Variation and Abnormality of Genital System in Littorina sitkana Philippi (Mollusca, Gastropoda) in Northern Japan

The anatomy of Littorina sitkana was examined using specimens from nine localities in northern Japan. These localities are known to be genetically clustered into two geographic groups; the first group consists of localities on the northern coast of Hokkaido along the Seas of Japan and Okhotsk and the second one comprises localities on the Pacific coasts of Hokkaido and Honshu. Anatomical surveys revealed that the mean numbers of penial glands differ significantly between the northern and Pacific sites, in correlation with the genetic variation. On the other hand, the mean relative size of the capsule gland within the pallial oviduct was positively correlated with mean sea-surface temperature, almost independently of the genetic difference. In addition, imposex was found for the first time in L. sitkana. Females frequently lacked the capsule gland in the imposex-present populations. The mean relative size of capsule gland to the whole pallial oviduct was significantly smaller there than in the imposex-free populations. The number of penial glands in males tended to be small in the imposex-present populations. The penis of imposex females was much shorter than that of males, in contrast to Nucella species occurring sympatrically on these coasts, in which the female penis is nearly as long as the male one. Although the pallial oviduct is reported to be replaced by a prostate gland in imposex females of Littorina littorea, such replacement was not found in any imposex female of L. sitkana.