Spatiotemporal variations in rocky intertidal malacofauna throughout Japan in the 1970s and 1980s

Abstract

Spatial and temporal variations of benthic fauna over a wide region (>1,000 km) that includes complex ocean currents have seldom been quantitatively studied. The present study revealed malacofaunal variations on Japanese rocky shores affected by the Soya, Tsushima, Oyashio, and Kuroshio currents, and in the Inland Sea. On 29 shores in seven regions in these sea areas, previous researchers have recorded the number of molluscs in quadrats in spring and summer of 1978 and 1984–1986. Reanalyses of these records revealed the following patterns. (1) Shore-to-shore variation in malacofauna was correlated with shore-to-shore geographical distance less clearly for more southern regions. This might show that in southern regions many taxa with low relative abundance are haphazardly recorded on some shores and blur the effect of shore-to-shore distance on malacofaunal dissimilarity. (2) Region-to-region variation in malacofauna was significantly correlated with region-to-region geographical distance. This suggests the influences of environments and paleontological events specific to each current surrounding each region. (3) The pattern of dissimilarity between the Inland Sea and the other sea areas greatly changed from the 1970s to 1980s. This was due in part to the following density fluctuations on some shores in the Inland Sea: increases in the non-indigenous mussel Mytilus galloprovincialis, cultured oyster Crassostrea gigas, and littorinid Littorina brevicula reportedly abundant in sheltered areas; and decreases in the littorinid Nodilittorina radiata reportedly abundant in exposed areas and muricid gastropods Thais spp. showing imposex. This suggests an increase of anthropogenic perturbations during the study period in the Inland Sea which is surrounded by large cities and ports.