantitative and taxonomic composition of nematodes. In this study, we examined seasonal changes in the density and taxonomic composition (at the family level) of nematodes, as well their spatial distributions, on a small flat (the Tanakagawa tidal flat, 0.06 km²) at the mouth area of a short river (the Tanakagawa River) along the western coast of Ise Bay, central Japan. Sampling was undertaken at the low water of spring tides June 2013 and also every three months from July 2013 to April 2014. Three sediment samples were collected at each of 13 sampling sites set wholly on the flat, using a core sampler (4 cm in diameter, 20 cm in depth). Through the present investigation, nematodes were identified to belong to 18 families. Dominant nematodes were different among the sites, and four clusters of sampling sites were distinguished for nematodes on the flat based on cluster analysis. Cluster I and II included sampling sites occupied with oxidative sediment, dominated by epigrowth feeders in nematodes. It is showed that seasonal change of nematode assemblages of cluster I was larger than other clusters. Cluster III included sampling sites occupied with reductive sediment containing high organic matter, dominated by selective deposit feeders in nematodes. Cluster IV included sampling sites occupied with rather reductive sediment located close to the water route, dominated by non-selective deposit feeders in nematodes.

Ariake Sound is a highly productive area and a good nursery for fish and shellfish species. Mysids are essential prey organisms for a number of juvenile fishes in Ariake Sound, but their distribution and abundance are not well understood. To investigate species composition and distribution, we collected hyperbenthic mysids with a sledge-net in the inner and central parts of Ariake Sound from 2001 to 2004. Fifteen species of mysid from nine genera were recorded. The numerically dominant species were Orientomysis tenuicauda, Erythrops minuta, Orientomysis aspera, Neomysis japonica, Rhopalophthalmus orientalis, and Hyperacanthomysis longirostris in order. Of these, O. tenuicauda and E. minuta were mainly collected at offshore stations with relatively high salinity. In contrast, the majority of O. aspera and the other three species were collected from nearshore stations and/or river estuaries. The highest density, 7,236 indiv./m², was recorded for O. tenuicauda at a station with a sandy bottom at 20 m depth. Mean estimated density, 241 indiv./m², was low compared with mysid densities recorded in other Japanese estuaries, but exceeded the density, 100 indiv./m², known to maintain the maximum juvenile Japanese flounder growth rate in their nursery ground.

In Hakodate Bay, Hokkaido, the short-necked clam Ruditapes philippinarum shows highly depressed growth rate above 30 mm in shell length. In this area, clams often burrow into the sediment at an angle, which seems to be caused by boulders in the sediment. For benthic bivalves, burrowing into sediment is an important behavior to escape severe environmental conditions and predation. To examine possible effects of boulders on burrowing behavior and growth of the clam, we compared burrowing depth of the clams in contact with boulders and those clear of boulders. The ratio of burrowing depth to shell length was higher in those clear of boulders than those touching boulders. To understand this depressed growth mechanism, we carried out a field growth experiment and a laboratory feeding experiment. In the field experiment, we compared the shell growth increments of clams with restricted burrowing into sediment and those of unrestricted burrowing for three months. Shell growth increments of the clams with restricted burrowing were smaller regardless of their initial shell sizes. In the laboratory experiment, we compared the clearance rates of clams under six different situations: predatory crabs were present or not and burrowing treatments were unrestricted (deep sediment), restricted (shallow sediment) or impossible (i.e., no sediment). Both the presence of predators and restriction of burrowing decreased the clearance rate of clams; however their interaction was not significant. These results suggest boulders in the sediment restrict burrowing, and then depress shell growth of the clams. Sediment features must be taken into account for fishery analysis and stock recovery potential in clams with burrowing behavior.

The smooth cordgrass Spartina alterniflora, native to the east coast of North America and the Gulf of Mexico, was first found from Japanese tidal flats in 2008. The species has been reported from Aichi and Kumamoto Prefectures where it settles on tidal flats lower than native reed grass Phragmites australis and other saltmarsh plants. In 2014, the Ministry of Environment in Japan designated all Spartina species as the invasive alien species. While S. alterniflora has been exterminated extensively by local governments, it still has invaded other tidal flats in Japan. We review the situation of S. alterniflora in Japan with the comparison of morphological characteristics of S. alterniflora and P. australis native to Japan for early detection on Japanese tidal flats.