Morphological and ecological differences of the periwinkle Littorina sitkana (Philippi, 1845) were investigated between two adjacent habitats (inner and outer habitats) of a small harbor in southern Hokkaido, Japan. The inner side of the harbor was well sheltered, and many shore crabs Hemigrapsus sanguineus, a potential predator for gastropods, were present, whereas the outer side of the harbor was very exposed, with few shore crabs. In the inner habitat, most periwinkles possess a uniformly dark-colored, large and solid shell, which is sculptured only at the base of the last whorl, and a comparatively small foot. In the outer habitat, periwinkles possess a small, thin, and light-colored shell with a dark apex and spiral sculpture throughout the surface, and a comparatively large foot. Predation and flume experiments were conducted in the laboratory using snails from both habitats. Only snails from the outer habitat were predated by the shore crabs during the 120-h experiments, and snails from the outer habitat showed significantly stronger resistance to water flow. These morphological and ecological differences of L. sitkana exhibited in the present study may suggest a local adaptation of the same species to different habitats, created by an artificial structure.
During the period 1991–1992, we examined temporal fluctuations in densities of the native and exotic mytilids Musculista senhousia and Xenostrobus securis in the brackish Lake Hamana on the Pacific coast of central Japan, using cohort separation of planktonic stages and benthic stages (i.e., new settlers and small individuals). Temporal fluctuations in densities of both planktonic and benthic stages were similar between the two species. However, the densities of benthic stages were much higher for M. senhousia than for X. securis. There was a similar trend of variation in densities of benthic stages among sampling sites for each mytilid. This suggests that microhabitat separation of large individuals (≥ 1.0 mm in shell length) between these two species may be determined at the post-recruitment stage, when X. securis individuals with shell lengths of ≥ 2.0 mm probably immigrate into the study area from other areas.
Traditional taxonomy of shell-bearing mollusks has generally been dependent on the shell morphology including surface sculpture and color pattern. Nowadays, most researchers also use anatomical information and molecular analysis as the basis of the classification in addition to the shell morphology. In this study, we examined anatomical features in two presumed subspecies of Trishoplita eumenes, land snails distributed in Yamaguchi Prefecture. We then did a statistical comparison of the measured data. The results showed no significant differences between the two in the coloration of the dorsal soft-body, radular morphology, radula formula, jaw-plate morphology or general morphology of the genital system, including the form and microstructure of the penial verge. However, we did find distinct differences in the distribution pattern of black pigments in the dorsal mantle, the ratio of the length of the genitalia to shell diameter, the number of mucus glands, love dart morphology, and the ratio of the length of the love dart to the dart sac. These findings strongly suggest that the two forms represent two distinct species rather than subspecies.
We investigated size structure and reproduction of violet shell Janthina prolongata stranded on the coast of Iwami-cho in eastern part of Tottori Prefecture, southwestern Sea of Japan, on 27 September, 2010. Shell length (SL) exhibited a bimodal size distribution with a range of 6.08–40.36 mm. Our results also indicated that individuals of more than 27.4 mm SL laid egg capsules under their bubble raft. Moreover, there was a significant positive correlation between the length of egg capsules and SL. Larger individuals had shrunken and differently colored egg capsules on the forefront of bubble raft, suggesting that J. prolongata might lay egg capsules intermittently.