Molluscan thanatocoenoses have been studied in 16 areas of the seashore of Possjet Bay (northwestern part of the Sea of Japan), and 58 species of bivalves, referred to 42 genera and 23 families, were identified. This is about 60% of the total number of bivalve molluscs inhabiting the Bay and surrounding waters. Peculiarities in the composition and taphonomy of the thanatocoenoses are described. The taxonomic composition of the beach thanatocoenoses of bivalve molluscs commonly reflects environmental variability in the shallow waters of Possjet Bay. There seems to be minimal postmortem migration of molluscan shells in Possjet Bay.

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Freshwater shrimp, Macrobrachium nipponense, were collected in summer from an artificial brackish-water lake, Lake Koj ima, in Okayama Prefecture, western Japan. Their hatched larvae were reared in the laboratory at 25°C in different salinities ranging from 0 to 32 ‰. The pelagic larvae successfully metamorphosed to the 1st postlarva only in salinities of 4 and 16 ‰ with survival ratios of 15 to 20 %. In the laboratory, an increase in salinity from 4 to 16 ‰ accelerated the settlement of middle-stage pelagic larvae. This, however, resulted in a shorter larval period and smaller 1st postlarvae than those obtained in 4 ‰. A field survey in late summer showed an aggregation of 7th-to 9th-stage zoeae and 1st post larvae in more saline (1-20 ‰) deep water, while 1st- to 3rd-stage zoeae were distributed in the less saline (0-1 ‰) surface water. Thus, both the laboratory experiment and field observation indicate that high salinity water induces larval settlement of M. nipponense in this brackish-water lake.

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Two discrete morphological types were recognized among the populations of a boring bivalve, Penitella kamakurensis, based mainly on whether the anterior end of the shell (beak) is pointed (P-type) or rounded (R-type). The relative abundance of the two types varies from place to place according to the hardness of the rock in each locality. P-type individuals are abundant in softer rocks and R-type in harder rocks. In R-type individuals, the width of growth band is narrower, the size of posterior adductor muscle scar is larger, and the shell is thicker than in P-type individuals from the same locality. Individuals of either shell type living in harder rocks show the same tendencies in comparison with those of the same shell type from softer rocks.

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[in Japanese]

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