著者

鎮西 清高

出版者

日本古生物学会

雑誌

化石 (ISSN:00229202)

巻号頁・発行日

vol.31, pp.27-34, 1982-06-21 (Released:2017-10-02)

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Some paleoecologic aspects of oysters are discussed, giving special emphasis on the adaptive strategy of Crassostrea to the soft muddy bottom. Crassostrea is most abundant in the muddy intertidal facies despite their sessile and suspension feeding habit. Muddy bottom environment seems unfavorable for oysters because it offers very few basis of attachment, and gives continuous danger of suffocation by rapidly accumulating mud. Crassostrea surmounts these difficulties in living on mud, and becomes dominant in this environment. The basic strategy of Crassostrea for the survival in the muddy bottom is considered to be their gregarious tendency to constitute the densely aggregated colony. The colony supports the individual shell to stand above the surface of mud, and gives the firm basis for the attachment of later generations. The later generations grow on the colony, thus manage to keep living animals atop the rising mud level. Formation of the biohermal reef is the consequence of this characteristic behavior of Crassostrea. Nongenetic variations in shell morphology are conspicuous among Crassostrea individuals. The shell outline of fossil and living Crassostrea gigas is not controlled by the population density, but is probably related to the position of the shell : individuals in the upright position tend to be elongated irrespective of their density, and those lying on the bottom are usually more rounded. The population density apparently affects the individual shell size. Where the density is low, shells are very large, while they are small in densely populated reefs. The relationship may be explained by the competitive effect in the allocation of limited food supply.