Evidence of a sharper decrease in a non-indigenous mussel Mytilus galloprovincialis than in indigenous bivalves from 1978 to 2006 on Japanese rocky shores

In various studies encompassing large spatiotemporal scales, the densities of non-indigenous species and indigenous species in the same habitat increased and decreased, respectively, prior to reaching an equilibrium. We examined whether this is true for a non-indigenous mussel Mytilus galloprovincialis, introduced into Japan before 1934, and the sympatric indigenous bivalves: Septifer virgatus, Septifer bilocularis, Hormomya mutabilis, Saccostrea kegaki, Saccostrea mordax and Barbatia (Savignyarca) virescens. Seven quadrat surveys during 1978–2006 on 19 rocky shores (26.6–41.4°N) showed unexpected results. Density of M. galloprovincialis decreased on 89% of the shores where this mussel had previously been found. In contrast, densities of the indigenous bivalves decreased on lower percentages (17–71%) of shores where the bivalves had been found. Shore-to-shore mean density of M. galloprovincialis decreased from the 1970s (27.0–88.7 m⁻²) to the 2000s (0.0–1.1 m⁻²), more sharply than any of the indigenous bivalves (from 4.6–9.1 m⁻² to 0.7–1.5 m⁻² for S. bilocularis, which showed the most marked reduction). The conspicuous decrease for M. galloprovincialis suggests that some factors on the studied shores have become disadvantageous especially for this non-indigenous mussel from the earlier to the later surveys.