福井県浦底湾におけるミズクラゲの生態-IV
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抄録
So far as we know, very few reports are available on the year-round growth of the jelly-fish, Aurelia aurita (LINNÉ), excepting for reports confined to partial or limited seasons. In continued studies on the ecology of the Aurelia aurita in Urazoko Bay, a small inlet of the Wakasa Bay on the south-western coast of the Japan Sea, the author was able to collect fairly sufficient data on the monthly changes in the bell-length composition of this Scyphomedusa, enough to discuss its breeding season, growth and longevity. Field sampling was carried out every month by vertical and horizontal hauls with two types of plankton-nets (100cm in mouth aperture, 2mm in mesh and 45cm in mouth aperture 0.33mm in mesh) and also hand-nets (40×20cm opening, 2mm in mesh), together with hydrographic observations throughout the period from April 1969 to April 1970.
About 3, 600 individuals of the medusae were collected and their bell diameter was measured while alive. Also the condition of the fertilized eggs or planula larvae attached to the oral-arms or subumbrella was simultaneously checked. The results obtained are summarized as follows:
1) From monthly changes in the bell-length composition, season of appearance of planula and also from the results of the previous reports, the longevity of Aurelia aurita was supposed to be from more than one year to less than two years (Fig. 1).
2) A number of young medusae which appeared in May or June were 1cm in bell-length, 0.2 to 0.9 in body weight. Their growth was most rapid in summer (July to August) and spring (March to April). They attained 22cm in bell-length and 400 g in body weight in 11 months.
3) A close correlation was observed between the increase in bell-length or body weight and the seasonal change in water temperature.
4) The fertilized eggs or planulae were observed from January to June in water temperature of 8 to 19°C and its peak from January to May with temperature of 8 to 18°C.
5) The medusae with fertilized eggs or planulae ranged from 7 to 31cm in bell diameter.