I studied an alien bivalve’s (Xenostrobus securis) distribution to understand its occurrence range better. I compared the bivalve’s horizontal seasonal change, vertical distribution, and population dynamics between periods of heavy eutrophication (1991–1992) vs. periods when water conditions were improved (2010–2011) in Dokai Bay, northern Kyushu, Japan.

Xenostrobus securis was the dominant species in both periods. The total number of sessile animal species increased in the latter period, although this mussel’s biomass in the latter period decreased to about 29％ of the former period. I observed X. securis’ wide distribution all year round, and the small mussels’ recruitment occurred in the bay’s inner and central areas in the former period. Meanwhile, in the latter period, small mussels were similarly recruited to a wide area in summer, but the population declined significantly after summer, except in the bay’s inner part. As a result, this mussel’s distribution became uneven in most inner and intertidal zones, where the environmental conditions were harsh. I found a negative correlation between X. securis’ biomass and the sessile animal species richness. This result suggests that improved water environment bears richer biodiversity and is not suitable for X. securis.