三河湾の貝類遺骸群集 : 第 2 部巻貝類・角貝類

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Mikawa Bay is located in the Pacific coast of central Japan and it is one of the representatives of Japanese shallow embayment. It consists of two branched bays ; Chita Bay in the north and Atsumi Bay in the south. On June 24 and 25, 1962, molluscan remains and living benthos were collected at 31 sampling points in Mikawa Bay by Itoigawa et al. They used Smith-MacIntyre type sampler which is capable of scooping sediments about an area of 32×35 cm. This study is the second report on molluscan thanatocoenoses of Mikawa Bay using the material. In the previous report (Nobuhara et al., 1991), we studied on thanatocoenoses of bivalves and classified them into six types indicating marine environments of each part in Mikawa Bay. In this report, we study shell remains of gastropods and scaphopods. We identified 113 species in 93 genera of gastropods and 4 species in 3 genera of scaphopods. In particular, we examine quantitative distributions of gastropod shell remains. We adopt the following two ways of counting the shell remains of gastropods ; counting an apex part as one individual and counting a specimen having more than half of shell. As a result, we recognize three types of distributional pattern common to many species as follows ; seaweed bed type, embayment type, and outer bay type. The species group of seaweed bed type mainly deposits its shell remains in the eastern central to inner part of Atsumi Bay, where the depth is less than 10m. This species group is characterized by phytal species such as Diffalaba picta vitrea. The species of embayment type deposit much shell remains in the eastern central part of Atsumi Bay and the remains radically decrease in Chita Bay, the inner part (13A, 14A) and the western central part (the depression of the central bay area) of Atsumi Bay, where oxygen content becomes seasonally very low. The species of outer bay type show the distribution restricted to the outer side and the mouth of Mikawa Bay. The distributional pattern of the gastropod shell remains generally corresponds to embayment degree indicated by the thanatocoenoses of bivalves. But there is no gastropod species which adopt to water mass of low oxygen content, such as Theora fragilis in bivalves. Moreover, we cannot recognize oceanic gastropod species which extend their distribution to the inner side of the bay, such as Microcirce dilecta in bivalves. However, changes of quantity and species composition of gastropods in thanatocoenoses correspond to seaweed bed, water mass of low oxygen content and oceanic water. These changes serve to make a cross-check of environmental analyses with index species of bivalves.
日本貝類学会の論文
1992-07-31

三河湾の貝類遺骸群集 : 第 2 部巻貝類・角貝類

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