大阪湾におけるスピオ科の多毛類Paraprinospio sp.(A型)個体群の季節変動と成長

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Distribution, seasonal fluctuation and growht in a population of the spionid polychaete Paraprionospio sp. (type A) have been studied in relation to bottom environment and the structure of benthic assemblage at Osaka Bay where organic pollution is conspicuous. Type A was distributed in the region shallower than 15 m in depth and the high population density more than 1000 individuals per m2 was found from July t November. Type A showed an annual reproductive cycle. The main spawning season ranged from late June to middle of August, and newly recruited juveniles were observed from late July to early September. The longevity was about a year. The reasons why type A was numerically dominant and showed high populatin density from summer to autumn in Osaka Bay are mainly as follows; recruitment of type A takes place during the decreasing period of the many other benthic animals due to oxygen deficiency in summer, strong tolerance to the oxygen deficiency, rapid growth, few competitive species with type A for food and space, and presence of a large amount of food maintaining high population density of type A. Low oxygen concentration in summer and a great deal of organic input are favorable physico-chemical conditions for type A. So, it is considered that dominancy and population density of type A can be used as indices for organic enriched zone.
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