We examined the spatial and temporal changes in percent cover of the Arcuatula senhousia Benson (Mytilidae, Bivalvia) population by SCUBA diving in the brackish Ohashi-River in Shimane Prefecture, southwestern Japan. From spring to summer, the numbers of A. senhousia increased, and the mussel mats spread from the deep, central part of the river to the shallower river banks. In summer, the mats spread over the entire transect line, but in late summer and autumn, the mats reduced in the deep central part. The factors that induced the decrease of population were different every year; they included low salinity, low dissolved oxygen, high water temperature, flood (physical destruction) and the combination of these. The seasonal pattern of population dynamics of A. senhousia in the Ohashi-River was closely related to the riverbed topography as well as seasonal trends in salinity and dissolved oxygen of river water.

Genetic population structure of the rare brackish-water crab Ptychognathus ishii Sakai, 1939 (Varunidae), was investigated based on specimens from ten localities in southwestern Japan from Shizuoka Prefecture to Okinawa Prefecture. Significant genetic differentiation was detected among many local populations, with the most remarkable difference between populations on the Japanese mainland and those on smaller islands, such as Tsushima Island, Kakeroma Island, and Okinawa Island. Additionally, genetic differentiation was found between some local populations on the mainland. Results of a neutrality test, divergence time estimation, and mismatch distribution results suggest that mainland populations diverged from the population in Okinawa after divergence of the Kakeroma and Tsushima populations.