びわ湖産イボカワニナ類 3 種の研究 : I) 殼形態及び生息場所の比較

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Semisulcospira decipiens is an endemic species in Lake Biwa and its water system. This gastropod has been said to contain three types which are classified as two subspecies and one forma ; S. d. decipiens, S. d. reticulata and S. d. d. forma multigranosa. Recently some researchers proposed on the basis of the results of chromosome studies that these three types are different species. The purpose of a series of the studies is to determine the taxonomic category of these three types and to investigate the affinity among them. As the first stage I deal with comparisons of shell form and of habitat preference among them in this paper. As regards several shell characteristics of adult, decipiens is considerably different from reticulata, and multigranosa seems to be intermediate between decipiens and reticulata. Therefore, decipiens can be easily separated from reticulata, but some specimens of multigranosa can be hardly separated from the two others because of taking intermediate characteristics. On the relation between the shell length and the shell width of embryos in the brood pouch, reticulata is significantly different from the two others. However, that of decipiens and of multigranosa are fairly overlapped. On the other hand decipiens can be distinguished more easily from multigranosa by comparing the shell length of the largest embryo in a brood pouch. Habitat preference of the three types seems to differ from each other as far as depth and particle size of substratum concerned ; decipiens live in a shallow and coarsely sedimented area, reticulata in a rather deep and finely sedimented area and multigranosa in an intermediate area, although fairly overlapped. This tendency holds good in the north basin except bays. In the south basin and the bays of the north basin where multigranosa is extremely rare, reticulata occupies an area where multigranosa should live as far as its depth concerned. Thus I would suggest that in addition to abiotic factors in general, the interaction among the three types could not be neglected as a controlling factor of microdistribution. The fact of different characteristics, especially in embryonic stage, in spite of the overlapped living site, added informations regarding the overlapped reproductive period and others show that these three types make the reproductive isolation each other. Therefore, I may conclude that either three types are considered as different species or at least they are on the final part of the course of speciation. This supports some author's suggestion on the basis of chromosome studies. From the fact described above of adult shell characteristics, embryonic shell characteristics and habitat of the three types, I propose the assumption that the affinity between decipiens and multigranosa is stronger than that between decipiens and reticulata.
1970-05-15

びわ湖産イボカワニナ類 3 種の研究 : I) 殼形態及び生息場所の比較

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