Changes in a common benthic community of mobile Japanese scallops Patinopecten (=Mizuhopecten) yessoensis and predatory sea stars under introduction of mariculture were assessed by comparing the distribution and abundance of each species, and the relationships between densities of these community elements in natural biotope and after twice-repeated seeding of young scallops on the bottom. Under mariculture, at high scallop density (up to 6 ind./m²), the scallop growth rates noticeably decreased. The density of the sea star Asterina (=Patiria) pectinifera markedly decreased, and its distribution was not dependent on scallop density both before and during mariculture. Mean sea star Distolasterias nipon density was also diminished, but aggregate response to high prey density increased. In contrast, the sea star Asterias amurensis density noticeably increased, and it positively correlated with scallop density before and during mariculture. The reduction of densities of sea stars A. pectinifera and D. nipon in mariculture conditions is likely to be explained by the increase in abundance of the large and mobile sea star A. amurensis as the competition for food resources seems to occur. Even big 2-y-old scallops (85.4 ± 1.5 mm shell height) at high density (>2 or 4 scallops per m², for different sea stars) attract such predatory sea stars as A. amurensis and D. nipon. The results indicate that seeding of young scallops on the bottom and especially reseeding markedly increase the attraction of the biotope to predatory sea stars for a long time.