コンスタンチン・ルタエンコ 日本海,ポシェット湾における貝類遺骸群集―特に波浪の強い海岸と弱い海岸における比較 日本海西北部のポシェット湾(ピョートル大帝湾の南西部)16地点の海岸の貝類遺骸群集について調べ,23科,42属,58種の二枚貝同定した.これは,この湾と周辺地域に生息する貝類の約60%にあたる.その種構成の特徴と遺骸群集の化石化過程について記述した.海岸性の貝類遺骸群集の種構成は,その湾の海域の環境に影響されていた.ポシェット湾における死後運搬作用は僅かであることが示唆された.

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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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ヌマエビとヌカエビそれぞれの個体の最終胸節腹柄に二型がみられた。ひとつは先端部が突出し(突出型)、もうひとつは突出せず丸く膨らんでいること(丸型)が特徴であった.第2腹肢の観察により、これらの個体の雌雄を判別したところ、突出型はすべて雄、丸型はすべて雌であった.したがって、本種の最終胸節腹柄の二型は性的二型であると考えられた.

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