Abstract: I investigated the process of growth, migration, and reproduction of the Japanese mitten crab Eriocheir japonica in the Saigo River, Fukuoka Prefecture, Japan. Samples were collected by hand-held nets in the lower freshwater, tidal river, and sea coast areas. Above the upper tidal river area, males and juvenile females appeared nearly all year round, but adult females only from August to February. Exuviae and post molt crabs were collected only in this area, from March to November. Below the middle tidal river area, adult females of >38mm CW and males of >28mm CW appeared from September to June. Duration of occurrence was longer in the lower tidal river and sea coast areas than in the middle tidal river area. In the lower tidal river and sea coast areas, postcopulatory guarding pairs were collected in December, January, and March, and ovigerous females appeared from October to the following June. Carcasses(>38mm CW and >34mm CW for females and males, respectively)were collected mostly in the tidal area. Therefore, growth molts of juvenile crabs appeared continuously except in the winter season, and the puberty molt to the adult stage from August to November, mainly in the freshwater area. Adult crabs migrated downstream from September to February, and reproduction took place mainly in the lower tidal river and sea coast areas.

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Abstract: Growth and age structure of the lancelet Branchiostoma belcheri was studied in the Seto Inland Sea, Japan, throughouthe year. A total of 3, 601 lancelets were collected by means of dredging. Spawning of the lancelets occurred from late June to late July. Changes in modal size on the length-frequency histograms showed that the lancelets settled on sandy sediment after four to five months of planktonic life when they grew to 7.0 mm BL. Mean body lengths of the lancelets at an age of one, two and three years appeared to be 10.9, 28.0and 33.0 mm, respectively. Most individuals developed gonads after an age of two years at 25 mm BL. The sex ratio(males: females)was 0.97: 1, and the size composition was not diffbrent between sexes. Growth rate and life span of lancelet species increased and decreased at the high temperature habitats, respectively.

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Abstract: Genetical differences were examined among three geographically different populations of a lancelet, Branchiostoma belcheri in western Japan. Electrophoresis of PCR products with six different primers detected 13-19 DNA bands from sixteen individuals of these three populations. Among them, two bands were common to all individuals, and three bands were shared with the populations of Hiroshima Bay and the Sea of Genkai. One band was specific to the population of the Sea of Genkai, and three bands were specific to the population of Osaka Bay. In cluster analysis on the genetic similarity, the individuals collected at the Sea of Genkai are relatively close to those of Hiroshima Bay, but both populations are relatively distant from those of Osaka Bay. It might be inferred that the three populations have not originated from a common larval recruitment and there are no genetical exchanges via larval recruitment at least between the populations of Hiroshima Bay and Osaka Bay.

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Abstract: A short neck clam, Ruditapes philippinarum, is one of the most dominant fauna in the benthic communities on the sand tidal flats across the Japanese coast. On the tidal flats facing to Ariake Bay in Kumamoto prefecture, more than 60, 000tons of the clam were harvested per year in the 1970s. However, the dense populations collapsed on the tidal flats in the 1980s to 1990s. The results of the recent studies on the clam showed that its population could recover only at the sand covers on the tidal flats, which were created with the sand collected from the sea floor of the offshore areas, while the young juveniles of the clam still suffered from extremely high mortality on the sediment of the sand flats. We focussed on the concentrations of heavy metals(Mn, Zn, Cu, Cd, Pb)in the sediments as contaminants imposing a negative impact on the physiology of the clam. The sediments on the tidal flats where the clam populations collapsed contained 1, 700 to 2, 900 μg/g of manganese, although the concentrations of the other four heavy metals of the sediments were lower than 63 μg/g.The results of this study indicated that the dense patches of the clam were not established on the sediments containing(at least)more than 1, 000 μg/g of manganese. It is very likely that the elevation of the concentration of manganese in the sediment is responsible for the collapse of the Ruditapes populations on the tidal flats.

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