On the Morphological Variation of Sagitta crassa

Abstract

The present paper dealt with the seasonal variation of Sagitta crassa on the basis of the appearance of the collarette, and dealt with the effect of the water temperature and the chlorinity of the sea water that have the important effect upon such variation. The materials for this study were collected from November ('54) to March ('58) at one station (St. 22), from December ('54) to February ('55) at 20 stations and from November ('55) to October ('56) at 6 stations in the neighbouring sea of Mukaishima (Inland Sea of Japan).
In the case of the study, the collarette of S. crassa is distinguished four classes as shown in Fig. 2 according to the degree of the distribution. Accordingly, the collarette of type form of S. crassa is A class and the collarette of f. naikaiensis is D class.
S. crassa occurred neighbouring sea of Mukaishima has three main spawing seasons (February-March, May-July and November-December) in which the generation alternates. In two generations that the individuals mainly spawn from May to July and from November to December, the collarette of S. crassa is D class or C class of low degree. But in the generation that the individuals mainly spawn from February to March. the collarette is ableto develop into C, B and A class in order. In that case, the collarette of S. crassa comes into C class from December to January, into B class from January to February and into A class on and after February.
The water temperature has the important effect upon the development of the collarette mentioned above. The occurring of the individuals that the collarette developed into A and B class is limited in the case of the water temperature dropped under 11.5°C, and the rate of the occurring is raised when the water temperature has dropped under 9.5°C. In other hand, the individuals that the collarette has developed into C class rarely occur even if the water temperature has rised upper 20°C, but the rate of the occurring is raised when the water temperature has dropped under 11.5°C and into C class within 17.4-18.6‰.
The chlorinity of sea water has the important effect upon the development of the collarette of S. crassa, too. The collarette develops into A class when the chlorinity of sea water is within 17.5-17.8 ‰, into B class within 17.4-18.1‰ and into C class within 17.4 18.6 ‰.
Thus, the collarette of S. crassa develops when the water temperature and the chlorinity are confined within fixed limits.