Where the river bed is completely covered with organic deposit or sewage, the main inhabitants are always the so-called sludge worms of the family Tubificidae. In general, it has been supposed that its sexual maturity occurs throughout the year. An investigation of oviposition of Tubifex hattai was made by breeding in the laboratory, from June 1962 to June 1965,comparing with the results of sampling in the channel carrying polluted waters through Yamagata City. The general trend of the study suggests that there are two breeding periods in a year, the one in March to July, and the other September to October. That time of breeding may be related to the temperature conditions is inferred from that egglaying was seen under the minimum temperature reading of 8-10℃ and the maximum of 23-25℃, and the high temperature conditions, above 30℃, inhibit their oviposition, therefore the breeding activity shows a marked decline in the middle of July and oviposition ceased in August. As the water temperature declines to 25℃ in September, autumn breeding commences and continues for about two months. Thereafter they suspend sexual activity for long term. The aquatic oligochaete produces a cocoon with a transparent covering and the number of eggs brooded by a worm varies from 8-15,with a mean of 12,during the March-June breeding. Therefore for every individual about 38 cocoons were bred during the season, containing 38×12,i.e. 456 eggs. It is clear from the survival curve that there appears to be a low mortality during the autumn months, and high mortality occurs immediately after the breeding of March to June, and the mortality becomes especially high after the spring breeding season of the second year. The difference of mortality of the first and the second year may reasonably be attributed to the conditions of density of the population and their longevity in the laboratory. Among the factors controlling the reproductive activity, the correlations with body size or nutrients are not important, but the temperature is rather the main. If enviromental factors are favourable, it may be possible that the worm might actually survive for 4-6 years, but the physiological longevity is controlled by the various ecological conditions such as population density, food requirement, soil conditions and so.
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